Rollup merge of rust-lang#132395 - Zalathar:coverage-cx-ice, r=jieyouxu

coverage: Avoid ICE when `coverage_cx` is unexpectedly unavailable

In rust-lang#132124, `coverage_cx()` was changed to panic if the context was unavailable, under the assumption that it would always be available whenever coverage instrumentation is enabled.

However, there have been reports of this change causing ICEs in `polars` CI.

I don't yet understand why this is happening, but for now it seems wisest to revert that part of the change, restoring the two early returns that had been replaced with panics.

compiler/rustc_codegen_llvm/src/context.rs

@@ -554,6 +554,7 @@ impl<'ll, 'tcx> CodegenCx<'ll, 'tcx> {
 
     /// Extra state that is only available when coverage instrumentation is enabled.
     #[inline]
+    #[track_caller]
     pub(crate) fn coverage_cx(&self) -> &coverageinfo::CrateCoverageContext<'ll, 'tcx> {
         self.coverage_cx.as_ref().expect("only called when coverage instrumentation is enabled")
     }

compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs

@@ -54,7 +54,11 @@ pub(crate) fn finalize(cx: &CodegenCx<'_, '_>) {
         add_unused_functions(cx);
     }
 
-    let function_coverage_map = cx.coverage_cx().take_function_coverage_map();
+    // FIXME(#132395): Can this be none even when coverage is enabled?
+    let function_coverage_map = match cx.coverage_cx {
+        Some(ref cx) => cx.take_function_coverage_map(),
+        None => return,
+    };
     if function_coverage_map.is_empty() {
         // This module has no functions with coverage instrumentation
         return;

compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs

@@ -152,7 +152,12 @@ impl<'tcx> CoverageInfoBuilderMethods<'tcx> for Builder<'_, '_, 'tcx> {
             return;
         };
 
-        let mut coverage_map = bx.coverage_cx().function_coverage_map.borrow_mut();
+        // FIXME(#132395): Unwrapping `coverage_cx` here has led to ICEs in the
+        // wild, so keep this early-return until we understand why.
+        let mut coverage_map = match bx.coverage_cx {
+            Some(ref cx) => cx.function_coverage_map.borrow_mut(),
+            None => return,
+        };
         let func_coverage = coverage_map
             .entry(instance)
             .or_insert_with(|| FunctionCoverageCollector::new(instance, function_coverage_info));