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Add comments about predicate folding.

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This explains why the predicate folding code looks different to the
ty/const folding code, something I was wondering.
This commit is contained in:
Nicholas Nethercote 2026-01-07 17:19:36 +11:00
parent eb1645653b
commit a07f71704a
1 changed files with 8 additions and 0 deletions
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8
compiler/rustc_middle/src/ty/structural_impls.rs
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@ -586,11 +586,18 @@ impl<'tcx> TypeSuperFoldable<TyCtxt<'tcx>> for ty::Predicate<'tcx> {
self,
folder: &mut F,
) -> Result<Self, F::Error> {
// This method looks different to `Ty::try_super_fold_with` and `Const::super_fold_with`.
// Why is that? `PredicateKind` provides little scope for optimized folding, unlike
// `TyKind` and `ConstKind` (which have common variants that don't require recursive
// `fold_with` calls on their fields). So we just derive the `TypeFoldable` impl for
// `PredicateKind` and call it here because the derived code is as fast as hand-written
// code would be.
let new = self.kind().try_fold_with(folder)?;
Ok(folder.cx().reuse_or_mk_predicate(self, new))
}
fn super_fold_with<F: TypeFolder<TyCtxt<'tcx>>>(self, folder: &mut F) -> Self {
// See comment in `Predicate::try_super_fold_with`.
let new = self.kind().fold_with(folder);
folder.cx().reuse_or_mk_predicate(self, new)
}
@ -598,6 +605,7 @@ impl<'tcx> TypeSuperFoldable<TyCtxt<'tcx>> for ty::Predicate<'tcx> {
impl<'tcx> TypeSuperVisitable<TyCtxt<'tcx>> for ty::Predicate<'tcx> {
fn super_visit_with<V: TypeVisitor<TyCtxt<'tcx>>>(&self, visitor: &mut V) -> V::Result {
// See comment in `Predicate::try_super_fold_with`.
self.kind().visit_with(visitor)
}
}