Use coverage-dump --demangle as the demangler for coverage-run tests

This avoids the need to build `rust-demangler` when running coverage tests,
since we typically need to build `coverage-dump` anyway.

Author: Zalathar

src/bootstrap/src/core/build_steps/test.rs

@@ -1781,25 +1781,14 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
                 .arg(builder.ensure(tool::JsonDocLint { compiler: json_compiler, target }));
         }
 
-        if mode == "coverage-map" {
+        if matches!(mode, "coverage-map" | "coverage-run") {
             let coverage_dump = builder.ensure(tool::CoverageDump {
                 compiler: compiler.with_stage(0),
                 target: compiler.host,
             });
             cmd.arg("--coverage-dump-path").arg(coverage_dump);
         }
 
-        if mode == "coverage-run" {
-            // The demangler doesn't need the current compiler, so we can avoid
-            // unnecessary rebuilds by using the bootstrap compiler instead.
-            let rust_demangler = builder.ensure(tool::RustDemangler {
-                compiler: compiler.with_stage(0),
-                target: compiler.host,
-                extra_features: Vec::new(),
-            });
-            cmd.arg("--rust-demangler-path").arg(rust_demangler);
-        }
-
         cmd.arg("--src-base").arg(builder.src.join("tests").join(suite));
         cmd.arg("--build-base").arg(testdir(builder, compiler.host).join(suite));
 

src/tools/compiletest/src/runtest/coverage.rs

@@ -10,10 +10,15 @@ use crate::common::{UI_COVERAGE, UI_COVERAGE_MAP};
 use crate::runtest::{static_regex, Emit, ProcRes, TestCx, WillExecute};
 
 impl<'test> TestCx<'test> {
+    fn coverage_dump_path(&self) -> &Path {
+        self.config
+            .coverage_dump_path
+            .as_deref()
+            .unwrap_or_else(|| self.fatal("missing --coverage-dump"))
+    }
+
     pub(crate) fn run_coverage_map_test(&self) {
-        let Some(coverage_dump_path) = &self.config.coverage_dump_path else {
-            self.fatal("missing --coverage-dump");
-        };
+        let coverage_dump_path = self.coverage_dump_path();
 
         let (proc_res, llvm_ir_path) = self.compile_test_and_save_ir();
         if !proc_res.status.success() {
@@ -102,8 +107,10 @@ impl<'test> TestCx<'test> {
         let proc_res = self.run_llvm_tool("llvm-cov", |cmd| {
             cmd.args(["show", "--format=text", "--show-line-counts-or-regions"]);
 
-            cmd.arg("--Xdemangler");
-            cmd.arg(self.config.rust_demangler_path.as_ref().unwrap());
+            // Specify the demangler binary and its arguments.
+            let coverage_dump_path = self.coverage_dump_path();
+            cmd.arg("--Xdemangler").arg(coverage_dump_path);
+            cmd.arg("--Xdemangler").arg("--demangle");
 
             cmd.arg("--instr-profile");
             cmd.arg(&profdata_path);