bazel: Switch to pre-built standard library

Now that we have the pre-built copies of the Rust standard library
for x86_64-unknown-none, we can use those instead of fetching libcore
and liballoc with the host copy of Rust and fetching compiler_builtins
directly. We also don't need to build them any more.

This takes us part of the way to getting cross-compiling for free
with rules_rust.

See #14.

Signed-off-by: SlyMarbo <[email protected]>

bazel/cross-compiling/BUILD

@@ -14,35 +14,42 @@
 # command line option implicitly, rather than the
 # user needing to specify it explicitly.
 
-load("//bazel/cross-compiling:librust.bzl", "find_librust_entrypoint", "rust_no_std")
+load("@rules_rust//rust:defs.bzl", "rust_stdlib_filegroup")
+load("//bazel:copy_file.bzl", "copy_file")
 load("//bazel/cross-compiling:x86_64_cc_toolchain.bzl", "x86_64_cc_toolchain")
 load("//bazel/cross-compiling:x86_64_rust_toolchain.bzl", "x86_64_rust_toolchain")
 
-# Find the entrypoint (lib.rs) for
-# the alloc and core crates.
+# Package the core, compiler-builtins,
+# and alloc crates.
 
-find_librust_entrypoint(
-    name = "liballoc_entry",
-    crate = "alloc",
-    rustc_srcs = "@rust_linux_x86_64//lib/rustlib/src:rustc_srcs",
+copy_file(
+    name = "liballoc.rlib",
+    src = "@rust_none_x86_64//:liballoc",
 )
 
-find_librust_entrypoint(
-    name = "libcore_entry",
-    crate = "core",
-    rustc_srcs = "@rust_linux_x86_64//lib/rustlib/src:rustc_srcs",
+copy_file(
+    name = "libcompiler_builtins.rlib",
+    src = "@rust_none_x86_64//:libcompiler_builtins",
 )
 
-# Build the core, compiler-builtins,
-# and alloc crates.
+copy_file(
+    name = "libcore.rlib",
+    src = "@rust_none_x86_64//:libcore",
+)
 
-rust_no_std(
+copy_file(
+    name = "librustc_std_workspace_core.rlib",
+    src = "@rust_none_x86_64//:librustc_std_workspace_core",
+)
+
+rust_stdlib_filegroup(
     name = "bare_metal_base",
-    liballoc_entry = ":liballoc_entry",
-    libcore_entry = ":libcore_entry",
-    rustc = "@rust_linux_x86_64//:rustc",
-    rustc_srcs = "@rust_linux_x86_64//lib/rustlib/src:rustc_srcs",
-    target = "x86_64-unknown-none",
+    srcs = [
+        ":liballoc.rlib",
+        ":libcompiler_builtins.rlib",
+        ":libcore.rlib",
+        ":librustc_std_workspace_core.rlib",
+    ],
 )
 
 # Set up our toolchains now that we have

bazel/cross-compiling/x86_64_rust_toolchain.bzl

@@ -32,7 +32,6 @@ def x86_64_rust_toolchain(name, target_triple, stdlib_filegroup):
         rust_std = stdlib_filegroup,
         rustc = "@rust_linux_x86_64//:rustc",
         rustc_lib = "@rust_linux_x86_64//:rustc_lib",
-        rustc_srcs = "@rust_linux_x86_64//lib/rustlib/src:rustc_srcs",
         rustfmt = "@rust_linux_x86_64//:rustfmt_bin",
         staticlib_ext = ".a",
         stdlib_linkflags = [],

bazel/deps/rust.bzl

@@ -160,17 +160,6 @@ def rust_deps():
         ],
     )
 
-    http_archive(
-        name = "compiler_builtins",
-        build_file = "//bazel/third_party:compiler_builtins.BUILD",
-        sha256 = "80873f979f0a344a4ade87c2f70d9ccf5720b83b10c97ec7cd745895d021e85a",
-        strip_prefix = "compiler_builtins-0.1.70",
-        type = "tgz",
-        urls = [
-            "https://static.crates.io/crates/compiler_builtins/compiler_builtins-0.1.70.crate",
-        ],
-    )
-
     # Fetch libcore, liballoc, and libcompiler-builtins
     # for the x86_64-unknown-none build target.
 
@@ -190,7 +179,6 @@ def rust_deps():
 
     rust_register_toolchains(
         edition = "2021",
-        include_rustc_srcs = True,
         iso_date = RUST_ISO_DATE,
         rustfmt_version = RUST_VERSION,
         sha256s = {