Rollup merge of #92142 - wesleywiser:fix_codecoverage_partitioning, r=tmandry

[code coverage] Fix missing dead code in modules that are never called

The issue here is that the logic used to determine which CGU to put the dead function stubs in doesn't handle cases where a module is never assigned to a CGU (which is what happens when all of the code in the module is dead).

The partitioning logic also caused issues in #85461 where inline functions were duplicated into multiple CGUs resulting in duplicate symbols.

This commit fixes the issue by removing the complex logic used to assign dead code stubs to CGUs and replaces it with a much simpler model: we pick one CGU to hold all the dead code stubs. We pick a CGU which has exported items which increases the likelihood the linker won't throw away our dead functions and we pick the smallest to minimize the impact on compilation times for crates with very large CGUs.

Fixes #91661
Fixes #86177
Fixes #85718
Fixes #79622

r? ```@tmandry```
cc ```@richkadel```

This PR is not urgent so please don't let it interrupt your holidays! 🎄 🎁

1 files changed, 0 insertions, 20 deletions

diff --git a/compiler/rustc_mir_transform/src/coverage/query.rs b/compiler/rustc_mir_transform/src/coverage/query.rs
index 1721fb5cde0..46de6d939a1 100644
--- a/compiler/rustc_mir_transform/src/coverage/query.rs
+++ b/compiler/rustc_mir_transform/src/coverage/query.rs
@@ -9,7 +9,6 @@ use rustc_span::def_id::DefId;
 /// A `query` provider for retrieving coverage information injected into MIR.
 pub(crate) fn provide(providers: &mut Providers) {
     providers.coverageinfo = |tcx, def_id| coverageinfo(tcx, def_id);
-    providers.covered_file_name = |tcx, def_id| covered_file_name(tcx, def_id);
     providers.covered_code_regions = |tcx, def_id| covered_code_regions(tcx, def_id);
 }
 
@@ -137,25 +136,6 @@ fn coverageinfo<'tcx>(tcx: TyCtxt<'tcx>, instance_def: ty::InstanceDef<'tcx>) ->
     coverage_visitor.info
 }
 
-fn covered_file_name(tcx: TyCtxt<'_>, def_id: DefId) -> Option<Symbol> {
-    if tcx.is_mir_available(def_id) {
-        let body = mir_body(tcx, def_id);
-        for bb_data in body.basic_blocks().iter() {
-            for statement in bb_data.statements.iter() {
-                if let StatementKind::Coverage(box ref coverage) = statement.kind {
-                    if let Some(code_region) = coverage.code_region.as_ref() {
-                        if is_inlined(body, statement) {
-                            continue;
-                        }
-                        return Some(code_region.file_name);
-                    }
-                }
-            }
-        }
-    }
-    return None;
-}
-
 fn covered_code_regions<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> Vec<&'tcx CodeRegion> {
     let body = mir_body(tcx, def_id);
     body.basic_blocks()