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).

While testing, I also noticed two other issues:

* spanview debug file output ICEd on a function with no body. The
workaround for this is included in this PR.
* `assert_*!()` macro coverage can appear covered if followed by another
`assert_*!()` macro. Normally they appear uncovered. I submitted a new
Issue #84561, and added a coverage test to demonstrate this issue.

compiler/rustc_builtin_macros/src/deriving/cmp/eq.rs

@@ -16,9 +16,10 @@ pub fn expand_deriving_eq(
     push: &mut dyn FnMut(Annotatable),
 ) {
     let inline = cx.meta_word(span, sym::inline);
+    let no_coverage = cx.meta_word(span, sym::no_coverage);
     let hidden = rustc_ast::attr::mk_nested_word_item(Ident::new(sym::hidden, span));
     let doc = rustc_ast::attr::mk_list_item(Ident::new(sym::doc, span), vec![hidden]);
-    let attrs = vec![cx.attribute(inline), cx.attribute(doc)];
+    let attrs = vec![cx.attribute(inline), cx.attribute(no_coverage), cx.attribute(doc)];
     let trait_def = TraitDef {
         span,
         attributes: Vec::new(),