Auto merge of #52993 - alexcrichton:fix-some-vis, r=michaelwoerister

rustc: Tweak visibility of some lang items

This commit tweaks the linker-level visibility of some lang items that rustc
uses and defines. Notably this means that `#[panic_implementation]` and
`#[alloc_error_handler]` functions are never marked as `internal`. It's up to
the linker to eliminate these, not rustc.

Additionally `#[global_allocator]` generated symbols are no longer forced to
`Default` visibility (fully exported), but rather they're relaxed to `Hidden`
visibility). This symbols are *not* needed across DLL boundaries, only as a
local implementation detail of the compiler-injected allocator symbols, so
`Hidden` should suffice.

Closes #51342
Closes #52795

src/librustc/middle/reachable.rs

@@ -15,7 +15,7 @@
 // makes all other generics or inline functions that it references
 // reachable as well.
 
-use hir::CodegenFnAttrs;
+use hir::{CodegenFnAttrs, CodegenFnAttrFlags};
 use hir::map as hir_map;
 use hir::def::Def;
 use hir::def_id::{DefId, CrateNum};
@@ -28,7 +28,6 @@ use util::nodemap::{NodeSet, FxHashSet};
 
 use rustc_target::spec::abi::Abi;
 use syntax::ast;
-use syntax::attr;
 use hir;
 use hir::def_id::LOCAL_CRATE;
 use hir::intravisit::{Visitor, NestedVisitorMap};
@@ -359,8 +358,12 @@ struct CollectPrivateImplItemsVisitor<'a, 'tcx: 'a> {
 impl<'a, 'tcx: 'a> ItemLikeVisitor<'tcx> for CollectPrivateImplItemsVisitor<'a, 'tcx> {
     fn visit_item(&mut self, item: &hir::Item) {
         // Anything which has custom linkage gets thrown on the worklist no
-        // matter where it is in the crate.
-        if attr::contains_name(&item.attrs, "linkage") {
+        // matter where it is in the crate, along with "special std symbols"
+        // which are currently akin to allocator symbols.
+        let def_id = self.tcx.hir.local_def_id(item.id);
+        let codegen_attrs = self.tcx.codegen_fn_attrs(def_id);
+        if codegen_attrs.linkage.is_some() ||
+            codegen_attrs.flags.contains(CodegenFnAttrFlags::RUSTC_STD_INTERNAL_SYMBOL) {
             self.worklist.push(item.id);
         }
 

src/librustc_allocator/expand.rs

@@ -13,7 +13,7 @@ use rustc_errors;
 use syntax::{
     ast::{
         self, Arg, Attribute, Crate, Expr, FnHeader, Generics, Ident, Item, ItemKind,
-        LitKind, Mac, Mod, Mutability, StrStyle, Ty, TyKind, Unsafety, VisibilityKind,
+        Mac, Mod, Mutability, Ty, TyKind, Unsafety, VisibilityKind,
     },
     attr,
     codemap::{
@@ -236,17 +236,12 @@ impl<'a> AllocFnFactory<'a> {
     }
 
     fn attrs(&self) -> Vec<Attribute> {
-        let key = Symbol::intern("linkage");
-        let value = LitKind::Str(Symbol::intern("external"), StrStyle::Cooked);
-        let linkage = self.cx.meta_name_value(self.span, key, value);
-
         let no_mangle = Symbol::intern("no_mangle");
         let no_mangle = self.cx.meta_word(self.span, no_mangle);
 
         let special = Symbol::intern("rustc_std_internal_symbol");
         let special = self.cx.meta_word(self.span, special);
         vec![
-            self.cx.attribute(self.span, linkage),
             self.cx.attribute(self.span, no_mangle),
             self.cx.attribute(self.span, special),
         ]

src/librustc_codegen_llvm/allocator.rs

@@ -67,14 +67,15 @@ pub(crate) unsafe fn codegen(tcx: TyCtxt, mods: &ModuleLlvm, kind: AllocatorKind
         if tcx.sess.target.target.options.default_hidden_visibility {
             llvm::LLVMRustSetVisibility(llfn, llvm::Visibility::Hidden);
         }
-       if tcx.sess.target.target.options.requires_uwtable {
-           attributes::emit_uwtable(llfn, true);
-       }
+        if tcx.sess.target.target.options.requires_uwtable {
+            attributes::emit_uwtable(llfn, true);
+        }
 
         let callee = CString::new(kind.fn_name(method.name)).unwrap();
         let callee = llvm::LLVMRustGetOrInsertFunction(llmod,
                                                        callee.as_ptr(),
                                                        ty);
+        llvm::LLVMRustSetVisibility(callee, llvm::Visibility::Hidden);
 
         let llbb = llvm::LLVMAppendBasicBlockInContext(llcx,
                                                        llfn,

src/librustc_codegen_llvm/base.rs

@@ -809,8 +809,28 @@ pub fn codegen_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
         rx,
         codegen_units.len());
 
-    // Codegen an allocator shim, if any
-    let allocator_module = if let Some(kind) = *tcx.sess.allocator_kind.get() {
+    // Codegen an allocator shim, if necessary.
+    //
+    // If the crate doesn't have an `allocator_kind` set then there's definitely
+    // no shim to generate. Otherwise we also check our dependency graph for all
+    // our output crate types. If anything there looks like its a `Dynamic`
+    // linkage, then it's already got an allocator shim and we'll be using that
+    // one instead. If nothing exists then it's our job to generate the
+    // allocator!
+    let any_dynamic_crate = tcx.sess.dependency_formats.borrow()
+        .iter()
+        .any(|(_, list)| {
+            use rustc::middle::dependency_format::Linkage;
+            list.iter().any(|linkage| {
+                match linkage {
+                    Linkage::Dynamic => true,
+                    _ => false,
+                }
+            })
+        });
+    let allocator_module = if any_dynamic_crate {
+        None
+    } else if let Some(kind) = *tcx.sess.allocator_kind.get() {
         unsafe {
             let llmod_id = "allocator";
             let modules = ModuleLlvm::new(tcx.sess, llmod_id);