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Optimize the codegen for Span::from_expansion #140485

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May 3, 2025
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Optimize the codegen for Span::from_expansion #140485

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17 changes: 15 additions & 2 deletions compiler/rustc_span/src/span_encoding.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -306,8 +306,21 @@ impl Span {
/// Returns `true` if this span comes from any kind of macro, desugaring or inlining.
#[inline]
pub fn from_expansion(self) -> bool {
// If the span is fully inferred then ctxt > MAX_CTXT
self.inline_ctxt().map_or(true, |ctxt| !ctxt.is_root())
let ctxt = match_span_kind! {
self,
// All branches here, except `InlineParent`, actually return `span.ctxt_or_parent_or_marker`.
// Since `Interned` is selected if the field contains `CTXT_INTERNED_MARKER` returning that value
// as the context allows the compiler to optimize out the branch that selects between either
// `Interned` and `PartiallyInterned`.
//
// Interned contexts can never be the root context and `CTXT_INTERNED_MARKER` has a different value
// than the root context so this works for checking is this is an expansion.
InlineCtxt(span) => SyntaxContext::from_u16(span.ctxt),
InlineParent(_span) => SyntaxContext::root(),
PartiallyInterned(span) => SyntaxContext::from_u16(span.ctxt),
Interned(_span) => SyntaxContext::from_u16(CTXT_INTERNED_MARKER),
};
!ctxt.is_root()
}

/// Returns `true` if this is a dummy span with any hygienic context.
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