Fail candidate assembly for erroneous types

Trait predicates for types which have errors may still
evaluate to OK leading to downstream ICEs. Now we return
a selection error for such types in candidate assembly and
thereby prevent such issues

Author: gurry

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -87,6 +87,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             } else if lang_items.sized_trait() == Some(def_id) {
                 // Sized is never implementable by end-users, it is
                 // always automatically computed.
+
+                // Error type cannot possibly implement `Sized` (fixes #123154)
+                if let ty::Error(_) = obligation.predicate.skip_binder().self_ty().kind() {
+                    return Err(SelectionError::Unimplemented);
+                }
+
                 let sized_conditions = self.sized_conditions(obligation);
                 self.assemble_builtin_bound_candidates(sized_conditions, &mut candidates);
             } else if lang_items.unsize_trait() == Some(def_id) {

compiler/rustc_ty_utils/src/ty.rs

@@ -92,7 +92,8 @@ fn adt_sized_constraint<'tcx>(
 
     let constraint_ty = sized_constraint_for_ty(tcx, tail_ty)?;
     if constraint_ty.references_error() {
-        return None;
+        let e = tcx.dcx().span_delayed_bug(DUMMY_SP, "ADT tail has error, but no error reported");
+        return Some(ty::EarlyBinder::bind(Ty::new_error(tcx, e)));
     }
 
     // perf hack: if there is a `constraint_ty: Sized` bound, then we know

tests/ui/associated-inherent-types/issue-109071.no_gate.stderr

@@ -43,7 +43,35 @@ LL |     type Item = &[T];
    = help: add `#![feature(inherent_associated_types)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-error: aborting due to 4 previous errors
+error[E0223]: ambiguous associated type
+  --> $DIR/issue-109071.rs:15:22
+   |
+LL |     fn T() -> Option<Self::Item> {}
+   |                      ^^^^^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+help: use fully-qualified syntax
+   |
+LL |     fn T() -> Option<<Windows<T> as IntoAsyncIterator>::Item> {}
+   |                      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+LL |     fn T() -> Option<<Windows<T> as IntoIterator>::Item> {}
+   |                      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+error[E0223]: ambiguous associated type
+  --> $DIR/issue-109071.rs:15:22
+   |
+LL |     fn T() -> Option<Self::Item> {}
+   |                      ^^^^^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+help: use fully-qualified syntax
+   |
+LL |     fn T() -> Option<<Windows<T> as IntoAsyncIterator>::Item> {}
+   |                      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+LL |     fn T() -> Option<<Windows<T> as IntoIterator>::Item> {}
+   |                      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+error: aborting due to 6 previous errors
 
 Some errors have detailed explanations: E0107, E0223, E0637, E0658.
 For more information about an error, try `rustc --explain E0107`.

tests/ui/associated-inherent-types/issue-109071.rs

@@ -14,6 +14,8 @@ impl<T> Windows { //~ ERROR: missing generics for struct `Windows`
 impl<T> Windows<T> {
     fn T() -> Option<Self::Item> {}
     //~^ ERROR: ambiguous associated type
+    //~| ERROR: ambiguous associated type
+    //~| ERROR: ambiguous associated type
 }
 
 fn main() {}

tests/ui/associated-inherent-types/issue-109071.with_gate.stderr

@@ -33,7 +33,35 @@ LL |     fn T() -> Option<<Windows<T> as IntoAsyncIterator>::Item> {}
 LL |     fn T() -> Option<<Windows<T> as IntoIterator>::Item> {}
    |                      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-error: aborting due to 3 previous errors
+error[E0223]: ambiguous associated type
+  --> $DIR/issue-109071.rs:15:22
+   |
+LL |     fn T() -> Option<Self::Item> {}
+   |                      ^^^^^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+help: use fully-qualified syntax
+   |
+LL |     fn T() -> Option<<Windows<T> as IntoAsyncIterator>::Item> {}
+   |                      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+LL |     fn T() -> Option<<Windows<T> as IntoIterator>::Item> {}
+   |                      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+error[E0223]: ambiguous associated type
+  --> $DIR/issue-109071.rs:15:22
+   |
+LL |     fn T() -> Option<Self::Item> {}
+   |                      ^^^^^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+help: use fully-qualified syntax
+   |
+LL |     fn T() -> Option<<Windows<T> as IntoAsyncIterator>::Item> {}
+   |                      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+LL |     fn T() -> Option<<Windows<T> as IntoIterator>::Item> {}
+   |                      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+error: aborting due to 5 previous errors
 
 Some errors have detailed explanations: E0107, E0223, E0637.
 For more information about an error, try `rustc --explain E0107`.

tests/ui/closures/issue-78720.rs

@@ -1,6 +1,5 @@
 fn server() -> impl {
     //~^ ERROR at least one trait must be specified
-    //~| ERROR type annotations needed
     ().map2(|| "")
 }
 

tests/ui/closures/issue-78720.stderr

@@ -5,7 +5,7 @@ LL | fn server() -> impl {
    |                ^^^^
 
 error[E0412]: cannot find type `F` in this scope
-  --> $DIR/issue-78720.rs:14:12
+  --> $DIR/issue-78720.rs:13:12
    |
 LL |     _func: F,
    |            ^
@@ -22,14 +22,8 @@ help: you might be missing a type parameter
 LL | struct Map2<Segment2, F> {
    |                     +++
 
-error[E0282]: type annotations needed
-  --> $DIR/issue-78720.rs:1:16
-   |
-LL | fn server() -> impl {
-   |                ^^^^ cannot infer type
-
 error[E0308]: mismatched types
-  --> $DIR/issue-78720.rs:8:39
+  --> $DIR/issue-78720.rs:7:39
    |
 LL |     fn map2<F>(self, f: F) -> Map2<F> {}
    |                                       ^^ expected `Map2<F>`, found `()`
@@ -38,7 +32,7 @@ LL |     fn map2<F>(self, f: F) -> Map2<F> {}
            found unit type `()`
 
 error[E0277]: the size for values of type `Self` cannot be known at compilation time
-  --> $DIR/issue-78720.rs:8:16
+  --> $DIR/issue-78720.rs:7:16
    |
 LL |     fn map2<F>(self, f: F) -> Map2<F> {}
    |                ^^^^ doesn't have a size known at compile-time
@@ -53,7 +47,7 @@ help: function arguments must have a statically known size, borrowed types alway
 LL |     fn map2<F>(&self, f: F) -> Map2<F> {}
    |                +
 
-error: aborting due to 5 previous errors
+error: aborting due to 4 previous errors
 
-Some errors have detailed explanations: E0277, E0282, E0308, E0412.
+Some errors have detailed explanations: E0277, E0308, E0412.
 For more information about an error, try `rustc --explain E0277`.

tests/ui/const-generics/const-arg-type-arg-misordered.rs

@@ -2,6 +2,7 @@ type Array<T, const N: usize> = [T; N];
 
 fn foo<const N: usize>() -> Array<N, ()> {
     //~^ ERROR constant provided when a type was expected
+    //~| ERROR constant provided when a type was expected
     unimplemented!()
 }
 

tests/ui/const-generics/const-arg-type-arg-misordered.stderr

@@ -4,6 +4,14 @@ error[E0747]: constant provided when a type was expected
 LL | fn foo<const N: usize>() -> Array<N, ()> {
    |                                   ^
 
-error: aborting due to 1 previous error
+error[E0747]: constant provided when a type was expected
+  --> $DIR/const-arg-type-arg-misordered.rs:3:35
+   |
+LL | fn foo<const N: usize>() -> Array<N, ()> {
+   |                                   ^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0747`.

tests/ui/const-generics/generic_const_exprs/issue-102768.rs

@@ -13,6 +13,10 @@ const _: () = {
     //~| ERROR associated type takes 0 generic arguments but 1 generic argument
     //~| ERROR associated type takes 1 lifetime argument but 0 lifetime arguments
     //~| ERROR associated type takes 0 generic arguments but 1 generic argument
+    //~| ERROR associated type takes 1 lifetime argument but 0 lifetime arguments
+    //~| ERROR associated type takes 1 lifetime argument but 0 lifetime arguments
+    //~| ERROR associated type takes 0 generic arguments but 1 generic argument
+    //~| ERROR associated type takes 0 generic arguments but 1 generic argument
     //~| ERROR `X` cannot be made into an object
 };