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#91318 introduced a trait for infallible folders distinct from the fallible version.  For some reason (completely unfathomable to me now that I look at it with fresh eyes), the infallible trait was a supertrait of the fallible one: that is, all fallible folders were required to also be infallible.  Moreover the `Error` associated type was defined on the infallible trait!  It's so absurd that it has me questioning whether I was entirely sane.

This trait reverses the hierarchy, so that the fallible trait is a supertrait of the infallible one: all infallible folders are required to also be fallible (which is a trivial blanket implementation).  This of course makes much more sense!  It also enables the `Error` associated type to sit on the fallible trait, where it sensibly belongs.

There is one downside however: folders expose a `tcx` accessor method.  Since the blanket fallible implementation for infallible folders only has access to a generic `F: TypeFolder`, we need that trait to expose such an accessor to which we can delegate.  Alternatively it's possible to extract that accessor into a separate `HasTcx` trait (or similar) that would then be a supertrait of both the fallible and infallible folder traits: this would ensure that there's only one unambiguous `tcx` method, at the cost of a little additional boilerplate.  If desired, I can submit that as a separate PR.

r? @jackh726
This commit is contained in:
Alan Egerton 2022-06-20 21:10:43 +01:00
parent 72fd41a8b4
commit 6ac6866bec
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GPG key ID: 07CAC3CCA7E0643F
5 changed files with 25 additions and 42 deletions
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6
compiler/rustc_trait_selection/src/traits/query/normalize.rs
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@ -12,7 +12,7 @@ use rustc_data_structures::sso::SsoHashMap;
use rustc_data_structures::stack::ensure_sufficient_stack;
use rustc_infer::traits::Normalized;
use rustc_middle::mir;
use rustc_middle::ty::fold::{FallibleTypeFolder, TypeFoldable, TypeFolder, TypeSuperFoldable};
use rustc_middle::ty::fold::{FallibleTypeFolder, TypeFoldable, TypeSuperFoldable};
use rustc_middle::ty::subst::Subst;
use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitor};
@ -162,15 +162,13 @@ struct QueryNormalizer<'cx, 'tcx> {
universes: Vec<Option<ty::UniverseIndex>>,
}
impl<'cx, 'tcx> TypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
impl<'cx, 'tcx> FallibleTypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
type Error = NoSolution;
fn tcx<'c>(&'c self) -> TyCtxt<'tcx> {
self.infcx.tcx
}
}
impl<'cx, 'tcx> FallibleTypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
fn try_fold_binder<T: TypeFoldable<'tcx>>(
&mut self,
t: ty::Binder<'tcx, T>,