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An optimization for pest-2.1.3 #98654

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Jul 8, 2022
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Avoid constructing an unnecessary InferCtxt.
Currently, `search_for_structural_match_violation` constructs an `infcx`
from a `tcx` and then only uses the `tcx` within the `infcx`. This is
wasteful because `infcx` is a big type.

This commit changes it to use the `tcx` directly. When compiling
`pest-2.1.3`, this changes the memcpy stats reported by DHAT for a `check full`
build from this:
```
433,008,916 bytes (100%, 99,787.93/Minstr) in 2,148,668 blocks (100%, 495.17/Minstr), avg size 201.52 bytes
```
to this:
```
101,422,347 bytes (99.98%, 25,243.59/Minstr) in 1,318,407 blocks (99.96%, 328.15/Minstr), avg size 76.93 bytes
```
This translates to a 4.3% reduction in instruction counts.
nnethercote committed Jun 29, 2022

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This commit was signed with the committer’s verified signature.
frostming Frost Ming
commit 687e391bc3a9682b6fe956a2e39a8a3935047158
Original file line number Diff line number Diff line change
@@ -58,10 +58,7 @@ pub fn search_for_structural_match_violation<'tcx>(
tcx: TyCtxt<'tcx>,
ty: Ty<'tcx>,
) -> Option<NonStructuralMatchTy<'tcx>> {
// FIXME: we should instead pass in an `infcx` from the outside.
tcx.infer_ctxt().enter(|infcx| {
ty.visit_with(&mut Search { tcx: infcx.tcx, span, seen: FxHashSet::default() }).break_value()
})
ty.visit_with(&mut Search { tcx, span, seen: FxHashSet::default() }).break_value()
}

/// This method returns true if and only if `adt_ty` itself has been marked as