Change how compiler-builtins gets many CGUs

Cargo.toml

@@ -42,6 +42,19 @@ debug-assertions = false
 debug = false
 debug-assertions = false
 
+[profile.release.package.compiler_builtins]
+# For compiler-builtins we always use a high number of codegen units.
+# The goal here is to place every single intrinsic into its own object
+# file to avoid symbol clashes with the system libgcc if possible. Note
+# that this number doesn't actually produce this many object files, we
+# just don't create more than this number of object files.
+#
+# It's a bit of a bummer that we have to pass this here, unfortunately.
+# Ideally this would be specified through an env var to Cargo so Cargo
+# knows how many CGUs are for this specific crate, but for now
+# per-crate configuration isn't specifiable in the environment.
+codegen-units = 10000
+
 # We want the RLS to use the version of Cargo that we've got vendored in this
 # repository to ensure that the same exact version of Cargo is used by both the
 # RLS and the Cargo binary itself. The RLS depends on Cargo as a git repository

src/librustc_mir/monomorphize/partitioning.rs

@@ -454,18 +454,11 @@ fn default_visibility(tcx: TyCtxt<'_>, id: DefId, is_generic: bool) -> Visibilit
 fn merge_codegen_units<'tcx>(
     tcx: TyCtxt<'tcx>,
     initial_partitioning: &mut PreInliningPartitioning<'tcx>,
-    mut target_cgu_count: usize,
+    target_cgu_count: usize,
 ) {
     assert!(target_cgu_count >= 1);
     let codegen_units = &mut initial_partitioning.codegen_units;
 
-    if tcx.is_compiler_builtins(LOCAL_CRATE) {
-        // Compiler builtins require some degree of control over how mono items
-        // are partitioned into compilation units. Provide it by keeping the
-        // original partitioning when compiling the compiler builtins crate.
-        target_cgu_count = codegen_units.len();
-    }
-
     // Note that at this point in time the `codegen_units` here may not be in a
     // deterministic order (but we know they're deterministically the same set).
     // We want this merging to produce a deterministic ordering of codegen units