bjoernager/rust
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Improve comments about type folding/visiting.

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I have found this code confusing for years. I've always roughly
understood it, but never exactly. I just made my fourth(?) attempt and
finally cracked it.

This commit improves the comments. In particular, it explicitly
describes how you can't do a custom fold/visit of any type; there are
actually a handful of "types of interest" (e.g. `Ty`, `Predicate`,
`Region`, `Const`) that can be custom folded/visted, and all other types
just get a generic traversal. I think this was the part that eluded me
on all my prior attempts at understanding.

The commit also updates comments to account for some newer changes such
as the fallible/infallible folding distinction, does some minor
reorderings, and moves one `impl` to a better place.
This commit is contained in:
Nicholas Nethercote 2022-02-08 11:46:53 +11:00
parent bfb2856f27
commit 94f08492af
2 changed files with 113 additions and 77 deletions
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19
compiler/rustc_middle/src/ty/structural_impls.rs
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@ -8,6 +8,7 @@ use crate::ty::fold::{FallibleTypeFolder, TypeFoldable, TypeVisitor};
use crate::ty::print::{with_no_trimmed_paths, FmtPrinter, Printer};
use crate::ty::{self, InferConst, Lift, Term, Ty, TyCtxt};
use rustc_data_structures::functor::IdFunctor;
use rustc_hir as hir;
use rustc_hir::def::Namespace;
use rustc_hir::def_id::CRATE_DEF_INDEX;
use rustc_index::vec::{Idx, IndexVec};
@ -663,14 +664,6 @@ impl<'a, 'tcx> Lift<'tcx> for ty::InstanceDef<'a> {
///////////////////////////////////////////////////////////////////////////
// TypeFoldable implementations.
//
// Ideally, each type should invoke `folder.fold_foo(self)` and
// nothing else. In some cases, though, we haven't gotten around to
// adding methods on the `folder` yet, and thus the folding is
// hard-coded here. This is less-flexible, because folders cannot
// override the behavior, but there are a lot of random types and one
// can easily refactor the folding into the TypeFolder trait as
// needed.
/// AdtDefs are basically the same as a DefId.
impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::AdtDef {
@ -1270,3 +1263,13 @@ impl<'tcx> TypeFoldable<'tcx> for ty::Unevaluated<'tcx, ()> {
self.substs.visit_with(visitor)
}
}
impl<'tcx> TypeFoldable<'tcx> for hir::Constness {
fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(self, _: &mut F) -> Result<Self, F::Error> {
Ok(self)
}
fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> ControlFlow<V::BreakTy> {
ControlFlow::CONTINUE
}
}