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Rollup merge of #118198 - Zalathar:if-not, r=cjgillot

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coverage: Use `SpanMarker` to improve coverage spans for `if !` expressions

Coverage instrumentation works by extracting source code spans from MIR. However, some kinds of syntax are effectively erased during MIR building, so their spans don't necessarily exist anywhere in MIR, making them invisible to the coverage instrumentor (unless we resort to various heuristics and hacks to recover them).

This PR introduces `CoverageKind::SpanMarker`, which is a new variant of `StatementKind::Coverage`. Its sole purpose is to represent spans that would otherwise not appear in MIR, so that the coverage instrumentor can extract them.

When coverage is enabled, the MIR builder can insert these dummy statements as needed, to improve the accuracy of spans used by coverage mappings.

Fixes #115468.

---

```@rustbot``` label +A-code-coverage
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Jubilee 2023-12-09 00:48:08 -08:00 • committed by GitHub
commit a71ab454ac
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11 changed files with 168 additions and 24 deletions
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15
compiler/rustc_mir_transform/src/coverage/spans/from_mir.rs
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@ -92,13 +92,13 @@ fn is_closure(statement: &Statement<'_>) -> bool {
/// If the MIR `Statement` has a span contributive to computing coverage spans,
/// return it; otherwise return `None`.
fn filtered_statement_span(statement: &Statement<'_>) -> Option<Span> {
use mir::coverage::CoverageKind;
match statement.kind {
// These statements have spans that are often outside the scope of the executed source code
// for their parent `BasicBlock`.
StatementKind::StorageLive(_)
| StatementKind::StorageDead(_)
// Coverage should not be encountered, but don't inject coverage coverage
| StatementKind::Coverage(_)
// Ignore `ConstEvalCounter`s
| StatementKind::ConstEvalCounter
// Ignore `Nop`s
@ -122,9 +122,13 @@ fn filtered_statement_span(statement: &Statement<'_>) -> Option<Span> {
// If and when the Issue is resolved, remove this special case match pattern:
StatementKind::FakeRead(box (FakeReadCause::ForGuardBinding, _)) => None,
// Retain spans from all other statements
// Retain spans from most other statements.
StatementKind::FakeRead(box (_, _)) // Not including `ForGuardBinding`
| StatementKind::Intrinsic(..)
| StatementKind::Coverage(box mir::Coverage {
// The purpose of `SpanMarker` is to be matched and accepted here.
kind: CoverageKind::SpanMarker
})
| StatementKind::Assign(_)
| StatementKind::SetDiscriminant { .. }
| StatementKind::Deinit(..)
@ -133,6 +137,11 @@ fn filtered_statement_span(statement: &Statement<'_>) -> Option<Span> {
| StatementKind::AscribeUserType(_, _) => {
Some(statement.source_info.span)
}
StatementKind::Coverage(box mir::Coverage {
// These coverage statements should not exist prior to coverage instrumentation.
kind: CoverageKind::CounterIncrement { .. } | CoverageKind::ExpressionUsed { .. }
}) => bug!("Unexpected coverage statement found during coverage instrumentation: {statement:?}"),
}
}