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(),

compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs

@@ -8,6 +8,7 @@ use rustc_codegen_ssa::traits::{ConstMethods, CoverageInfoMethods};
 use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexSet};
 use rustc_hir::def_id::{DefId, DefIdSet, LOCAL_CRATE};
 use rustc_llvm::RustString;
+use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use rustc_middle::mir::coverage::CodeRegion;
 use rustc_span::Symbol;
 
@@ -280,6 +281,10 @@ fn add_unused_functions<'ll, 'tcx>(cx: &CodegenCx<'ll, 'tcx>) {
 
     let mut unused_def_ids_by_file: FxHashMap<Symbol, Vec<DefId>> = FxHashMap::default();
     for &non_codegenned_def_id in all_def_ids.difference(codegenned_def_ids) {
+        let codegen_fn_attrs = tcx.codegen_fn_attrs(non_codegenned_def_id);
+        if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::NO_COVERAGE) {
+            continue;
+        }
         // Make sure the non-codegenned (unused) function has a file_name
         if let Some(non_codegenned_file_name) = tcx.covered_file_name(non_codegenned_def_id) {
             let def_ids =

compiler/rustc_feature/src/builtin_attrs.rs

@@ -264,6 +264,7 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
 
     // Code generation:
     ungated!(inline, AssumedUsed, template!(Word, List: "always|never")),
+    ungated!(no_coverage, AssumedUsed, template!(Word)),
     ungated!(cold, AssumedUsed, template!(Word)),
     ungated!(no_builtins, AssumedUsed, template!(Word)),
     ungated!(target_feature, AssumedUsed, template!(List: r#"enable = "name""#)),

compiler/rustc_middle/src/middle/codegen_fn_attrs.rs

@@ -89,6 +89,10 @@ bitflags! {
         /// #[cmse_nonsecure_entry]: with a TrustZone-M extension, declare a
         /// function as an entry function from Non-Secure code.
         const CMSE_NONSECURE_ENTRY      = 1 << 14;
+        /// `#[no_coverage]`: indicates that the function should be ignored by
+        /// the MIR `InstrumentCoverage` pass and not added to the coverage map
+        /// during codegen.
+        const NO_COVERAGE               = 1 << 15;
     }
 }
 

compiler/rustc_mir/src/transform/coverage/mod.rs

@@ -23,6 +23,7 @@ use rustc_index::vec::IndexVec;
 use rustc_middle::hir;
 use rustc_middle::hir::map::blocks::FnLikeNode;
 use rustc_middle::ich::StableHashingContext;
+use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use rustc_middle::mir::coverage::*;
 use rustc_middle::mir::{
     self, BasicBlock, BasicBlockData, Coverage, SourceInfo, Statement, StatementKind, Terminator,
@@ -87,6 +88,11 @@ impl<'tcx> MirPass<'tcx> for InstrumentCoverage {
             _ => {}
         }
 
+        let codegen_fn_attrs = tcx.codegen_fn_attrs(mir_source.def_id());
+        if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::NO_COVERAGE) {
+            return;
+        }
+
         trace!("InstrumentCoverage starting for {:?}", mir_source.def_id());
         Instrumentor::new(&self.name(), tcx, mir_body).inject_counters();
         trace!("InstrumentCoverage starting for {:?}", mir_source.def_id());

compiler/rustc_span/src/symbol.rs

@@ -781,6 +781,7 @@ symbols! {
         no,
         no_builtins,
         no_core,
+        no_coverage,
         no_crate_inject,
         no_debug,
         no_default_passes,

compiler/rustc_typeck/src/collect.rs

@@ -2724,6 +2724,8 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, id: DefId) -> CodegenFnAttrs {
             codegen_fn_attrs.flags |= CodegenFnAttrFlags::NAKED;
         } else if tcx.sess.check_name(attr, sym::no_mangle) {
             codegen_fn_attrs.flags |= CodegenFnAttrFlags::NO_MANGLE;
+        } else if tcx.sess.check_name(attr, sym::no_coverage) {
+            codegen_fn_attrs.flags |= CodegenFnAttrFlags::NO_COVERAGE;
         } else if tcx.sess.check_name(attr, sym::rustc_std_internal_symbol) {
             codegen_fn_attrs.flags |= CodegenFnAttrFlags::RUSTC_STD_INTERNAL_SYMBOL;
         } else if tcx.sess.check_name(attr, sym::used) {