Collapse placeholders to root universe in canonicalizer if not preserving universes

Author: compiler-errors

compiler/rustc_infer/src/infer/canonical/canonicalizer.rs

@@ -418,10 +418,15 @@ impl<'cx, 'tcx> TypeFolder<TyCtxt<'tcx>> for Canonicalizer<'cx, 'tcx> {
                 bug!("encountered a fresh type during canonicalization")
             }
 
-            ty::Placeholder(placeholder) => self.canonicalize_ty_var(
-                CanonicalVarInfo { kind: CanonicalVarKind::PlaceholderTy(placeholder) },
-                t,
-            ),
+            ty::Placeholder(mut placeholder) => {
+                if !self.canonicalize_mode.preserve_universes() {
+                    placeholder.universe = ty::UniverseIndex::ROOT;
+                }
+                self.canonicalize_ty_var(
+                    CanonicalVarInfo { kind: CanonicalVarKind::PlaceholderTy(placeholder) },
+                    t,
+                )
+            }
 
             ty::Bound(debruijn, _) => {
                 if debruijn >= self.binder_index {

tests/ui/traits/non_lifetime_binders/bad-sized-cond.rs

@@ -7,7 +7,15 @@ where
 {
 }
 
+pub fn bar()
+where
+    for<V> V: IntoIterator,
+{
+}
+
 fn main() {
     foo();
     //~^ ERROR the size for values of type `V` cannot be known at compilation time
+
+    bar();
 }

tests/ui/traits/non_lifetime_binders/bad-sized-cond.stderr

@@ -8,7 +8,7 @@ LL | #![feature(non_lifetime_binders)]
    = note: `#[warn(incomplete_features)]` on by default
 
 error[E0277]: the size for values of type `V` cannot be known at compilation time
-  --> $DIR/bad-sized-cond.rs:11:5
+  --> $DIR/bad-sized-cond.rs:17:5
    |
 LL |     foo();
    |     ^^^ doesn't have a size known at compile-time
@@ -23,6 +23,40 @@ LL | where
 LL |     for<V> V: Sized,
    |               ^^^^^ required by this bound in `foo`
 
-error: aborting due to previous error; 1 warning emitted
+error[E0277]: the size for values of type `Placeholder(Placeholder { universe: U3, name: Param(DefId(0:6 ~ bad_sized_cond[9450]::bar::V), "V") })` cannot be known at compilation time
+  --> $DIR/bad-sized-cond.rs:20:5
+   |
+LL |     bar();
+   |     ^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `Placeholder(Placeholder { universe: U3, name: Param(DefId(0:6 ~ bad_sized_cond[9450]::bar::V), "V") })`
+   = note: required for `V` to implement `IntoIterator`
+note: required by a bound in `bar`
+  --> $DIR/bad-sized-cond.rs:12:15
+   |
+LL | pub fn bar()
+   |        --- required by a bound in this
+LL | where
+LL |     for<V> V: IntoIterator,
+   |               ^^^^^^^^^^^^ required by this bound in `bar`
+
+error[E0277]: `Placeholder(Placeholder { universe: U3, name: Param(DefId(0:6 ~ bad_sized_cond[9450]::bar::V), "V") })` is not an iterator
+  --> $DIR/bad-sized-cond.rs:20:5
+   |
+LL |     bar();
+   |     ^^^ `Placeholder(Placeholder { universe: U3, name: Param(DefId(0:6 ~ bad_sized_cond[9450]::bar::V), "V") })` is not an iterator
+   |
+   = help: the trait `Iterator` is not implemented for `Placeholder(Placeholder { universe: U3, name: Param(DefId(0:6 ~ bad_sized_cond[9450]::bar::V), "V") })`
+   = note: required for `V` to implement `IntoIterator`
+note: required by a bound in `bar`
+  --> $DIR/bad-sized-cond.rs:12:15
+   |
+LL | pub fn bar()
+   |        --- required by a bound in this
+LL | where
+LL |     for<V> V: IntoIterator,
+   |               ^^^^^^^^^^^^ required by this bound in `bar`
+
+error: aborting due to 3 previous errors; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0277`.