Handle trait/impl GAC mismatches when inferring missing/placeholder types

Author: fmease

compiler/rustc_hir_typeck/src/lib.rs

@@ -244,7 +244,8 @@ fn infer_type_if_missing<'tcx>(fcx: &FnCtxt<'_, 'tcx>, node: Node<'tcx>) -> Opti
                 impl_def_id,
                 impl_trait_ref.args,
             );
-            Some(tcx.type_of(trait_item_def_id).instantiate(tcx, args))
+            tcx.check_args_compatible(trait_item_def_id, args)
+                .then(|| tcx.type_of(trait_item_def_id).instantiate(tcx, args))
         } else {
             Some(fcx.next_ty_var(span))
         }

tests/crashes/124833.rs

@@ -5,13 +5,17 @@
 
 trait Trait {
     const K<T>: T;
+    const Q<'a>: &'a str;
 }
 
 impl Trait for () {
     const K<T> = ();
     //~^ ERROR missing type for `const` item
     //~| ERROR mismatched types
     //~| ERROR mismatched types
+    const Q = "";
+    //~^ ERROR missing type for `const` item
+    //~| ERROR lifetime parameters or bounds on const `Q` do not match the trait declaration
 }
 
 fn main() {}

tests/ui/generic-const-items/assoc-const-missing-type.rs

@@ -1,5 +1,5 @@
 error[E0308]: mismatched types
-  --> $DIR/assoc-const-missing-type.rs:11:18
+  --> $DIR/assoc-const-missing-type.rs:12:18
    |
 LL |     const K<T> = ();
    |             -    ^^ expected type parameter `T`, found `()`
@@ -10,13 +10,28 @@ LL |     const K<T> = ();
                    found unit type `()`
 
 error: missing type for `const` item
-  --> $DIR/assoc-const-missing-type.rs:11:15
+  --> $DIR/assoc-const-missing-type.rs:12:15
    |
 LL |     const K<T> = ();
    |               ^ help: provide a type for the associated constant: `()`
 
+error[E0195]: lifetime parameters or bounds on const `Q` do not match the trait declaration
+  --> $DIR/assoc-const-missing-type.rs:16:12
+   |
+LL |     const Q<'a>: &'a str;
+   |            ---- lifetimes in impl do not match this const in trait
+...
+LL |     const Q = "";
+   |            ^ lifetimes do not match const in trait
+
+error: missing type for `const` item
+  --> $DIR/assoc-const-missing-type.rs:16:12
+   |
+LL |     const Q = "";
+   |            ^ help: provide a type for the associated constant: `: &str`
+
 error[E0308]: mismatched types
-  --> $DIR/assoc-const-missing-type.rs:11:18
+  --> $DIR/assoc-const-missing-type.rs:12:18
    |
 LL |     const K<T> = ();
    |             -    ^^ expected type parameter `T`, found `()`
@@ -27,6 +42,7 @@ LL |     const K<T> = ();
                    found unit type `()`
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
-error: aborting due to 3 previous errors
+error: aborting due to 5 previous errors
 
-For more information about this error, try `rustc --explain E0308`.
+Some errors have detailed explanations: E0195, E0308.
+For more information about an error, try `rustc --explain E0195`.