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),