From adfda1f5909dfc5352b5e3e88b14b8e80c911d74 Mon Sep 17 00:00:00 2001
From: Alex Crichton <alex@alexcrichton.com>
Date: Tue, 22 Oct 2019 08:51:35 -0700
Subject: [PATCH 1/5] rustc: Link LLVM directly into rustc again

This commit builds on #65501 continue to simplify the build system and
compiler now that we no longer have multiple LLVM backends to ship by
default. Here this switches the compiler back to what it once was long
long ago, which is linking LLVM directly to the compiler rather than
dynamically loading it at runtime. The `codegen-backends` directory of
the sysroot no longer exists and all relevant support in the build
system is removed. Note that `rustc` still supports a dynamically loaded
codegen backend as it did previously, it just no longer supports
dynamically loaded codegen backends in its own sysroot.

Additionally as part of this the `librustc_codegen_llvm` crate now once
again explicitly depends on all of its crates instead of implicitly
loading them through the sysroot. This involved filling out its
`Cargo.toml` and deleting all the now-unnecessary `extern crate`
annotations in the header of the crate. (this in turn required adding a
number of imports for names of macros too).

The end results of this change are:

* Rustbuild's build process for the compiler as all the "oh don't forget
  the codegen backend" checks can be easily removed.
* Building `rustc_codegen_llvm` is much simpler since it's simply
  another compiler crate.
* Managing the dependencies of `rustc_codegen_llvm` is much simpler since
  it's "just another `Cargo.toml` to edit"
* The build process should be a smidge faster because there's more
  parallelism in the main rustc build step rather than splitting
  `librustc_codegen_llvm` out to its own step.
* The compiler is expected to be slightly faster by default because the
  codegen backend does not need to be dynamically loaded.
* Disabling LLVM as part of rustbuild is still supported, supporting
  multiple codegen backends is still supported, and dynamic loading of a
  codegen backend is still supported.
---
 Cargo.lock                                    |  19 ++
 config.toml.example                           |   3 -
 src/bootstrap/builder.rs                      |  22 --
 src/bootstrap/check.rs                        |  66 +----
 src/bootstrap/compile.rs                      | 267 ++++--------------
 src/bootstrap/config.rs                       |   5 -
 src/bootstrap/dist.rs                         |  10 -
 src/bootstrap/doc.rs                          |   2 +-
 src/bootstrap/lib.rs                          |   3 +
 src/bootstrap/test.rs                         |   2 +-
 src/librustc_codegen_llvm/Cargo.toml          |  20 +-
 src/librustc_codegen_llvm/abi.rs              |   2 +
 src/librustc_codegen_llvm/allocator.rs        |   1 +
 src/librustc_codegen_llvm/asm.rs              |   2 +-
 src/librustc_codegen_llvm/attributes.rs       |   1 +
 src/librustc_codegen_llvm/back/lto.rs         |   4 +-
 src/librustc_codegen_llvm/back/write.rs       |   8 +-
 src/librustc_codegen_llvm/builder.rs          |   2 +
 src/librustc_codegen_llvm/callee.rs           |   1 +
 src/librustc_codegen_llvm/common.rs           |   2 +
 src/librustc_codegen_llvm/consts.rs           |   3 +
 src/librustc_codegen_llvm/context.rs          |   2 +
 src/librustc_codegen_llvm/debuginfo/gdb.rs    |   1 +
 .../debuginfo/metadata.rs                     |   3 +
 src/librustc_codegen_llvm/debuginfo/mod.rs    |   1 +
 .../debuginfo/source_loc.rs                   |   1 +
 src/librustc_codegen_llvm/declare.rs          |   1 +
 src/librustc_codegen_llvm/intrinsic.rs        |   1 +
 src/librustc_codegen_llvm/lib.rs              |  11 +-
 src/librustc_codegen_llvm/llvm/ffi.rs         |   1 +
 src/librustc_codegen_llvm/llvm_util.rs        |   1 +
 src/librustc_codegen_llvm/metadata.rs         |   2 +
 src/librustc_codegen_llvm/mono_item.rs        |   1 +
 src/librustc_codegen_llvm/type_.rs            |   1 +
 src/librustc_codegen_llvm/type_of.rs          |   2 +
 src/librustc_driver/Cargo.toml                |   3 +
 src/librustc_driver/lib.rs                    |   6 +-
 src/librustc_interface/Cargo.toml             |   4 +
 src/librustc_interface/util.rs                |  85 +-----
 src/rustc/Cargo.toml                          |   1 +
 src/tools/tidy/src/deps.rs                    |   1 +
 41 files changed, 159 insertions(+), 415 deletions(-)

diff --git a/Cargo.lock b/Cargo.lock
index 26727c5c1db60..4d523b18a9d65 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3496,7 +3496,25 @@ dependencies = [
 name = "rustc_codegen_llvm"
 version = "0.0.0"
 dependencies = [
+ "bitflags",
+ "flate2",
+ "libc",
+ "log",
+ "rustc",
+ "rustc-demangle",
+ "rustc_codegen_ssa",
+ "rustc_codegen_utils",
+ "rustc_data_structures",
+ "rustc_errors",
+ "rustc_fs_util",
+ "rustc_incremental",
+ "rustc_index",
  "rustc_llvm",
+ "rustc_target",
+ "smallvec 0.6.10",
+ "syntax",
+ "syntax_expand",
+ "syntax_pos",
 ]
 
 [[package]]
@@ -3658,6 +3676,7 @@ dependencies = [
  "once_cell",
  "rustc",
  "rustc-rayon 0.3.0",
+ "rustc_codegen_llvm",
  "rustc_codegen_ssa",
  "rustc_codegen_utils",
  "rustc_data_structures",
diff --git a/config.toml.example b/config.toml.example
index e832570ed982e..5152a6c988582 100644
--- a/config.toml.example
+++ b/config.toml.example
@@ -379,9 +379,6 @@
 # and currently the only standard option supported is `"llvm"`
 #codegen-backends = ["llvm"]
 
-# This is the name of the directory in which codegen backends will get installed
-#codegen-backends-dir = "codegen-backends"
-
 # Indicates whether LLD will be compiled and made available in the sysroot for
 # rustc to execute.
 #lld = false
diff --git a/src/bootstrap/builder.rs b/src/bootstrap/builder.rs
index 99b8ddf7db1f0..fc58fed2b0686 100644
--- a/src/bootstrap/builder.rs
+++ b/src/bootstrap/builder.rs
@@ -339,7 +339,6 @@ impl<'a> Builder<'a> {
             Kind::Build => describe!(
                 compile::Std,
                 compile::Rustc,
-                compile::CodegenBackend,
                 compile::StartupObjects,
                 tool::BuildManifest,
                 tool::Rustbook,
@@ -364,7 +363,6 @@ impl<'a> Builder<'a> {
             Kind::Check | Kind::Clippy | Kind::Fix => describe!(
                 check::Std,
                 check::Rustc,
-                check::CodegenBackend,
                 check::Rustdoc
             ),
             Kind::Test => describe!(
@@ -631,11 +629,6 @@ impl<'a> Builder<'a> {
         self.ensure(Libdir { compiler, target })
     }
 
-    pub fn sysroot_codegen_backends(&self, compiler: Compiler) -> PathBuf {
-        self.sysroot_libdir(compiler, compiler.host)
-            .with_file_name(self.config.rust_codegen_backends_dir.clone())
-    }
-
     /// Returns the compiler's libdir where it stores the dynamic libraries that
     /// it itself links against.
     ///
@@ -706,15 +699,6 @@ impl<'a> Builder<'a> {
         }
     }
 
-    /// Gets the paths to all of the compiler's codegen backends.
-    fn codegen_backends(&self, compiler: Compiler) -> impl Iterator<Item = PathBuf> {
-        fs::read_dir(self.sysroot_codegen_backends(compiler))
-            .into_iter()
-            .flatten()
-            .filter_map(Result::ok)
-            .map(|entry| entry.path())
-    }
-
     pub fn rustdoc(&self, compiler: Compiler) -> PathBuf {
         self.ensure(tool::Rustdoc { compiler })
     }
@@ -758,12 +742,6 @@ impl<'a> Builder<'a> {
         let mut cargo = Command::new(&self.initial_cargo);
         let out_dir = self.stage_out(compiler, mode);
 
-        // Codegen backends are not yet tracked by -Zbinary-dep-depinfo,
-        // so we need to explicitly clear out if they've been updated.
-        for backend in self.codegen_backends(compiler) {
-            self.clear_if_dirty(&out_dir, &backend);
-        }
-
         if cmd == "doc" || cmd == "rustdoc" {
             let my_out = match mode {
                 // This is the intended out directory for compiler documentation.
diff --git a/src/bootstrap/check.rs b/src/bootstrap/check.rs
index df1c72575846b..f5c427d870e71 100644
--- a/src/bootstrap/check.rs
+++ b/src/bootstrap/check.rs
@@ -1,11 +1,10 @@
 //! Implementation of compiling the compiler and standard library, in "check"-based modes.
 
-use crate::compile::{run_cargo, std_cargo, rustc_cargo, rustc_cargo_env,
-                     add_to_sysroot};
+use crate::compile::{run_cargo, std_cargo, rustc_cargo, add_to_sysroot};
 use crate::builder::{RunConfig, Builder, Kind, ShouldRun, Step};
 use crate::tool::{prepare_tool_cargo, SourceType};
 use crate::{Compiler, Mode};
-use crate::cache::{INTERNER, Interned};
+use crate::cache::Interned;
 use std::path::PathBuf;
 
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
@@ -97,7 +96,7 @@ impl Step for Rustc {
 
         let mut cargo = builder.cargo(compiler, Mode::Rustc, target,
             cargo_subcommand(builder.kind));
-        rustc_cargo(builder, &mut cargo);
+        rustc_cargo(builder, &mut cargo, target);
 
         builder.info(&format!("Checking compiler artifacts ({} -> {})", &compiler.host, target));
         run_cargo(builder,
@@ -113,55 +112,6 @@ impl Step for Rustc {
     }
 }
 
-#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
-pub struct CodegenBackend {
-    pub target: Interned<String>,
-    pub backend: Interned<String>,
-}
-
-impl Step for CodegenBackend {
-    type Output = ();
-    const ONLY_HOSTS: bool = true;
-    const DEFAULT: bool = true;
-
-    fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
-        run.all_krates("rustc_codegen_llvm")
-    }
-
-    fn make_run(run: RunConfig<'_>) {
-        let backend = run.builder.config.rust_codegen_backends.get(0);
-        let backend = backend.cloned().unwrap_or_else(|| {
-            INTERNER.intern_str("llvm")
-        });
-        run.builder.ensure(CodegenBackend {
-            target: run.target,
-            backend,
-        });
-    }
-
-    fn run(self, builder: &Builder<'_>) {
-        let compiler = builder.compiler(0, builder.config.build);
-        let target = self.target;
-        let backend = self.backend;
-
-        builder.ensure(Rustc { target });
-
-        let mut cargo = builder.cargo(compiler, Mode::Codegen, target,
-            cargo_subcommand(builder.kind));
-        cargo.arg("--manifest-path").arg(builder.src.join("src/librustc_codegen_llvm/Cargo.toml"));
-        rustc_cargo_env(builder, &mut cargo);
-
-        // We won't build LLVM if it's not available, as it shouldn't affect `check`.
-
-        run_cargo(builder,
-                  cargo,
-                  args(builder.kind),
-                  &codegen_backend_stamp(builder, compiler, target, backend),
-                  vec![],
-                  true);
-    }
-}
-
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
 pub struct Rustdoc {
     pub target: Interned<String>,
@@ -231,16 +181,6 @@ pub fn librustc_stamp(
     builder.cargo_out(compiler, Mode::Rustc, target).join(".librustc-check.stamp")
 }
 
-/// Cargo's output path for librustc_codegen_llvm in a given stage, compiled by a particular
-/// compiler for the specified target and backend.
-fn codegen_backend_stamp(builder: &Builder<'_>,
-                         compiler: Compiler,
-                         target: Interned<String>,
-                         backend: Interned<String>) -> PathBuf {
-    builder.cargo_out(compiler, Mode::Codegen, target)
-         .join(format!(".librustc_codegen_llvm-{}-check.stamp", backend))
-}
-
 /// Cargo's output path for rustdoc in a given stage, compiled by a particular
 /// compiler for the specified target.
 pub fn rustdoc_stamp(
diff --git a/src/bootstrap/compile.rs b/src/bootstrap/compile.rs
index f686dfe71b937..9d5e3db449890 100644
--- a/src/bootstrap/compile.rs
+++ b/src/bootstrap/compile.rs
@@ -27,7 +27,7 @@ use crate::{Compiler, Mode, GitRepo};
 use crate::native;
 
 use crate::cache::{INTERNER, Interned};
-use crate::builder::{Step, RunConfig, ShouldRun, Builder};
+use crate::builder::{Step, RunConfig, ShouldRun, Builder, Kind};
 
 #[derive(Debug, PartialOrd, Ord, Copy, Clone, PartialEq, Eq, Hash)]
 pub struct Std {
@@ -445,7 +445,7 @@ impl Step for Rustc {
         });
 
         let mut cargo = builder.cargo(compiler, Mode::Rustc, target, "build");
-        rustc_cargo(builder, &mut cargo);
+        rustc_cargo(builder, &mut cargo, target);
 
         builder.info(&format!("Building stage{} compiler artifacts ({} -> {})",
                  compiler.stage, &compiler.host, target));
@@ -464,21 +464,20 @@ impl Step for Rustc {
     }
 }
 
-pub fn rustc_cargo(builder: &Builder<'_>, cargo: &mut Cargo) {
+pub fn rustc_cargo(builder: &Builder<'_>, cargo: &mut Cargo, target: Interned<String>) {
     cargo.arg("--features").arg(builder.rustc_features())
          .arg("--manifest-path")
          .arg(builder.src.join("src/rustc/Cargo.toml"));
-    rustc_cargo_env(builder, cargo);
+    rustc_cargo_env(builder, cargo, target);
 }
 
-pub fn rustc_cargo_env(builder: &Builder<'_>, cargo: &mut Cargo) {
+pub fn rustc_cargo_env(builder: &Builder<'_>, cargo: &mut Cargo, target: Interned<String>) {
     // Set some configuration variables picked up by build scripts and
     // the compiler alike
     cargo.env("CFG_RELEASE", builder.rust_release())
          .env("CFG_RELEASE_CHANNEL", &builder.config.channel)
          .env("CFG_VERSION", builder.rust_version())
-         .env("CFG_PREFIX", builder.config.prefix.clone().unwrap_or_default())
-         .env("CFG_CODEGEN_BACKENDS_DIR", &builder.config.rust_codegen_backends_dir);
+         .env("CFG_PREFIX", builder.config.prefix.clone().unwrap_or_default());
 
     let libdir_relative = builder.config.libdir_relative().unwrap_or(Path::new("lib"));
     cargo.env("CFG_LIBDIR_RELATIVE", libdir_relative);
@@ -501,6 +500,49 @@ pub fn rustc_cargo_env(builder: &Builder<'_>, cargo: &mut Cargo) {
     if builder.config.rust_verify_llvm_ir {
         cargo.env("RUSTC_VERIFY_LLVM_IR", "1");
     }
+
+    // Pass down configuration from the LLVM build into the build of
+    // librustc_llvm and librustc_codegen_llvm.
+    //
+    // Note that this is disabled if LLVM itself is disabled or we're in a check
+    // build, where if we're in a check build there's no need to build all of
+    // LLVM and such.
+    if builder.config.llvm_enabled() && builder.kind != Kind::Check {
+        if builder.is_rust_llvm(target) {
+            cargo.env("LLVM_RUSTLLVM", "1");
+        }
+        let llvm_config = builder.ensure(native::Llvm { target });
+        cargo.env("LLVM_CONFIG", &llvm_config);
+        let target_config = builder.config.target_config.get(&target);
+        if let Some(s) = target_config.and_then(|c| c.llvm_config.as_ref()) {
+            cargo.env("CFG_LLVM_ROOT", s);
+        }
+        // Some LLVM linker flags (-L and -l) may be needed to link librustc_llvm.
+        if let Some(ref s) = builder.config.llvm_ldflags {
+            cargo.env("LLVM_LINKER_FLAGS", s);
+        }
+        // Building with a static libstdc++ is only supported on linux right now,
+        // not for MSVC or macOS
+        if builder.config.llvm_static_stdcpp &&
+           !target.contains("freebsd") &&
+           !target.contains("windows") &&
+           !target.contains("apple") {
+            let file = compiler_file(builder,
+                                     builder.cxx(target).unwrap(),
+                                     target,
+                                     "libstdc++.a");
+            cargo.env("LLVM_STATIC_STDCPP", file);
+        }
+        if builder.config.llvm_link_shared || builder.config.llvm_thin_lto {
+            cargo.env("LLVM_LINK_SHARED", "1");
+        }
+        if builder.config.llvm_use_libcxx {
+            cargo.env("LLVM_USE_LIBCXX", "1");
+        }
+        if builder.config.llvm_optimize && !builder.config.llvm_release_debuginfo {
+            cargo.env("LLVM_NDEBUG", "1");
+        }
+    }
 }
 
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
@@ -537,197 +579,6 @@ impl Step for RustcLink {
     }
 }
 
-#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
-pub struct CodegenBackend {
-    pub compiler: Compiler,
-    pub target: Interned<String>,
-    pub backend: Interned<String>,
-}
-
-impl Step for CodegenBackend {
-    type Output = ();
-    const ONLY_HOSTS: bool = true;
-    const DEFAULT: bool = true;
-
-    fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
-        run.all_krates("rustc_codegen_llvm")
-    }
-
-    fn make_run(run: RunConfig<'_>) {
-        let backend = run.builder.config.rust_codegen_backends.get(0);
-        let backend = backend.cloned().unwrap_or_else(|| {
-            INTERNER.intern_str("llvm")
-        });
-        run.builder.ensure(CodegenBackend {
-            compiler: run.builder.compiler(run.builder.top_stage, run.host),
-            target: run.target,
-            backend,
-        });
-    }
-
-    fn run(self, builder: &Builder<'_>) {
-        let compiler = self.compiler;
-        let target = self.target;
-        let backend = self.backend;
-
-        builder.ensure(Rustc { compiler, target });
-
-        if builder.config.keep_stage.contains(&compiler.stage) {
-            builder.info("Warning: Using a potentially old codegen backend. \
-                This may not behave well.");
-            // Codegen backends are linked separately from this step today, so we don't do
-            // anything here.
-            return;
-        }
-
-        let compiler_to_use = builder.compiler_for(compiler.stage, compiler.host, target);
-        if compiler_to_use != compiler {
-            builder.ensure(CodegenBackend {
-                compiler: compiler_to_use,
-                target,
-                backend,
-            });
-            return;
-        }
-
-        let out_dir = builder.cargo_out(compiler, Mode::Codegen, target);
-
-        let mut cargo = builder.cargo(compiler, Mode::Codegen, target, "build");
-        cargo.arg("--manifest-path")
-            .arg(builder.src.join("src/librustc_codegen_llvm/Cargo.toml"));
-        rustc_cargo_env(builder, &mut cargo);
-
-        let features = build_codegen_backend(&builder, &mut cargo, &compiler, target, backend);
-        cargo.arg("--features").arg(features);
-
-        let tmp_stamp = out_dir.join(".tmp.stamp");
-
-        let files = run_cargo(builder, cargo, vec![], &tmp_stamp, vec![], false);
-        if builder.config.dry_run {
-            return;
-        }
-        let mut files = files.into_iter()
-            .filter(|f| {
-                let filename = f.file_name().unwrap().to_str().unwrap();
-                is_dylib(filename) && filename.contains("rustc_codegen_llvm-")
-            });
-        let codegen_backend = match files.next() {
-            Some(f) => f,
-            None => panic!("no dylibs built for codegen backend?"),
-        };
-        if let Some(f) = files.next() {
-            panic!("codegen backend built two dylibs:\n{}\n{}",
-                   codegen_backend.display(),
-                   f.display());
-        }
-        let stamp = codegen_backend_stamp(builder, compiler, target, backend);
-        let codegen_backend = codegen_backend.to_str().unwrap();
-        t!(fs::write(&stamp, &codegen_backend));
-    }
-}
-
-pub fn build_codegen_backend(builder: &Builder<'_>,
-                             cargo: &mut Cargo,
-                             compiler: &Compiler,
-                             target: Interned<String>,
-                             backend: Interned<String>) -> String {
-    match &*backend {
-        "llvm" => {
-            // Build LLVM for our target. This will implicitly build the
-            // host LLVM if necessary.
-            let llvm_config = builder.ensure(native::Llvm {
-                target,
-            });
-
-            builder.info(&format!("Building stage{} codegen artifacts ({} -> {}, {})",
-                     compiler.stage, &compiler.host, target, backend));
-
-            // Pass down configuration from the LLVM build into the build of
-            // librustc_llvm and librustc_codegen_llvm.
-            if builder.is_rust_llvm(target) {
-                cargo.env("LLVM_RUSTLLVM", "1");
-            }
-
-            cargo.env("LLVM_CONFIG", &llvm_config);
-            let target_config = builder.config.target_config.get(&target);
-            if let Some(s) = target_config.and_then(|c| c.llvm_config.as_ref()) {
-                cargo.env("CFG_LLVM_ROOT", s);
-            }
-            // Some LLVM linker flags (-L and -l) may be needed to link librustc_llvm.
-            if let Some(ref s) = builder.config.llvm_ldflags {
-                cargo.env("LLVM_LINKER_FLAGS", s);
-            }
-            // Building with a static libstdc++ is only supported on linux and mingw right now,
-            // not for MSVC or macOS
-            if builder.config.llvm_static_stdcpp &&
-               !target.contains("freebsd") &&
-               !target.contains("msvc") &&
-               !target.contains("apple") {
-                let file = compiler_file(builder,
-                                         builder.cxx(target).unwrap(),
-                                         target,
-                                         "libstdc++.a");
-                cargo.env("LLVM_STATIC_STDCPP", file);
-            }
-            if builder.config.llvm_link_shared || builder.config.llvm_thin_lto {
-                cargo.env("LLVM_LINK_SHARED", "1");
-            }
-            if builder.config.llvm_use_libcxx {
-                cargo.env("LLVM_USE_LIBCXX", "1");
-            }
-            if builder.config.llvm_optimize && !builder.config.llvm_release_debuginfo {
-                cargo.env("LLVM_NDEBUG", "1");
-            }
-        }
-        _ => panic!("unknown backend: {}", backend),
-    }
-    String::new()
-}
-
-/// Creates the `codegen-backends` folder for a compiler that's about to be
-/// assembled as a complete compiler.
-///
-/// This will take the codegen artifacts produced by `compiler` and link them
-/// into an appropriate location for `target_compiler` to be a functional
-/// compiler.
-fn copy_codegen_backends_to_sysroot(builder: &Builder<'_>,
-                                    compiler: Compiler,
-                                    target_compiler: Compiler) {
-    let target = target_compiler.host;
-
-    // Note that this step is different than all the other `*Link` steps in
-    // that it's not assembling a bunch of libraries but rather is primarily
-    // moving the codegen backend into place. The codegen backend of rustc is
-    // not linked into the main compiler by default but is rather dynamically
-    // selected at runtime for inclusion.
-    //
-    // Here we're looking for the output dylib of the `CodegenBackend` step and
-    // we're copying that into the `codegen-backends` folder.
-    let dst = builder.sysroot_codegen_backends(target_compiler);
-    t!(fs::create_dir_all(&dst));
-
-    if builder.config.dry_run {
-        return;
-    }
-
-    for backend in builder.config.rust_codegen_backends.iter() {
-        let stamp = codegen_backend_stamp(builder, compiler, target, *backend);
-        let dylib = t!(fs::read_to_string(&stamp));
-        let file = Path::new(&dylib);
-        let filename = file.file_name().unwrap().to_str().unwrap();
-        // change `librustc_codegen_llvm-xxxxxx.so` to `librustc_codegen_llvm-llvm.so`
-        let target_filename = {
-            let dash = filename.find('-').unwrap();
-            let dot = filename.find('.').unwrap();
-            format!("{}-{}{}",
-                    &filename[..dash],
-                    backend,
-                    &filename[dot..])
-        };
-        builder.copy(&file, &dst.join(target_filename));
-    }
-}
-
 fn copy_lld_to_sysroot(builder: &Builder<'_>,
                        target_compiler: Compiler,
                        lld_install_root: &Path) {
@@ -766,16 +617,6 @@ pub fn librustc_stamp(
     builder.cargo_out(compiler, Mode::Rustc, target).join(".librustc.stamp")
 }
 
-/// Cargo's output path for librustc_codegen_llvm in a given stage, compiled by a particular
-/// compiler for the specified target and backend.
-fn codegen_backend_stamp(builder: &Builder<'_>,
-                         compiler: Compiler,
-                         target: Interned<String>,
-                         backend: Interned<String>) -> PathBuf {
-    builder.cargo_out(compiler, Mode::Codegen, target)
-        .join(format!(".librustc_codegen_llvm-{}.stamp", backend))
-}
-
 pub fn compiler_file(
     builder: &Builder<'_>,
     compiler: &Path,
@@ -879,13 +720,6 @@ impl Step for Assemble {
             compiler: build_compiler,
             target: target_compiler.host,
         });
-        for &backend in builder.config.rust_codegen_backends.iter() {
-            builder.ensure(CodegenBackend {
-                compiler: build_compiler,
-                target: target_compiler.host,
-                backend,
-            });
-        }
 
         let lld_install = if builder.config.lld_enabled {
             Some(builder.ensure(native::Lld {
@@ -911,9 +745,6 @@ impl Step for Assemble {
             }
         }
 
-        copy_codegen_backends_to_sysroot(builder,
-                                         build_compiler,
-                                         target_compiler);
         if let Some(lld_install) = lld_install {
             copy_lld_to_sysroot(builder, target_compiler, &lld_install);
         }
diff --git a/src/bootstrap/config.rs b/src/bootstrap/config.rs
index 0c03b95c7b251..5f2ef01bd5c45 100644
--- a/src/bootstrap/config.rs
+++ b/src/bootstrap/config.rs
@@ -105,7 +105,6 @@ pub struct Config {
     pub rust_optimize_tests: bool,
     pub rust_dist_src: bool,
     pub rust_codegen_backends: Vec<Interned<String>>,
-    pub rust_codegen_backends_dir: String,
     pub rust_verify_llvm_ir: bool,
     pub rust_remap_debuginfo: bool,
 
@@ -316,7 +315,6 @@ struct Rust {
     dist_src: Option<bool>,
     save_toolstates: Option<String>,
     codegen_backends: Option<Vec<String>>,
-    codegen_backends_dir: Option<String>,
     lld: Option<bool>,
     llvm_tools: Option<bool>,
     lldb: Option<bool>,
@@ -372,7 +370,6 @@ impl Config {
         config.ignore_git = false;
         config.rust_dist_src = true;
         config.rust_codegen_backends = vec![INTERNER.intern_str("llvm")];
-        config.rust_codegen_backends_dir = "codegen-backends".to_owned();
         config.deny_warnings = true;
         config.missing_tools = false;
 
@@ -575,8 +572,6 @@ impl Config {
                     .collect();
             }
 
-            set(&mut config.rust_codegen_backends_dir, rust.codegen_backends_dir.clone());
-
             config.rust_codegen_units = rust.codegen_units.map(threads_from_config);
             config.rust_codegen_units_std = rust.codegen_units_std.map(threads_from_config);
         }
diff --git a/src/bootstrap/dist.rs b/src/bootstrap/dist.rs
index d0c9e0dbaf458..4c49913bab989 100644
--- a/src/bootstrap/dist.rs
+++ b/src/bootstrap/dist.rs
@@ -498,16 +498,6 @@ impl Step for Rustc {
                 }
             }
 
-            // Copy over the codegen backends
-            let backends_src = builder.sysroot_codegen_backends(compiler);
-            let backends_rel = backends_src.strip_prefix(&src).unwrap()
-                .strip_prefix(builder.sysroot_libdir_relative(compiler)).unwrap();
-            // Don't use custom libdir here because ^lib/ will be resolved again with installer
-            let backends_dst = image.join("lib").join(&backends_rel);
-
-            t!(fs::create_dir_all(&backends_dst));
-            builder.cp_r(&backends_src, &backends_dst);
-
             // Copy libLLVM.so to the lib dir as well, if needed. While not
             // technically needed by rustc itself it's needed by lots of other
             // components like the llvm tools and LLD. LLD is included below and
diff --git a/src/bootstrap/doc.rs b/src/bootstrap/doc.rs
index 4ee8cd2485c02..6d12d5239f0b4 100644
--- a/src/bootstrap/doc.rs
+++ b/src/bootstrap/doc.rs
@@ -541,7 +541,7 @@ impl Step for Rustc {
         // Build cargo command.
         let mut cargo = builder.cargo(compiler, Mode::Rustc, target, "doc");
         cargo.env("RUSTDOCFLAGS", "--document-private-items --passes strip-hidden");
-        compile::rustc_cargo(builder, &mut cargo);
+        compile::rustc_cargo(builder, &mut cargo, target);
 
         // Only include compiler crates, no dependencies of those, such as `libc`.
         cargo.arg("--no-deps");
diff --git a/src/bootstrap/lib.rs b/src/bootstrap/lib.rs
index 39d7ea922bced..732658bd97204 100644
--- a/src/bootstrap/lib.rs
+++ b/src/bootstrap/lib.rs
@@ -501,6 +501,9 @@ impl Build {
         if self.config.jemalloc {
             features.push_str("jemalloc");
         }
+        if self.config.llvm_enabled() {
+            features.push_str(" llvm");
+        }
         features
     }
 
diff --git a/src/bootstrap/test.rs b/src/bootstrap/test.rs
index a858ed42badfd..f3b2a73d3c5dc 100644
--- a/src/bootstrap/test.rs
+++ b/src/bootstrap/test.rs
@@ -1773,7 +1773,7 @@ impl Step for Crate {
             }
             Mode::Rustc => {
                 builder.ensure(compile::Rustc { compiler, target });
-                compile::rustc_cargo(builder, &mut cargo);
+                compile::rustc_cargo(builder, &mut cargo, target);
             }
             _ => panic!("can only test libraries"),
         };
diff --git a/src/librustc_codegen_llvm/Cargo.toml b/src/librustc_codegen_llvm/Cargo.toml
index 867bbd22cfbbb..0559aac28497e 100644
--- a/src/librustc_codegen_llvm/Cargo.toml
+++ b/src/librustc_codegen_llvm/Cargo.toml
@@ -7,8 +7,26 @@ edition = "2018"
 [lib]
 name = "rustc_codegen_llvm"
 path = "lib.rs"
-crate-type = ["dylib"]
 test = false
+doctest = false
 
 [dependencies]
+bitflags = "1.0"
+flate2 = "1.0"
+libc = "0.2"
+log = "0.4"
+rustc = { path = "../librustc" }
+rustc-demangle = "0.1"
+rustc_codegen_ssa = { path = "../librustc_codegen_ssa" }
+rustc_codegen_utils = { path = "../librustc_codegen_utils" }
+rustc_data_structures = { path = "../librustc_data_structures" }
+rustc_errors = { path = "../librustc_errors" }
+rustc_fs_util = { path = "../librustc_fs_util" }
+rustc_incremental = { path = "../librustc_incremental" }
+rustc_index = { path = "../librustc_index" }
 rustc_llvm = { path = "../librustc_llvm" }
+rustc_target = { path = "../librustc_target" }
+smallvec = { version = "0.6.7", features = ["union", "may_dangle"] }
+syntax = { path = "../libsyntax" }
+syntax_expand = { path = "../libsyntax_expand" }
+syntax_pos = { path = "../libsyntax_pos" }
diff --git a/src/librustc_codegen_llvm/abi.rs b/src/librustc_codegen_llvm/abi.rs
index 1f3c8e1953e4f..24a6c12d48cd5 100644
--- a/src/librustc_codegen_llvm/abi.rs
+++ b/src/librustc_codegen_llvm/abi.rs
@@ -8,6 +8,8 @@ use crate::value::Value;
 use rustc_codegen_ssa::MemFlags;
 use rustc_codegen_ssa::mir::place::PlaceRef;
 use rustc_codegen_ssa::mir::operand::OperandValue;
+use rustc_target::abi::call::ArgAbi;
+use rustc::bug;
 use rustc_codegen_ssa::traits::*;
 use rustc_target::abi::call::ArgAbi;
 use rustc_target::abi::{HasDataLayout, LayoutOf};
diff --git a/src/librustc_codegen_llvm/allocator.rs b/src/librustc_codegen_llvm/allocator.rs
index 11b6e0befa1b1..e1d56b9be7a27 100644
--- a/src/librustc_codegen_llvm/allocator.rs
+++ b/src/librustc_codegen_llvm/allocator.rs
@@ -4,6 +4,7 @@ use crate::attributes;
 use libc::c_uint;
 use rustc::ty::TyCtxt;
 use syntax::expand::allocator::{AllocatorKind, AllocatorTy, ALLOCATOR_METHODS};
+use rustc::bug;
 
 use crate::ModuleLlvm;
 use crate::llvm::{self, False, True};
diff --git a/src/librustc_codegen_llvm/asm.rs b/src/librustc_codegen_llvm/asm.rs
index abdd2e3e8dbd7..fa43e0829190b 100644
--- a/src/librustc_codegen_llvm/asm.rs
+++ b/src/librustc_codegen_llvm/asm.rs
@@ -12,7 +12,7 @@ use syntax_pos::Span;
 
 use std::ffi::{CStr, CString};
 use libc::{c_uint, c_char};
-
+use log::debug;
 
 impl AsmBuilderMethods<'tcx> for Builder<'a, 'll, 'tcx> {
     fn codegen_inline_asm(
diff --git a/src/librustc_codegen_llvm/attributes.rs b/src/librustc_codegen_llvm/attributes.rs
index 33dc2513de584..405a685afe287 100644
--- a/src/librustc_codegen_llvm/attributes.rs
+++ b/src/librustc_codegen_llvm/attributes.rs
@@ -12,6 +12,7 @@ use rustc::ty::query::Providers;
 use rustc_data_structures::small_c_str::SmallCStr;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_target::abi::call::Conv;
+use rustc_data_structures::const_cstr;
 use rustc_target::spec::PanicStrategy;
 use rustc_codegen_ssa::traits::*;
 
diff --git a/src/librustc_codegen_llvm/back/lto.rs b/src/librustc_codegen_llvm/back/lto.rs
index 858dd59b26148..0e4e4e2f983f6 100644
--- a/src/librustc_codegen_llvm/back/lto.rs
+++ b/src/librustc_codegen_llvm/back/lto.rs
@@ -4,11 +4,12 @@ use crate::back::write::{self, DiagnosticHandlers, with_llvm_pmb, save_temp_bitc
 use crate::llvm::archive_ro::ArchiveRO;
 use crate::llvm::{self, True, False};
 use crate::{ModuleLlvm, LlvmCodegenBackend};
+use rustc::bug;
 use rustc_codegen_ssa::back::symbol_export;
 use rustc_codegen_ssa::back::write::{ModuleConfig, CodegenContext, FatLTOInput};
 use rustc_codegen_ssa::back::lto::{SerializedModule, LtoModuleCodegen, ThinShared, ThinModule};
 use rustc_codegen_ssa::traits::*;
-use errors::{FatalError, Handler};
+use rustc_errors::{FatalError, Handler};
 use rustc::dep_graph::WorkProduct;
 use rustc_session::cgu_reuse_tracker::CguReuse;
 use rustc::hir::def_id::LOCAL_CRATE;
@@ -17,6 +18,7 @@ use rustc::session::config::{self, Lto};
 use rustc::util::common::time_ext;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_codegen_ssa::{RLIB_BYTECODE_EXTENSION, ModuleCodegen, ModuleKind};
+use log::{info, debug};
 
 use std::ffi::{CStr, CString};
 use std::ptr;
diff --git a/src/librustc_codegen_llvm/back/write.rs b/src/librustc_codegen_llvm/back/write.rs
index a05ba9c78e0d2..458976a1d0b81 100644
--- a/src/librustc_codegen_llvm/back/write.rs
+++ b/src/librustc_codegen_llvm/back/write.rs
@@ -10,6 +10,7 @@ use crate::type_::Type;
 use crate::context::{is_pie_binary, get_reloc_model};
 use crate::common;
 use crate::LlvmCodegenBackend;
+use rustc::bug;
 use rustc::hir::def_id::LOCAL_CRATE;
 use rustc_codegen_ssa::back::write::{CodegenContext, ModuleConfig, run_assembler};
 use rustc_codegen_ssa::traits::*;
@@ -20,7 +21,8 @@ use rustc_codegen_ssa::{RLIB_BYTECODE_EXTENSION, ModuleCodegen, CompiledModule};
 use rustc::util::common::time_ext;
 use rustc_fs_util::{path_to_c_string, link_or_copy};
 use rustc_data_structures::small_c_str::SmallCStr;
-use errors::{Handler, FatalError};
+use rustc_errors::{Handler, FatalError};
+use log::debug;
 
 use std::ffi::{CString, CStr};
 use std::fs;
@@ -55,7 +57,7 @@ pub const TLS_MODEL_ARGS : [(&str, llvm::ThreadLocalMode); 4] = [
     ("local-exec", llvm::ThreadLocalMode::LocalExec),
 ];
 
-pub fn llvm_err(handler: &errors::Handler, msg: &str) -> FatalError {
+pub fn llvm_err(handler: &rustc_errors::Handler, msg: &str) -> FatalError {
     match llvm::last_error() {
         Some(err) => handler.fatal(&format!("{}: {}", msg, err)),
         None => handler.fatal(&msg),
@@ -63,7 +65,7 @@ pub fn llvm_err(handler: &errors::Handler, msg: &str) -> FatalError {
 }
 
 pub fn write_output_file(
-        handler: &errors::Handler,
+        handler: &rustc_errors::Handler,
         target: &'ll llvm::TargetMachine,
         pm: &llvm::PassManager<'ll>,
         m: &'ll llvm::Module,
diff --git a/src/librustc_codegen_llvm/builder.rs b/src/librustc_codegen_llvm/builder.rs
index 6f72466c559dc..7509584df277e 100644
--- a/src/librustc_codegen_llvm/builder.rs
+++ b/src/librustc_codegen_llvm/builder.rs
@@ -23,6 +23,8 @@ use std::ffi::CStr;
 use std::ops::{Deref, Range};
 use std::ptr;
 use std::iter::TrustedLen;
+use rustc_data_structures::const_cstr;
+use log::debug;
 
 // All Builders must have an llfn associated with them
 #[must_use]
diff --git a/src/librustc_codegen_llvm/callee.rs b/src/librustc_codegen_llvm/callee.rs
index e0db7cae99e1a..c0be87b117d65 100644
--- a/src/librustc_codegen_llvm/callee.rs
+++ b/src/librustc_codegen_llvm/callee.rs
@@ -10,6 +10,7 @@ use crate::llvm;
 use crate::context::CodegenCx;
 use crate::value::Value;
 use rustc_codegen_ssa::traits::*;
+use log::debug;
 
 use rustc::ty::{TypeFoldable, Instance};
 use rustc::ty::layout::{FnAbiExt, HasTyCtxt};
diff --git a/src/librustc_codegen_llvm/common.rs b/src/librustc_codegen_llvm/common.rs
index f38f9dfecd387..075f54b9e613f 100644
--- a/src/librustc_codegen_llvm/common.rs
+++ b/src/librustc_codegen_llvm/common.rs
@@ -8,6 +8,8 @@ use crate::type_::Type;
 use crate::type_of::LayoutLlvmExt;
 use crate::value::Value;
 use rustc_codegen_ssa::traits::*;
+use rustc::bug;
+use log::debug;
 
 use crate::consts::const_alloc_to_llvm;
 use rustc::ty::layout::{HasDataLayout, LayoutOf, self, TyLayout, Size};
diff --git a/src/librustc_codegen_llvm/consts.rs b/src/librustc_codegen_llvm/consts.rs
index 541b3d9476b33..d51efd2833c71 100644
--- a/src/librustc_codegen_llvm/consts.rs
+++ b/src/librustc_codegen_llvm/consts.rs
@@ -16,6 +16,9 @@ use rustc::ty::{self, Ty, Instance};
 use rustc_codegen_ssa::traits::*;
 use syntax::symbol::{Symbol, sym};
 use syntax_pos::Span;
+use rustc::{bug, span_bug};
+use rustc_data_structures::const_cstr;
+use log::debug;
 
 use rustc::ty::layout::{self, Size, Align, LayoutOf};
 
diff --git a/src/librustc_codegen_llvm/context.rs b/src/librustc_codegen_llvm/context.rs
index 39ea1f6f5dccf..2c894a5d7403c 100644
--- a/src/librustc_codegen_llvm/context.rs
+++ b/src/librustc_codegen_llvm/context.rs
@@ -12,6 +12,7 @@ use rustc_codegen_ssa::traits::*;
 
 use rustc_data_structures::base_n;
 use rustc_data_structures::small_c_str::SmallCStr;
+use rustc::bug;
 use rustc::mir::mono::CodegenUnit;
 use rustc::session::config::{self, DebugInfo};
 use rustc::session::Session;
@@ -23,6 +24,7 @@ use rustc::util::nodemap::FxHashMap;
 use rustc_target::spec::{HasTargetSpec, Target};
 use rustc_codegen_ssa::base::wants_msvc_seh;
 use crate::callee::get_fn;
+use rustc_data_structures::const_cstr;
 
 use std::ffi::CStr;
 use std::cell::{Cell, RefCell};
diff --git a/src/librustc_codegen_llvm/debuginfo/gdb.rs b/src/librustc_codegen_llvm/debuginfo/gdb.rs
index 9ed1c1730a697..739437ac27b82 100644
--- a/src/librustc_codegen_llvm/debuginfo/gdb.rs
+++ b/src/librustc_codegen_llvm/debuginfo/gdb.rs
@@ -7,6 +7,7 @@ use crate::builder::Builder;
 use crate::value::Value;
 use rustc::session::config::DebugInfo;
 use rustc_codegen_ssa::traits::*;
+use rustc::bug;
 
 use syntax::attr;
 use syntax::symbol::sym;
diff --git a/src/librustc_codegen_llvm/debuginfo/metadata.rs b/src/librustc_codegen_llvm/debuginfo/metadata.rs
index 1847e4e9fa951..8327ff257c210 100644
--- a/src/librustc_codegen_llvm/debuginfo/metadata.rs
+++ b/src/librustc_codegen_llvm/debuginfo/metadata.rs
@@ -35,10 +35,13 @@ use rustc::session::config::{self, DebugInfo};
 use rustc::util::nodemap::FxHashMap;
 use rustc_fs_util::path_to_c_string;
 use rustc_data_structures::small_c_str::SmallCStr;
+use rustc_data_structures::const_cstr;
 use rustc_target::abi::HasDataLayout;
 use syntax::ast;
 use syntax::symbol::{Interner, Symbol};
 use syntax_pos::{self, Span, FileName};
+use rustc::{bug, span_bug};
+use log::debug;
 
 use libc::{c_uint, c_longlong};
 use std::collections::hash_map::Entry;
diff --git a/src/librustc_codegen_llvm/debuginfo/mod.rs b/src/librustc_codegen_llvm/debuginfo/mod.rs
index c2359a2fe6d94..f669699aa9ba3 100644
--- a/src/librustc_codegen_llvm/debuginfo/mod.rs
+++ b/src/librustc_codegen_llvm/debuginfo/mod.rs
@@ -33,6 +33,7 @@ use rustc_codegen_ssa::mir::debuginfo::{FunctionDebugContext, DebugScope,
 use libc::c_uint;
 use std::cell::RefCell;
 use std::ffi::{CStr, CString};
+use log::debug;
 
 use smallvec::SmallVec;
 use syntax_pos::{self, BytePos, Span, Pos};
diff --git a/src/librustc_codegen_llvm/debuginfo/source_loc.rs b/src/librustc_codegen_llvm/debuginfo/source_loc.rs
index ccb3bde1cbe4e..82183fa9bd7bf 100644
--- a/src/librustc_codegen_llvm/debuginfo/source_loc.rs
+++ b/src/librustc_codegen_llvm/debuginfo/source_loc.rs
@@ -8,6 +8,7 @@ use crate::llvm;
 use crate::llvm::debuginfo::DIScope;
 use crate::builder::Builder;
 use rustc_codegen_ssa::traits::*;
+use log::debug;
 
 use libc::c_uint;
 use syntax_pos::{Span, Pos};
diff --git a/src/librustc_codegen_llvm/declare.rs b/src/librustc_codegen_llvm/declare.rs
index 8b6fedc87db96..1131f3939c48b 100644
--- a/src/librustc_codegen_llvm/declare.rs
+++ b/src/librustc_codegen_llvm/declare.rs
@@ -22,6 +22,7 @@ use rustc::ty::Ty;
 use rustc::session::config::Sanitizer;
 use rustc_data_structures::small_c_str::SmallCStr;
 use rustc_codegen_ssa::traits::*;
+use log::debug;
 
 /// Declare a function.
 ///
diff --git a/src/librustc_codegen_llvm/intrinsic.rs b/src/librustc_codegen_llvm/intrinsic.rs
index 9df75a800f123..eb66813b89357 100644
--- a/src/librustc_codegen_llvm/intrinsic.rs
+++ b/src/librustc_codegen_llvm/intrinsic.rs
@@ -19,6 +19,7 @@ use rustc_codegen_ssa::common::{IntPredicate, TypeKind};
 use rustc::hir;
 use rustc_target::abi::HasDataLayout;
 use syntax::ast;
+use rustc::{bug, span_bug};
 
 use rustc_codegen_ssa::common::span_invalid_monomorphization_error;
 use rustc_codegen_ssa::traits::*;
diff --git a/src/librustc_codegen_llvm/lib.rs b/src/librustc_codegen_llvm/lib.rs
index 2ff5872370fd1..63b3e8da1fcc3 100644
--- a/src/librustc_codegen_llvm/lib.rs
+++ b/src/librustc_codegen_llvm/lib.rs
@@ -23,6 +23,7 @@
 use back::write::{create_target_machine, create_informational_target_machine};
 use syntax_pos::symbol::Symbol;
 
+<<<<<<< HEAD
 extern crate rustc_demangle;
 extern crate flate2;
 #[macro_use] extern crate bitflags;
@@ -45,11 +46,13 @@ extern crate syntax_pos;
 extern crate rustc_errors as errors;
 extern crate rustc_session;
 
+=======
+>>>>>>> rustc: Link LLVM directly into rustc again
 use rustc_codegen_ssa::traits::*;
 use rustc_codegen_ssa::back::write::{CodegenContext, ModuleConfig, FatLTOInput};
 use rustc_codegen_ssa::back::lto::{SerializedModule, LtoModuleCodegen, ThinModule};
 use rustc_codegen_ssa::CompiledModule;
-use errors::{FatalError, Handler};
+use rustc_errors::{FatalError, Handler};
 use rustc::dep_graph::WorkProduct;
 use syntax::expand::allocator::AllocatorKind;
 pub use llvm_util::target_features;
@@ -338,12 +341,6 @@ impl CodegenBackend for LlvmCodegenBackend {
     }
 }
 
-/// This is the entrypoint for a hot plugged rustc_codegen_llvm
-#[no_mangle]
-pub fn __rustc_codegen_backend() -> Box<dyn CodegenBackend> {
-    LlvmCodegenBackend::new()
-}
-
 pub struct ModuleLlvm {
     llcx: &'static mut llvm::Context,
     llmod_raw: *const llvm::Module,
diff --git a/src/librustc_codegen_llvm/llvm/ffi.rs b/src/librustc_codegen_llvm/llvm/ffi.rs
index 5362d180d1b78..d8f11b4861965 100644
--- a/src/librustc_codegen_llvm/llvm/ffi.rs
+++ b/src/librustc_codegen_llvm/llvm/ffi.rs
@@ -544,6 +544,7 @@ pub type InlineAsmDiagHandler = unsafe extern "C" fn(&SMDiagnostic, *const c_voi
 
 pub mod debuginfo {
     use super::{InvariantOpaque, Metadata};
+    use bitflags::bitflags;
 
     #[repr(C)]
     pub struct DIBuilder<'a>(InvariantOpaque<'a>);
diff --git a/src/librustc_codegen_llvm/llvm_util.rs b/src/librustc_codegen_llvm/llvm_util.rs
index 72612c4704e66..40739387b00c4 100644
--- a/src/librustc_codegen_llvm/llvm_util.rs
+++ b/src/librustc_codegen_llvm/llvm_util.rs
@@ -8,6 +8,7 @@ use libc::c_int;
 use std::ffi::CString;
 use rustc_feature::UnstableFeatures;
 use syntax::symbol::sym;
+use rustc::bug;
 
 use std::str;
 use std::slice;
diff --git a/src/librustc_codegen_llvm/metadata.rs b/src/librustc_codegen_llvm/metadata.rs
index cd7255888118c..bbe42e3b50a2c 100644
--- a/src/librustc_codegen_llvm/metadata.rs
+++ b/src/librustc_codegen_llvm/metadata.rs
@@ -6,6 +6,8 @@ use rustc_target::spec::Target;
 
 use rustc_data_structures::owning_ref::OwningRef;
 use rustc_codegen_ssa::METADATA_FILENAME;
+use log::debug;
+use rustc_data_structures::rustc_erase_owner;
 
 use std::path::Path;
 use std::slice;
diff --git a/src/librustc_codegen_llvm/mono_item.rs b/src/librustc_codegen_llvm/mono_item.rs
index cbc8af4fd2710..9f6bdd2390082 100644
--- a/src/librustc_codegen_llvm/mono_item.rs
+++ b/src/librustc_codegen_llvm/mono_item.rs
@@ -9,6 +9,7 @@ use rustc::mir::mono::{Linkage, Visibility};
 use rustc::ty::{TypeFoldable, Instance};
 use rustc::ty::layout::{FnAbiExt, LayoutOf};
 use rustc_codegen_ssa::traits::*;
+use log::debug;
 
 pub use rustc::mir::mono::MonoItem;
 
diff --git a/src/librustc_codegen_llvm/type_.rs b/src/librustc_codegen_llvm/type_.rs
index f936367572ea0..e6677f3d25b9c 100644
--- a/src/librustc_codegen_llvm/type_.rs
+++ b/src/librustc_codegen_llvm/type_.rs
@@ -5,6 +5,7 @@ use crate::llvm::{Bool, False, True};
 use crate::context::CodegenCx;
 use crate::value::Value;
 use rustc_codegen_ssa::traits::*;
+use rustc::bug;
 
 use crate::common;
 use crate::type_of::LayoutLlvmExt;
diff --git a/src/librustc_codegen_llvm/type_of.rs b/src/librustc_codegen_llvm/type_of.rs
index d77bbb279216a..f9cbf4bbe4502 100644
--- a/src/librustc_codegen_llvm/type_of.rs
+++ b/src/librustc_codegen_llvm/type_of.rs
@@ -6,6 +6,8 @@ use rustc::ty::layout::{self, Align, LayoutOf, FnAbiExt, PointeeInfo, Size, TyLa
 use rustc_target::abi::TyLayoutMethods;
 use rustc::ty::print::obsolete::DefPathBasedNames;
 use rustc_codegen_ssa::traits::*;
+use log::debug;
+use rustc::bug;
 
 use std::fmt::Write;
 
diff --git a/src/librustc_driver/Cargo.toml b/src/librustc_driver/Cargo.toml
index d1cb4cbeb9b31..043cfc58974b7 100644
--- a/src/librustc_driver/Cargo.toml
+++ b/src/librustc_driver/Cargo.toml
@@ -32,3 +32,6 @@ rustc_serialize = { path = "../libserialize", package = "serialize" }
 rustc_resolve = { path = "../librustc_resolve" }
 syntax = { path = "../libsyntax" }
 syntax_pos = { path = "../libsyntax_pos" }
+
+[features]
+llvm = ['rustc_interface/llvm']
diff --git a/src/librustc_driver/lib.rs b/src/librustc_driver/lib.rs
index 05945504db237..d435309ab3d22 100644
--- a/src/librustc_driver/lib.rs
+++ b/src/librustc_driver/lib.rs
@@ -43,6 +43,8 @@ use rustc_codegen_utils::codegen_backend::CodegenBackend;
 use errors::{PResult, registry::Registry};
 use rustc_interface::{interface, Queries};
 use rustc_interface::util::get_codegen_sysroot;
+use rustc_interface::interface;
+use rustc_interface::util::get_builtin_codegen_backend;
 use rustc_data_structures::sync::SeqCst;
 use rustc_feature::{find_gated_cfg, UnstableFeatures};
 use rustc_serialize::json::ToJson;
@@ -765,7 +767,7 @@ pub fn version(binary: &str, matches: &getopts::Matches) {
         println!("commit-date: {}", unw(commit_date_str()));
         println!("host: {}", config::host_triple());
         println!("release: {}", unw(release_str()));
-        get_codegen_sysroot("llvm")().print_version();
+        get_builtin_codegen_backend("llvm")().print_version();
     }
 }
 
@@ -1059,7 +1061,7 @@ pub fn handle_options(args: &[String]) -> Option<getopts::Matches> {
     }
 
     if cg_flags.iter().any(|x| *x == "passes=list") {
-        get_codegen_sysroot("llvm")().print_passes();
+        get_builtin_codegen_backend("llvm")().print_passes();
         return None;
     }
 
diff --git a/src/librustc_interface/Cargo.toml b/src/librustc_interface/Cargo.toml
index 7ab5ec2b2329e..58fd92822e9a2 100644
--- a/src/librustc_interface/Cargo.toml
+++ b/src/librustc_interface/Cargo.toml
@@ -25,6 +25,7 @@ rustc_traits = { path = "../librustc_traits" }
 rustc_data_structures = { path = "../librustc_data_structures" }
 rustc_codegen_ssa = { path = "../librustc_codegen_ssa" }
 rustc_codegen_utils = { path = "../librustc_codegen_utils" }
+rustc_codegen_llvm = { path = "../librustc_codegen_llvm", optional = true }
 rustc_metadata = { path = "../librustc_metadata" }
 rustc_mir = { path = "../librustc_mir" }
 rustc_passes = { path = "../librustc_passes" }
@@ -39,3 +40,6 @@ once_cell = "1"
 
 [dev-dependencies]
 rustc_target = { path = "../librustc_target" }
+
+[features]
+llvm = ['rustc_codegen_llvm']
diff --git a/src/librustc_interface/util.rs b/src/librustc_interface/util.rs
index d8e20e17ce835..669021deac908 100644
--- a/src/librustc_interface/util.rs
+++ b/src/librustc_interface/util.rs
@@ -16,11 +16,9 @@ use rustc_errors::registry::Registry;
 use rustc_metadata::dynamic_lib::DynamicLibrary;
 use rustc_resolve::{self, Resolver};
 use std::env;
-use std::env::consts::{DLL_PREFIX, DLL_SUFFIX};
 use std::io::{self, Write};
 use std::mem;
 use std::path::{Path, PathBuf};
-use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::{Arc, Mutex, Once};
 use std::ops::DerefMut;
 use smallvec::SmallVec;
@@ -253,7 +251,7 @@ pub fn get_codegen_backend(sess: &Session) -> Box<dyn CodegenBackend> {
             filename if filename.contains(".") => {
                 load_backend_from_dylib(filename.as_ref())
             }
-            codegen_name => get_codegen_sysroot(codegen_name),
+            codegen_name => get_builtin_codegen_backend(codegen_name),
         };
 
         unsafe {
@@ -392,83 +390,16 @@ fn sysroot_candidates() -> Vec<PathBuf> {
     }
 }
 
-pub fn get_codegen_sysroot(backend_name: &str) -> fn() -> Box<dyn CodegenBackend> {
-    // For now we only allow this function to be called once as it'll dlopen a
-    // few things, which seems to work best if we only do that once. In
-    // general this assertion never trips due to the once guard in `get_codegen_backend`,
-    // but there's a few manual calls to this function in this file we protect
-    // against.
-    static LOADED: AtomicBool = AtomicBool::new(false);
-    assert!(!LOADED.fetch_or(true, Ordering::SeqCst),
-            "cannot load the default codegen backend twice");
-
-    let target = session::config::host_triple();
-    let sysroot_candidates = sysroot_candidates();
-
-    let sysroot = sysroot_candidates.iter()
-        .map(|sysroot| {
-            let libdir = filesearch::relative_target_lib_path(&sysroot, &target);
-            sysroot.join(libdir).with_file_name(
-                option_env!("CFG_CODEGEN_BACKENDS_DIR").unwrap_or("codegen-backends"))
-        })
-        .filter(|f| {
-            info!("codegen backend candidate: {}", f.display());
-            f.exists()
-        })
-        .next();
-    let sysroot = sysroot.unwrap_or_else(|| {
-        let candidates = sysroot_candidates.iter()
-            .map(|p| p.display().to_string())
-            .collect::<Vec<_>>()
-            .join("\n* ");
-        let err = format!("failed to find a `codegen-backends` folder \
-                           in the sysroot candidates:\n* {}", candidates);
-        early_error(ErrorOutputType::default(), &err);
-    });
-    info!("probing {} for a codegen backend", sysroot.display());
-
-    let d = sysroot.read_dir().unwrap_or_else(|e| {
-        let err = format!("failed to load default codegen backend, couldn't \
-                           read `{}`: {}", sysroot.display(), e);
-        early_error(ErrorOutputType::default(), &err);
-    });
-
-    let mut file: Option<PathBuf> = None;
-
-    let expected_name = format!("rustc_codegen_llvm-{}", backend_name);
-    for entry in d.filter_map(|e| e.ok()) {
-        let path = entry.path();
-        let filename = match path.file_name().and_then(|s| s.to_str()) {
-            Some(s) => s,
-            None => continue,
-        };
-        if !(filename.starts_with(DLL_PREFIX) && filename.ends_with(DLL_SUFFIX)) {
-            continue
-        }
-        let name = &filename[DLL_PREFIX.len() .. filename.len() - DLL_SUFFIX.len()];
-        if name != expected_name {
-            continue
-        }
-        if let Some(ref prev) = file {
-            let err = format!("duplicate codegen backends found\n\
-                               first:  {}\n\
-                               second: {}\n\
-            ", prev.display(), path.display());
-            early_error(ErrorOutputType::default(), &err);
-        }
-        file = Some(path.clone());
-    }
-
-    match file {
-        Some(ref s) => return load_backend_from_dylib(s),
-        None => {
-            let err = format!("failed to load default codegen backend for `{}`, \
-                               no appropriate codegen dylib found in `{}`",
-                              backend_name, sysroot.display());
-            early_error(ErrorOutputType::default(), &err);
+pub fn get_builtin_codegen_backend(backend_name: &str) -> fn() -> Box<dyn CodegenBackend> {
+    #[cfg(feature = "llvm")]
+    {
+        if backend_name == "llvm" {
+            return rustc_codegen_llvm::LlvmCodegenBackend::new;
         }
     }
 
+    let err = format!("unsupported builtin codegen backend `{}`", backend_name);
+    early_error(ErrorOutputType::default(), &err);
 }
 
 pub(crate) fn compute_crate_disambiguator(session: &Session) -> CrateDisambiguator {
diff --git a/src/rustc/Cargo.toml b/src/rustc/Cargo.toml
index 997d139383798..86a93d7d0cbfe 100644
--- a/src/rustc/Cargo.toml
+++ b/src/rustc/Cargo.toml
@@ -23,3 +23,4 @@ features = ['unprefixed_malloc_on_supported_platforms']
 
 [features]
 jemalloc = ['jemalloc-sys']
+llvm = ['rustc_driver/llvm']
diff --git a/src/tools/tidy/src/deps.rs b/src/tools/tidy/src/deps.rs
index 7b928a2e7c274..36e412975b931 100644
--- a/src/tools/tidy/src/deps.rs
+++ b/src/tools/tidy/src/deps.rs
@@ -135,6 +135,7 @@ const WHITELIST: &[Crate<'_>] = &[
     Crate("polonius-engine"),
     Crate("ppv-lite86"),
     Crate("proc-macro2"),
+    Crate("punycode"),
     Crate("quick-error"),
     Crate("quote"),
     Crate("rand"),

From da50f76a340642f64b34436ff2e4bbe953bf7d1b Mon Sep 17 00:00:00 2001
From: Alex Crichton <alex@alexcrichton.com>
Date: Mon, 4 Nov 2019 11:16:30 -0800
Subject: [PATCH 2/5] Fix some linking of LLVM's dynamic library

Ensure it shows up in the same places it did before so tools can find it
at runtime.
---
 src/bootstrap/compile.rs | 28 ++++++++--------------------
 src/bootstrap/dist.rs    | 19 +++++++++++++++----
 src/bootstrap/doc.rs     |  2 +-
 3 files changed, 24 insertions(+), 25 deletions(-)

diff --git a/src/bootstrap/compile.rs b/src/bootstrap/compile.rs
index 9d5e3db449890..c7b012f869f42 100644
--- a/src/bootstrap/compile.rs
+++ b/src/bootstrap/compile.rs
@@ -579,24 +579,6 @@ impl Step for RustcLink {
     }
 }
 
-fn copy_lld_to_sysroot(builder: &Builder<'_>,
-                       target_compiler: Compiler,
-                       lld_install_root: &Path) {
-    let target = target_compiler.host;
-
-    let dst = builder.sysroot_libdir(target_compiler, target)
-        .parent()
-        .unwrap()
-        .join("bin");
-    t!(fs::create_dir_all(&dst));
-
-    let src_exe = exe("lld", &target);
-    let dst_exe = exe("rust-lld", &target);
-    // we prepend this bin directory to the user PATH when linking Rust binaries. To
-    // avoid shadowing the system LLD we rename the LLD we provide to `rust-lld`.
-    builder.copy(&lld_install_root.join("bin").join(&src_exe), &dst.join(&dst_exe));
-}
-
 /// Cargo's output path for the standard library in a given stage, compiled
 /// by a particular compiler for the specified target.
 pub fn libstd_stamp(
@@ -745,10 +727,16 @@ impl Step for Assemble {
             }
         }
 
+        let libdir = builder.sysroot_libdir(target_compiler, target_compiler.host);
         if let Some(lld_install) = lld_install {
-            copy_lld_to_sysroot(builder, target_compiler, &lld_install);
+            let src_exe = exe("lld", &target_compiler.host);
+            let dst_exe = exe("rust-lld", &target_compiler.host);
+            // we prepend this bin directory to the user PATH when linking Rust binaries. To
+            // avoid shadowing the system LLD we rename the LLD we provide to `rust-lld`.
+            let dst = libdir.parent().unwrap().join("bin");
+            t!(fs::create_dir_all(&dst));
+            builder.copy(&lld_install.join("bin").join(&src_exe), &dst.join(&dst_exe));
         }
-
         dist::maybe_install_llvm_dylib(builder, target_compiler.host, &sysroot);
 
         // Link the compiler binary itself into place
diff --git a/src/bootstrap/dist.rs b/src/bootstrap/dist.rs
index 4c49913bab989..02533944fc28f 100644
--- a/src/bootstrap/dist.rs
+++ b/src/bootstrap/dist.rs
@@ -2124,6 +2124,10 @@ impl Step for HashSign {
 
 // Maybe add libLLVM.so to the lib-dir. It will only have been built if
 // LLVM tools are linked dynamically.
+//
+// We add this to both the libdir of the rustc binary itself (for it to load at
+// runtime) and also to the target directory so it can find it at link-time.
+//
 // Note: This function does no yet support Windows but we also don't support
 //       linking LLVM tools dynamically on Windows yet.
 pub fn maybe_install_llvm_dylib(builder: &Builder<'_>,
@@ -2132,13 +2136,19 @@ pub fn maybe_install_llvm_dylib(builder: &Builder<'_>,
     let src_libdir = builder
         .llvm_out(target)
         .join("lib");
-    let dst_libdir = sysroot.join("lib/rustlib").join(&*target).join("lib");
-    t!(fs::create_dir_all(&dst_libdir));
+    let dst_libdir1 = sysroot.join("lib/rustlib").join(&*target).join("lib");
+    let dst_libdir2 = sysroot.join(builder.sysroot_libdir_relative(Compiler {
+        stage: 1,
+        host: target,
+    }));
+    t!(fs::create_dir_all(&dst_libdir1));
+    t!(fs::create_dir_all(&dst_libdir2));
 
     if target.contains("apple-darwin") {
         let llvm_dylib_path = src_libdir.join("libLLVM.dylib");
         if llvm_dylib_path.exists() {
-            builder.install(&llvm_dylib_path, &dst_libdir, 0o644);
+            builder.install(&llvm_dylib_path, &dst_libdir1, 0o644);
+            builder.install(&llvm_dylib_path, &dst_libdir2, 0o644);
         }
         return
     }
@@ -2154,7 +2164,8 @@ pub fn maybe_install_llvm_dylib(builder: &Builder<'_>,
         });
 
 
-        builder.install(&llvm_dylib_path, &dst_libdir, 0o644);
+        builder.install(&llvm_dylib_path, &dst_libdir1, 0o644);
+        builder.install(&llvm_dylib_path, &dst_libdir2, 0o644);
     }
 }
 
diff --git a/src/bootstrap/doc.rs b/src/bootstrap/doc.rs
index 6d12d5239f0b4..608cee0a80bfc 100644
--- a/src/bootstrap/doc.rs
+++ b/src/bootstrap/doc.rs
@@ -433,7 +433,7 @@ impl Step for Std {
         builder.info(&format!("Documenting stage{} std ({})", stage, target));
         let out = builder.doc_out(target);
         t!(fs::create_dir_all(&out));
-        let compiler = builder.compiler_for(stage, builder.config.build, target);
+        let compiler = builder.compiler(stage, builder.config.build);
 
         builder.ensure(compile::Std { compiler, target });
         let out_dir = builder.stage_out(compiler, Mode::Std)

From 428c95ea2ec37b995267fb522185f459a442e462 Mon Sep 17 00:00:00 2001
From: Alex Crichton <alex@alexcrichton.com>
Date: Wed, 20 Nov 2019 07:03:13 -0800
Subject: [PATCH 3/5] Fix a test in the bootstrap test suite

---
 src/bootstrap/builder/tests.rs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/bootstrap/builder/tests.rs b/src/bootstrap/builder/tests.rs
index 2bb90fdb04edc..b9d97fb8b760a 100644
--- a/src/bootstrap/builder/tests.rs
+++ b/src/bootstrap/builder/tests.rs
@@ -363,6 +363,10 @@ fn dist_with_same_targets_and_hosts() {
                 compiler: Compiler { host: a, stage: 1 },
                 target: b,
             },
+            compile::Std {
+                compiler: Compiler { host: a, stage: 2 },
+                target: b,
+            },
         ]
     );
     assert_eq!(

From 949f4104df6b37f219de7a479252fab79f8f641b Mon Sep 17 00:00:00 2001
From: Aaron Hill <aa1ronham@gmail.com>
Date: Thu, 5 Dec 2019 22:04:35 -0500
Subject: [PATCH 4/5] Fix fallout from rebase

---
 Cargo.lock                           | 2 ++
 src/librustc_codegen_llvm/Cargo.toml | 2 ++
 src/librustc_codegen_llvm/abi.rs     | 1 -
 src/librustc_codegen_llvm/lib.rs     | 5 -----
 src/librustc_driver/lib.rs           | 2 --
 5 files changed, 4 insertions(+), 8 deletions(-)

diff --git a/Cargo.lock b/Cargo.lock
index 4d523b18a9d65..afa8dffa83156 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3506,10 +3506,12 @@ dependencies = [
  "rustc_codegen_utils",
  "rustc_data_structures",
  "rustc_errors",
+ "rustc_feature",
  "rustc_fs_util",
  "rustc_incremental",
  "rustc_index",
  "rustc_llvm",
+ "rustc_session",
  "rustc_target",
  "smallvec 0.6.10",
  "syntax",
diff --git a/src/librustc_codegen_llvm/Cargo.toml b/src/librustc_codegen_llvm/Cargo.toml
index 0559aac28497e..71cfacfa56058 100644
--- a/src/librustc_codegen_llvm/Cargo.toml
+++ b/src/librustc_codegen_llvm/Cargo.toml
@@ -21,10 +21,12 @@ rustc_codegen_ssa = { path = "../librustc_codegen_ssa" }
 rustc_codegen_utils = { path = "../librustc_codegen_utils" }
 rustc_data_structures = { path = "../librustc_data_structures" }
 rustc_errors = { path = "../librustc_errors" }
+rustc_feature = { path = "../librustc_feature" }
 rustc_fs_util = { path = "../librustc_fs_util" }
 rustc_incremental = { path = "../librustc_incremental" }
 rustc_index = { path = "../librustc_index" }
 rustc_llvm = { path = "../librustc_llvm" }
+rustc_session = { path = "../librustc_session" }
 rustc_target = { path = "../librustc_target" }
 smallvec = { version = "0.6.7", features = ["union", "may_dangle"] }
 syntax = { path = "../libsyntax" }
diff --git a/src/librustc_codegen_llvm/abi.rs b/src/librustc_codegen_llvm/abi.rs
index 24a6c12d48cd5..2607a497326f9 100644
--- a/src/librustc_codegen_llvm/abi.rs
+++ b/src/librustc_codegen_llvm/abi.rs
@@ -8,7 +8,6 @@ use crate::value::Value;
 use rustc_codegen_ssa::MemFlags;
 use rustc_codegen_ssa::mir::place::PlaceRef;
 use rustc_codegen_ssa::mir::operand::OperandValue;
-use rustc_target::abi::call::ArgAbi;
 use rustc::bug;
 use rustc_codegen_ssa::traits::*;
 use rustc_target::abi::call::ArgAbi;
diff --git a/src/librustc_codegen_llvm/lib.rs b/src/librustc_codegen_llvm/lib.rs
index 63b3e8da1fcc3..5123f8f386d5b 100644
--- a/src/librustc_codegen_llvm/lib.rs
+++ b/src/librustc_codegen_llvm/lib.rs
@@ -23,7 +23,6 @@
 use back::write::{create_target_machine, create_informational_target_machine};
 use syntax_pos::symbol::Symbol;
 
-<<<<<<< HEAD
 extern crate rustc_demangle;
 extern crate flate2;
 #[macro_use] extern crate bitflags;
@@ -37,17 +36,13 @@ extern crate rustc_incremental;
 extern crate rustc_codegen_utils;
 extern crate rustc_codegen_ssa;
 extern crate rustc_fs_util;
-extern crate rustc_driver as _;
 
 #[macro_use] extern crate log;
 extern crate smallvec;
 extern crate syntax;
 extern crate syntax_pos;
 extern crate rustc_errors as errors;
-extern crate rustc_session;
 
-=======
->>>>>>> rustc: Link LLVM directly into rustc again
 use rustc_codegen_ssa::traits::*;
 use rustc_codegen_ssa::back::write::{CodegenContext, ModuleConfig, FatLTOInput};
 use rustc_codegen_ssa::back::lto::{SerializedModule, LtoModuleCodegen, ThinModule};
diff --git a/src/librustc_driver/lib.rs b/src/librustc_driver/lib.rs
index d435309ab3d22..3230e048a3bf3 100644
--- a/src/librustc_driver/lib.rs
+++ b/src/librustc_driver/lib.rs
@@ -42,8 +42,6 @@ use rustc_metadata::locator;
 use rustc_codegen_utils::codegen_backend::CodegenBackend;
 use errors::{PResult, registry::Registry};
 use rustc_interface::{interface, Queries};
-use rustc_interface::util::get_codegen_sysroot;
-use rustc_interface::interface;
 use rustc_interface::util::get_builtin_codegen_backend;
 use rustc_data_structures::sync::SeqCst;
 use rustc_feature::{find_gated_cfg, UnstableFeatures};

From 408e3cfe53ea326714f91817d764ca869afc7703 Mon Sep 17 00:00:00 2001
From: Aaron Hill <aa1ronham@gmail.com>
Date: Thu, 5 Dec 2019 22:25:20 -0500
Subject: [PATCH 5/5] Remove `extern crate` declarations

---
 src/librustc_codegen_llvm/lib.rs | 20 --------------------
 1 file changed, 20 deletions(-)

diff --git a/src/librustc_codegen_llvm/lib.rs b/src/librustc_codegen_llvm/lib.rs
index 5123f8f386d5b..e252dee5e77c0 100644
--- a/src/librustc_codegen_llvm/lib.rs
+++ b/src/librustc_codegen_llvm/lib.rs
@@ -23,26 +23,6 @@
 use back::write::{create_target_machine, create_informational_target_machine};
 use syntax_pos::symbol::Symbol;
 
-extern crate rustc_demangle;
-extern crate flate2;
-#[macro_use] extern crate bitflags;
-extern crate libc;
-#[macro_use] extern crate rustc;
-extern crate rustc_target;
-#[macro_use] extern crate rustc_data_structures;
-extern crate rustc_feature;
-extern crate rustc_index;
-extern crate rustc_incremental;
-extern crate rustc_codegen_utils;
-extern crate rustc_codegen_ssa;
-extern crate rustc_fs_util;
-
-#[macro_use] extern crate log;
-extern crate smallvec;
-extern crate syntax;
-extern crate syntax_pos;
-extern crate rustc_errors as errors;
-
 use rustc_codegen_ssa::traits::*;
 use rustc_codegen_ssa::back::write::{CodegenContext, ModuleConfig, FatLTOInput};
 use rustc_codegen_ssa::back::lto::{SerializedModule, LtoModuleCodegen, ThinModule};