rustc: Continue to tweak "std internal symbols"

In investigating [an issue][1] with `panic_implementation` defined in an
executable that's optimized I once again got to rethinking a bit about the
`rustc_std_internal_symbol` attribute as well as weak lang items. We've sort of
been non-stop tweaking these items ever since their inception, and this
continues to the trend.

The crux of the bug was that in the reachability we have a [different branch][2]
for non-library builds which meant that weak lang items (and std internal
symbols) weren't considered reachable, causing them to get eliminiated by
ThinLTO passes. The fix was to basically tweak that branch to consider these
symbols to ensure that they're propagated all the way to the linker.

Along the way I've attempted to erode the distinction between std internal
symbols and weak lang items by having weak lang items automatically configure
fields of `CodegenFnAttrs`. That way most code no longer even considers weak
lang items and they're simply considered normal functions with attributes about
the ABI.

In the end this fixes the final comment of #51342

[1]: https://github.com/rust-lang/rust/issues/51342#issuecomment-414368019
[2]: https://github.com/rust-lang/rust/blob/35bf1ae25799a4e62131159f052e0a3cbd27c960/src/librustc/middle/reachable.rs#L225-L238

1 files changed, 5 insertions, 10 deletions

diff --git a/src/librustc_codegen_utils/symbol_names.rs b/src/librustc_codegen_utils/symbol_names.rs
index 33ce06217a4..4f5e76c24e3 100644
--- a/src/librustc_codegen_utils/symbol_names.rs
+++ b/src/librustc_codegen_utils/symbol_names.rs
@@ -99,9 +99,9 @@
 
 use rustc::hir::def_id::{DefId, LOCAL_CRATE};
 use rustc::hir::map as hir_map;
+use rustc::hir::CodegenFnAttrFlags;
 use rustc::hir::map::definitions::DefPathData;
 use rustc::ich::NodeIdHashingMode;
-use rustc::middle::weak_lang_items;
 use rustc::ty::item_path::{self, ItemPathBuffer, RootMode};
 use rustc::ty::query::Providers;
 use rustc::ty::subst::Substs;
@@ -111,7 +111,6 @@ use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_mir::monomorphize::item::{InstantiationMode, MonoItem, MonoItemExt};
 use rustc_mir::monomorphize::Instance;
 
-use syntax::attr;
 use syntax_pos::symbol::Symbol;
 
 use std::fmt::Write;
@@ -260,7 +259,6 @@ fn compute_symbol_name<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, instance: Instance
     }
 
     // FIXME(eddyb) Precompute a custom symbol name based on attributes.
-    let attrs = tcx.get_attrs(def_id);
     let is_foreign = if let Some(id) = node_id {
         match tcx.hir.get(id) {
             hir_map::NodeForeignItem(_) => true,
@@ -270,24 +268,21 @@ fn compute_symbol_name<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, instance: Instance
         tcx.is_foreign_item(def_id)
     };
 
-    if let Some(name) = weak_lang_items::link_name(&attrs) {
-        return name.to_string();
-    }
-
+    let attrs = tcx.codegen_fn_attrs(def_id);
     if is_foreign {
-        if let Some(name) = attr::first_attr_value_str_by_name(&attrs, "link_name") {
+        if let Some(name) = attrs.link_name {
             return name.to_string();
         }
         // Don't mangle foreign items.
         return tcx.item_name(def_id).to_string();
     }
 
-    if let Some(name) = tcx.codegen_fn_attrs(def_id).export_name {
+    if let Some(name) = &attrs.export_name {
         // Use provided name
         return name.to_string();
     }
 
-    if attr::contains_name(&attrs, "no_mangle") {
+    if attrs.flags.contains(CodegenFnAttrFlags::NO_MANGLE) {
         // Don't mangle
         return tcx.item_name(def_id).to_string();
     }