Rollup merge of rust-lang#110180 - lcnr:canonicalize, r=compiler-errors

don't uniquify regions when canonicalizing

uniquifying causes a bunch of issues, most notably it causes `AliasEq(<?x as Trait<'a>>::Assoc, <?x as Trait<'a>>::Assoc)` to result in ambiguity because both `normalizes-to` paths result in ambiguity and substs equate should trivially succeed but doesn't because we uniquified `'a` to two different regions.

I originally added uniquification to make it easier to deal with requirement 6 from the dev-guide: https://rustc-dev-guide.rust-lang.org/solve/trait-solving.html#requirements

> ### 6. Trait solving must be (free) lifetime agnostic
>
> Trait solving during codegen should have the same result as during typeck. As we erase
> all free regions during codegen we must not rely on them during typeck. A noteworthy example
> is special behavior for `'static`.

cc rust-lang/rustc-dev-guide#1671

Relying on regions being identical may cause ICE during MIR typeck, but even without this PR we can end up relying on that as type inference vars can resolve to types which contain an identical region. Let's land this and deal with any ICE that crop up as we go. Will look at this issue again before stabilization.

r? ``@compiler-errors``

Author: matthiaskrgr

compiler/rustc_trait_selection/src/solve/canonicalize.rs

@@ -125,8 +125,9 @@ impl<'a, 'tcx> Canonicalizer<'a, 'tcx> {
         // - var_infos: [E0, U1, E1, U1, E1, E6, U6], curr_compressed_uv: 1, next_orig_uv: 6
         // - var_infos: [E0, U1, E1, U1, E1, E2, U2], curr_compressed_uv: 2, next_orig_uv: -
         //
-        // This algorithm runs in `O(n²)` where `n` is the number of different universe
-        // indices in the input. This should be fine as `n` is expected to be small.
+        // This algorithm runs in `O(nm)` where `n` is the number of different universe
+        // indices in the input and `m` is the number of canonical variables.
+        // This should be fine as both `n` and `m` are expected to be small.
         let mut curr_compressed_uv = ty::UniverseIndex::ROOT;
         let mut existential_in_new_uv = false;
         let mut next_orig_uv = Some(ty::UniverseIndex::ROOT);
@@ -245,18 +246,14 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for Canonicalizer<'_, 'tcx> {
             ty::ReError(_) => return r,
         };
 
-        let existing_bound_var = match self.canonicalize_mode {
-            CanonicalizeMode::Input => None,
-            CanonicalizeMode::Response { .. } => {
-                self.variables.iter().position(|&v| v == r.into()).map(ty::BoundVar::from)
-            }
-        };
-        let var = existing_bound_var.unwrap_or_else(|| {
-            let var = ty::BoundVar::from(self.variables.len());
-            self.variables.push(r.into());
-            self.primitive_var_infos.push(CanonicalVarInfo { kind });
-            var
-        });
+        let var = ty::BoundVar::from(
+            self.variables.iter().position(|&v| v == r.into()).unwrap_or_else(|| {
+                let var = self.variables.len();
+                self.variables.push(r.into());
+                self.primitive_var_infos.push(CanonicalVarInfo { kind });
+                var
+            }),
+        );
         let br = ty::BoundRegion { var, kind: BrAnon(None) };
         self.interner().mk_re_late_bound(self.binder_index, br)
     }

tests/ui/regions/issue-2718.rs

@@ -0,0 +1,12 @@
+// compile-flags: -Ztrait-solver=next
+// check-pass
+
+use std::{iter, slice};
+
+struct Attr;
+
+fn test<'a, T: Iterator<Item = &'a Attr>>() {}
+
+fn main() {
+    test::<iter::Filter<slice::Iter<'_, Attr>, fn(&&Attr) -> bool>>();
+}