From 50fb40a987746e8847091ce354b649be4a44cde1 Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Sat, 23 Nov 2024 05:24:39 +0000
Subject: [PATCH] Delay a bug when encountering an impl with unconstrained
 generics in codegen_select

---
 compiler/rustc_traits/src/codegen.rs          | 21 +++++++++++++------
 tests/crashes/126646.rs                       | 18 ----------------
 .../resolve-impl-before-constrain-check.rs    | 20 ++++++++++++++++++
 ...resolve-impl-before-constrain-check.stderr |  9 ++++++++
 4 files changed, 44 insertions(+), 24 deletions(-)
 delete mode 100644 tests/crashes/126646.rs
 create mode 100644 tests/ui/traits/resolve-impl-before-constrain-check.rs
 create mode 100644 tests/ui/traits/resolve-impl-before-constrain-check.stderr

diff --git a/compiler/rustc_traits/src/codegen.rs b/compiler/rustc_traits/src/codegen.rs
index 57225df0819d8..e5276e6d51585 100644
--- a/compiler/rustc_traits/src/codegen.rs
+++ b/compiler/rustc_traits/src/codegen.rs
@@ -74,12 +74,21 @@ pub(crate) fn codegen_select_candidate<'tcx>(
     }
 
     let impl_source = infcx.resolve_vars_if_possible(impl_source);
-    let impl_source = infcx.tcx.erase_regions(impl_source);
-    if impl_source.has_infer() {
-        // Unused lifetimes on an impl get replaced with inference vars, but never resolved,
-        // causing the return value of a query to contain inference vars. We do not have a concept
-        // for this and will in fact ICE in stable hashing of the return value. So bail out instead.
-        infcx.tcx.dcx().has_errors().unwrap();
+    let impl_source = tcx.erase_regions(impl_source);
+    if impl_source.has_non_region_infer() {
+        // Unused generic types or consts on an impl get replaced with inference vars,
+        // but never resolved, causing the return value of a query to contain inference
+        // vars. We do not have a concept for this and will in fact ICE in stable hashing
+        // of the return value. So bail out instead.
+        match impl_source {
+            ImplSource::UserDefined(impl_) => {
+                tcx.dcx().span_delayed_bug(
+                    tcx.def_span(impl_.impl_def_id),
+                    "this impl has unconstrained generic parameters",
+                );
+            }
+            _ => unreachable!(),
+        }
         return Err(CodegenObligationError::FulfillmentError);
     }
 
diff --git a/tests/crashes/126646.rs b/tests/crashes/126646.rs
deleted file mode 100644
index 24e3530320a89..0000000000000
--- a/tests/crashes/126646.rs
+++ /dev/null
@@ -1,18 +0,0 @@
-//@ known-bug: rust-lang/rust#126646
-mod foo {
-    pub trait Callable {
-        type Output;
-        fn call() -> Self::Output;
-    }
-
-    impl<'a, V: ?Sized> Callable for &'a () {
-        type Output = ();
-    }
-}
-use foo::*;
-
-fn test<'a>() -> impl Sized {
-    <&'a () as Callable>::call()
-}
-
-fn main() {}
diff --git a/tests/ui/traits/resolve-impl-before-constrain-check.rs b/tests/ui/traits/resolve-impl-before-constrain-check.rs
new file mode 100644
index 0000000000000..87f9c241e402b
--- /dev/null
+++ b/tests/ui/traits/resolve-impl-before-constrain-check.rs
@@ -0,0 +1,20 @@
+// Need a different module so we try to build the mir for `test`
+// before analyzing `mod foo`.
+
+mod foo {
+    pub trait Callable {
+        fn call();
+    }
+
+    impl<V: ?Sized> Callable for () {
+    //~^ ERROR the type parameter `V` is not constrained by the impl trait, self type, or predicates
+        fn call() {}
+    }
+}
+use foo::*;
+
+fn test() -> impl Sized {
+    <() as Callable>::call()
+}
+
+fn main() {}
diff --git a/tests/ui/traits/resolve-impl-before-constrain-check.stderr b/tests/ui/traits/resolve-impl-before-constrain-check.stderr
new file mode 100644
index 0000000000000..e8e569ba625ee
--- /dev/null
+++ b/tests/ui/traits/resolve-impl-before-constrain-check.stderr
@@ -0,0 +1,9 @@
+error[E0207]: the type parameter `V` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/resolve-impl-before-constrain-check.rs:9:10
+   |
+LL |     impl<V: ?Sized> Callable for () {
+   |          ^ unconstrained type parameter
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0207`.