From 39d9b840bbd2ed663241f4800f4d07b7950df46f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Le=C3=B3n=20Orell=20Valerian=20Liehr?= <me@fmease.dev>
Date: Thu, 23 May 2024 20:05:28 +0200
Subject: [PATCH] Handle trait/impl GAC mismatches when inferring
 missing/placeholder types

---
 compiler/rustc_hir_typeck/src/lib.rs          |  3 ++-
 tests/crashes/124833.rs                       | 10 -------
 .../assoc-const-missing-type.rs               |  4 +++
 .../assoc-const-missing-type.stderr           | 26 +++++++++++++++----
 4 files changed, 27 insertions(+), 16 deletions(-)
 delete mode 100644 tests/crashes/124833.rs

diff --git a/compiler/rustc_hir_typeck/src/lib.rs b/compiler/rustc_hir_typeck/src/lib.rs
index 4f35acaa44755..86fe2756b59a0 100644
--- a/compiler/rustc_hir_typeck/src/lib.rs
+++ b/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))
         }
diff --git a/tests/crashes/124833.rs b/tests/crashes/124833.rs
deleted file mode 100644
index f1c4847b544a5..0000000000000
--- a/tests/crashes/124833.rs
+++ /dev/null
@@ -1,10 +0,0 @@
-//@ known-bug: rust-lang/rust#124833
-#![feature(generic_const_items)]
-
-trait Trait {
-    const C<'a>: &'a str;
-}
-
-impl Trait for () {
-    const C<'a>:  = "C";
-}
diff --git a/tests/ui/generic-const-items/assoc-const-missing-type.rs b/tests/ui/generic-const-items/assoc-const-missing-type.rs
index 80f282ea134dc..93160f0b5758d 100644
--- a/tests/ui/generic-const-items/assoc-const-missing-type.rs
+++ b/tests/ui/generic-const-items/assoc-const-missing-type.rs
@@ -5,6 +5,7 @@
 
 trait Trait {
     const K<T>: T;
+    const Q<'a>: &'a str;
 }
 
 impl Trait for () {
@@ -12,6 +13,9 @@ impl Trait for () {
     //~^ 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() {}
diff --git a/tests/ui/generic-const-items/assoc-const-missing-type.stderr b/tests/ui/generic-const-items/assoc-const-missing-type.stderr
index 158e9a068f657..6f35c0958d45c 100644
--- a/tests/ui/generic-const-items/assoc-const-missing-type.stderr
+++ b/tests/ui/generic-const-items/assoc-const-missing-type.stderr
@@ -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`.