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Auto merge of #119336 - Zalathar:find-ancestor, r=petrochenkov
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coverage: Unexpand spans with `find_ancestor_inside_same_ctxt`

Back in #118525 (comment) it was observed that our `unexpand_into_body_span` now looks very similar to `Span::find_ancestor_inside`.

At the time I tried switching over, but doing so resulted in incorrect coverage mappings (or assertion failures), so I left a `FIXME` comment instead.

After some investigation, I identified the two problems with my original approach:
- I should have been using `find_ancestor_inside_same_ctxt` instead, since we want a span that's inside the body and has the same context as the body.
- For async functions, we were actually using the post-expansion body span, which is why we needed to forcibly set the unexpanded span's context to match the body span. For body spans produced by macro-expansion, we already have special-case code to detect this and use the pre-expansion call site as the body span. By making this code also detect async desugaring, I was able to end up with a body span that works properly with `find_ancestor_inside_same_ctxt`, avoiding the need to forcibly change the span context.
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bors committed Dec 28, 2023
2 parents 89e2160 + be6b059 commit 8e34642
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Showing 2 changed files with 6 additions and 22 deletions.
21 changes: 5 additions & 16 deletions compiler/rustc_mir_transform/src/coverage/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -23,7 +23,7 @@ use rustc_middle::mir::{
use rustc_middle::ty::TyCtxt;
use rustc_span::def_id::LocalDefId;
use rustc_span::source_map::SourceMap;
use rustc_span::{ExpnKind, Span, Symbol};
use rustc_span::{Span, Symbol};

/// Inserts `StatementKind::Coverage` statements that either instrument the binary with injected
/// counters, via intrinsic `llvm.instrprof.increment`, and/or inject metadata used during codegen
Expand Down Expand Up @@ -346,21 +346,10 @@ fn get_body_span<'tcx>(
let mut body_span = hir_body.value.span;

if tcx.is_closure(def_id.to_def_id()) {
// If the MIR function is a closure, and if the closure body span
// starts from a macro, but it's content is not in that macro, try
// to find a non-macro callsite, and instrument the spans there
// instead.
loop {
let expn_data = body_span.ctxt().outer_expn_data();
if expn_data.is_root() {
break;
}
if let ExpnKind::Macro { .. } = expn_data.kind {
body_span = expn_data.call_site;
} else {
break;
}
}
// If the current function is a closure, and its "body" span was created
// by macro expansion or compiler desugaring, try to walk backwards to
// the pre-expansion call site or body.
body_span = body_span.source_callsite();
}

body_span
Expand Down
7 changes: 1 addition & 6 deletions compiler/rustc_mir_transform/src/coverage/spans/from_mir.rs
Original file line number Diff line number Diff line change
Expand Up @@ -204,10 +204,5 @@ fn filtered_terminator_span(terminator: &Terminator<'_>) -> Option<Span> {
/// etc.).
#[inline]
fn unexpand_into_body_span(span: Span, body_span: Span) -> Option<Span> {
use rustc_span::source_map::original_sp;

// FIXME(#118525): Consider switching from `original_sp` to `Span::find_ancestor_inside`,
// which is similar but gives slightly different results in some edge cases.
let original_span = original_sp(span, body_span).with_ctxt(body_span.ctxt());
body_span.contains(original_span).then_some(original_span)
span.find_ancestor_inside_same_ctxt(body_span)
}

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