Rollup merge of rust-lang#120378 - lcnr:normalize-ast, r=compiler-errors

always normalize `LoweredTy` in the new solver

I currently expect us to stop using alias bound candidates of normalizable aliases due to rust-lang/trait-system-refactor-initiative#77 by landing rust-lang#119744. At this point it mostly doesn't matter whether we eagerly normalize (and replace with infer vars in case of ambiguity). cc rust-lang#113473 previous attempt

The infer var replacement for ambiguous projections can in very rare cases:
- weaken inference rust-lang/trait-system-refactor-initiative#81
- strengthen inference rust-lang/trait-system-refactor-initiative#7

I do not expect this impact on inference to significantly affect real crates.

r? ``@compiler-errors``

Author: matthiaskrgr

compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs

@@ -369,6 +369,14 @@ pub struct LoweredTy<'tcx> {
 
 impl<'tcx> LoweredTy<'tcx> {
     pub fn from_raw(fcx: &FnCtxt<'_, 'tcx>, span: Span, raw: Ty<'tcx>) -> LoweredTy<'tcx> {
-        LoweredTy { raw, normalized: fcx.normalize(span, raw) }
+        // FIXME(-Znext-solver): We're still figuring out how to best handle
+        // normalization and this doesn't feel too great. We should look at this
+        // code again before stabilizing it.
+        let normalized = if fcx.next_trait_solver() {
+            fcx.try_structurally_resolve_type(span, raw)
+        } else {
+            fcx.normalize(span, raw)
+        };
+        LoweredTy { raw, normalized }
     }
 }

tests/ui/for/issue-20605.next.stderr

@@ -34,15 +34,11 @@ error: the type `&mut <dyn Iterator<Item = &'a mut u8> as IntoIterator>::IntoIte
 LL |     for item in *things { *item = 0 }
    |                 ^^^^^^^
 
-error[E0277]: the size for values of type `<<dyn Iterator<Item = &'a mut u8> as IntoIterator>::IntoIter as Iterator>::Item` cannot be known at compilation time
-  --> $DIR/issue-20605.rs:5:9
+error: the type `Option<<<dyn Iterator<Item = &'a mut u8> as IntoIterator>::IntoIter as Iterator>::Item>` is not well-formed
+  --> $DIR/issue-20605.rs:5:17
    |
 LL |     for item in *things { *item = 0 }
-   |         ^^^^ doesn't have a size known at compile-time
-   |
-   = help: the trait `Sized` is not implemented for `<<dyn Iterator<Item = &'a mut u8> as IntoIterator>::IntoIter as Iterator>::Item`
-   = note: all local variables must have a statically known size
-   = help: unsized locals are gated as an unstable feature
+   |                 ^^^^^^^
 
 error[E0277]: the size for values of type `<<dyn Iterator<Item = &'a mut u8> as IntoIterator>::IntoIter as Iterator>::Item` cannot be known at compilation time
   --> $DIR/issue-20605.rs:5:5
@@ -54,11 +50,15 @@ LL |     for item in *things { *item = 0 }
 note: required by a bound in `None`
   --> $SRC_DIR/core/src/option.rs:LL:COL
 
-error: the type `Option<<<dyn Iterator<Item = &'a mut u8> as IntoIterator>::IntoIter as Iterator>::Item>` is not well-formed
-  --> $DIR/issue-20605.rs:5:17
+error[E0277]: the size for values of type `<<dyn Iterator<Item = &'a mut u8> as IntoIterator>::IntoIter as Iterator>::Item` cannot be known at compilation time
+  --> $DIR/issue-20605.rs:5:9
    |
 LL |     for item in *things { *item = 0 }
-   |                 ^^^^^^^
+   |         ^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `<<dyn Iterator<Item = &'a mut u8> as IntoIterator>::IntoIter as Iterator>::Item`
+   = note: all local variables must have a statically known size
+   = help: unsized locals are gated as an unstable feature
 
 error[E0614]: type `<<dyn Iterator<Item = &'a mut u8> as IntoIterator>::IntoIter as Iterator>::Item` cannot be dereferenced
   --> $DIR/issue-20605.rs:5:27

tests/ui/traits/next-solver/alias_eq_cant_be_furthur_normalized.rs renamed to tests/ui/traits/next-solver/alias-relate/alias_eq_cant_be_furthur_normalized.rs

@@ -3,7 +3,7 @@
 
 #![feature(type_alias_impl_trait)]
 
-// Similar to tests/ui/traits/next-solver/tait-eq-proj.rs
+// Similar to tests/ui/traits/next-solver/alias-relate/tait-eq-proj.rs
 // but check the alias-sub relation in the other direction.
 
 type Tait = impl Iterator<Item = impl Sized>;