Rollup merge of #111042 - Zalathar:no-coverage, r=wesleywiser

Add `#[no_coverage]` to the test harness's `fn main`

There are two main motivations for adding `#[no_coverage]` to the test harness's entry point:

- The entry point is trivial compiler-generated code that doesn't correspond to user source, and it always runs, so there's no value in instrumenting it for coverage.
- Because it has dummy spans, it causes the instrumentor implementation to emit invalid coverage mappings that confuse `llvm-cov` and result in strange coverage reports.

Fixes #110749.

compiler/rustc_builtin_macros/src/test_harness.rs

@@ -232,7 +232,7 @@ fn generate_test_harness(
     let expn_id = ext_cx.resolver.expansion_for_ast_pass(
         DUMMY_SP,
         AstPass::TestHarness,
-        &[sym::test, sym::rustc_attrs],
+        &[sym::test, sym::rustc_attrs, sym::no_coverage],
         None,
     );
     let def_site = DUMMY_SP.with_def_site_ctxt(expn_id.to_expn_id());
@@ -313,6 +313,8 @@ fn mk_main(cx: &mut TestCtxt<'_>) -> P<ast::Item> {
 
     // #[rustc_main]
     let main_attr = ecx.attr_word(sym::rustc_main, sp);
+    // #[no_coverage]
+    let no_coverage_attr = ecx.attr_word(sym::no_coverage, sp);
 
     // pub fn main() { ... }
     let main_ret_ty = ecx.ty(sp, ast::TyKind::Tup(ThinVec::new()));
@@ -342,7 +344,7 @@ fn mk_main(cx: &mut TestCtxt<'_>) -> P<ast::Item> {
 
     let main = P(ast::Item {
         ident: main_id,
-        attrs: thin_vec![main_attr],
+        attrs: thin_vec![main_attr, no_coverage_attr],
         id: ast::DUMMY_NODE_ID,
         kind: main,
         vis: ast::Visibility { span: sp, kind: ast::VisibilityKind::Public, tokens: None },