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

Adds feature-gated `#[no_coverage]` function attribute, to fix derived Eq `0` coverage issue rust-lang#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: rust-lang#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 rust-lang#84605.

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

Author: bors

core/src/cmp.rs

@@ -274,6 +274,8 @@ pub trait Eq: PartialEq<Self> {
     //
     // This should never be implemented by hand.
     #[doc(hidden)]
+    #[cfg_attr(not(bootstrap), feature(no_coverage))]
+    #[cfg_attr(not(bootstrap), no_coverage)]
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn assert_receiver_is_total_eq(&self) {}