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Rollup merge of #101520 - oli-obk:transmute_lifetimes, r=compiler-errors

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Allow transmutes between the same types after erasing lifetimes

r? ````@compiler-errors````  on the impl

fixes #101081

See discussion in the issue and at https://rust-lang.zulipchat.com/#narrow/stream/326866-t-types.2Fnominated/topic/.23101081.3A.20Regression.20transmuting.20.60RwLockReadGuard.3CT.3A.20.3FSized.3E.E2.80.A6

I think this may need lang team signoff as its implications may go beyond the jurisdiction of T-types

I'll write up a proper summary later
This commit is contained in:
Matthias Krüger 2022-10-08 14:38:18 +02:00 • committed by GitHub
commit 6bcdf8aa74
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6 changed files with 43 additions and 27 deletions
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20
compiler/rustc_hir_analysis/src/check/intrinsicck.rs
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@ -44,13 +44,23 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
pub fn check_transmute(&self, from: Ty<'tcx>, to: Ty<'tcx>, hir_id: HirId) {
let tcx = self.tcx;
let span = tcx.hir().span(hir_id);
let convert = |ty: Ty<'tcx>| {
let normalize = |ty| {
let ty = self.resolve_vars_if_possible(ty);
let ty = tcx.normalize_erasing_regions(self.param_env, ty);
(SizeSkeleton::compute(ty, tcx, self.param_env), ty)
self.tcx.normalize_erasing_regions(self.param_env, ty)
};
let (sk_from, from) = convert(from);
let (sk_to, to) = convert(to);
let from = normalize(from);
let to = normalize(to);
trace!(?from, ?to);
// Transmutes that are only changing lifetimes are always ok.
if from == to {
return;
}
let skel = |ty| SizeSkeleton::compute(ty, tcx, self.param_env);
let sk_from = skel(from);
let sk_to = skel(to);
trace!(?sk_from, ?sk_to);
// Check for same size using the skeletons.
if let (Ok(sk_from), Ok(sk_to)) = (sk_from, sk_to) {