Auto merge of #84562 - richkadel:issue-83601, r=tmandry

Adds feature-gated `#[no_coverage]` function attribute, to fix derived Eq `0` coverage issue #83601

Derived Eq no longer shows uncovered

The Eq trait has a special hidden function. MIR `InstrumentCoverage`
would add this function to the coverage map, but it is never called, so
the `Eq` trait would always appear uncovered.

Fixes: #83601

The fix required creating a new function attribute `no_coverage` to mark
functions that should be ignored by `InstrumentCoverage` and the
coverage `mapgen` (during codegen).

Adding a `no_coverage` feature gate with tracking issue #84605.

r? `@tmandry`
cc: `@wesleywiser`

compiler/rustc_feature/src/active.rs

@@ -646,6 +646,10 @@ declare_features! (
     /// Allows `extern "wasm" fn`
     (active, wasm_abi, "1.53.0", Some(83788), None),
 
+    /// Allows function attribute `#[no_coverage]`, to bypass coverage
+    /// instrumentation of that function.
+    (active, no_coverage, "1.53.0", Some(84605), None),
+
     /// Allows trait bounds in `const fn`.
     (active, const_fn_trait_bound, "1.53.0", Some(57563), None),
 

compiler/rustc_feature/src/builtin_attrs.rs

@@ -273,6 +273,13 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         template!(List: "address, memory, thread"),
         experimental!(no_sanitize)
     ),
+    ungated!(
+        // Not exclusively gated at the crate level (though crate-level is
+        // supported). The feature can alternatively be enabled on individual
+        // functions.
+        no_coverage, AssumedUsed,
+        template!(Word),
+    ),
 
     // FIXME: #14408 assume docs are used since rustdoc looks at them.
     ungated!(doc, AssumedUsed, template!(List: "hidden|inline|...", NameValueStr: "string")),