Prevent name collisions with internal implementation details

The implementation of the linkage attribute inside extern blocks defines
symbols starting with _rust_extern_with_linkage_. If someone tries to
also define this symbol you will get a symbol conflict or even an ICE.
By adding an unpredictable component to the symbol name, this becomes
less of an issue.

compiler/rustc_codegen_llvm/src/consts.rs

@@ -5,7 +5,7 @@ use rustc_codegen_ssa::common;
 use rustc_codegen_ssa::traits::*;
 use rustc_hir::LangItem;
 use rustc_hir::def::DefKind;
-use rustc_hir::def_id::DefId;
+use rustc_hir::def_id::{DefId, LOCAL_CRATE};
 use rustc_middle::middle::codegen_fn_attrs::{CodegenFnAttrFlags, CodegenFnAttrs};
 use rustc_middle::mir::interpret::{
     Allocation, ConstAllocation, ErrorHandled, InitChunk, Pointer, Scalar as InterpScalar,
@@ -191,8 +191,8 @@ fn check_and_apply_linkage<'ll, 'tcx>(
         // linkage and there are no definitions), then
         // `extern_with_linkage_foo` will instead be initialized to
         // zero.
-        let mut real_name = "_rust_extern_with_linkage_".to_string();
-        real_name.push_str(sym);
+        let real_name =
+            format!("_rust_extern_with_linkage_{:016x}_{sym}", cx.tcx.stable_crate_id(LOCAL_CRATE));
         let g2 = cx.define_global(&real_name, llty).unwrap_or_else(|| {
             cx.sess().dcx().emit_fatal(SymbolAlreadyDefined {
                 span: cx.tcx.def_span(def_id),