coverage: Infer instances_used from pgo_func_name_var_map

In obscure circumstances, we would sometimes emit a covfun record for a
function with no physical coverage counters, causing `llvm-cov` to fail with
the cryptic error message:

    malformed instrumentation profile data: function name is empty

We can eliminate this mismatch by removing `instances_used` entirely, and
instead inferring its contents from the keys of `pgo_func_name_var_map`.

This makes it impossible for a "used" function to lack a PGO name entry.

Author: Zalathar

compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs

@@ -46,21 +46,17 @@ pub(crate) fn finalize(cx: &mut CodegenCx<'_, '_>) {
     debug!("Generating coverage map for CodegenUnit: `{}`", cx.codegen_unit.name());
 
     // FIXME(#132395): Can this be none even when coverage is enabled?
-    let instances_used = match cx.coverage_cx {
-        Some(ref cx) => cx.instances_used.borrow(),
-        None => return,
-    };
+    let Some(ref coverage_cx) = cx.coverage_cx else { return };
 
-    let mut covfun_records = instances_used
-        .iter()
-        .copied()
+    let mut covfun_records = coverage_cx
+        .instances_used()
+        .into_iter()
         // Sort by symbol name, so that the global file table is built in an
         // order that doesn't depend on the stable-hash-based order in which
         // instances were visited during codegen.
         .sorted_by_cached_key(|&instance| tcx.symbol_name(instance).name)
         .filter_map(|instance| prepare_covfun_record(tcx, instance, true))
         .collect::<Vec<_>>();
-    drop(instances_used);
 
     // In a single designated CGU, also prepare covfun records for functions
     // in this crate that were instrumented for coverage, but are unused.

compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs

@@ -5,7 +5,7 @@ use rustc_abi::Size;
 use rustc_codegen_ssa::traits::{
     BuilderMethods, ConstCodegenMethods, CoverageInfoBuilderMethods, MiscCodegenMethods,
 };
-use rustc_data_structures::fx::{FxHashMap, FxIndexSet};
+use rustc_data_structures::fx::{FxHashMap, FxIndexMap};
 use rustc_middle::mir::coverage::CoverageKind;
 use rustc_middle::ty::Instance;
 use tracing::{debug, instrument};
@@ -20,9 +20,14 @@ mod mapgen;
 
 /// Extra per-CGU context/state needed for coverage instrumentation.
 pub(crate) struct CguCoverageContext<'ll, 'tcx> {
-    /// Coverage data for each instrumented function identified by DefId.
-    pub(crate) instances_used: RefCell<FxIndexSet<Instance<'tcx>>>,
-    pub(crate) pgo_func_name_var_map: RefCell<FxHashMap<Instance<'tcx>, &'ll llvm::Value>>,
+    /// Associates function instances with an LLVM global that holds the
+    /// function's symbol name, as needed by LLVM coverage intrinsics.
+    ///
+    /// Instances in this map are also considered "used" for the purposes of
+    /// emitting covfun records. Every covfun record holds a hash of its
+    /// symbol name, and `llvm-cov` will exit fatally if it can't resolve that
+    /// hash back to an entry in the binary's `__llvm_prf_names` linker section.
+    pub(crate) pgo_func_name_var_map: RefCell<FxIndexMap<Instance<'tcx>, &'ll llvm::Value>>,
     pub(crate) mcdc_condition_bitmap_map: RefCell<FxHashMap<Instance<'tcx>, Vec<&'ll llvm::Value>>>,
 
     covfun_section_name: OnceCell<CString>,
@@ -31,7 +36,6 @@ pub(crate) struct CguCoverageContext<'ll, 'tcx> {
 impl<'ll, 'tcx> CguCoverageContext<'ll, 'tcx> {
     pub(crate) fn new() -> Self {
         Self {
-            instances_used: RefCell::<FxIndexSet<_>>::default(),
             pgo_func_name_var_map: Default::default(),
             mcdc_condition_bitmap_map: Default::default(),
             covfun_section_name: Default::default(),
@@ -53,6 +57,14 @@ impl<'ll, 'tcx> CguCoverageContext<'ll, 'tcx> {
             .and_then(|bitmap_map| bitmap_map.get(decision_depth as usize))
             .copied() // Dereference Option<&&Value> to Option<&Value>
     }
+
+    /// Returns the list of instances considered "used" in this CGU, as
+    /// inferred from the keys of `pgo_func_name_var_map`.
+    pub(crate) fn instances_used(&self) -> Vec<Instance<'tcx>> {
+        // Collecting into a Vec is way easier than trying to juggle RefCell
+        // projections, and this should only run once per CGU anyway.
+        self.pgo_func_name_var_map.borrow().keys().copied().collect::<Vec<_>>()
+    }
 }
 
 impl<'ll, 'tcx> CodegenCx<'ll, 'tcx> {
@@ -151,11 +163,6 @@ impl<'tcx> CoverageInfoBuilderMethods<'tcx> for Builder<'_, '_, 'tcx> {
             return;
         };
 
-        // Mark the instance as used in this CGU, for coverage purposes.
-        // This includes functions that were not partitioned into this CGU,
-        // but were MIR-inlined into one of this CGU's functions.
-        coverage_cx.instances_used.borrow_mut().insert(instance);
-
         match *kind {
             CoverageKind::SpanMarker | CoverageKind::BlockMarker { .. } => unreachable!(
                 "marker statement {kind:?} should have been removed by CleanupPostBorrowck"