From 96b8225d8dd971bc2ecc7aa12068adcda9a9f20f Mon Sep 17 00:00:00 2001
From: Veera <sveera.2001@gmail.com>
Date: Mon, 11 Mar 2024 23:05:43 -0400
Subject: [PATCH] Don't Create `ParamCandidate` When Obligation Contains Errors

Fixes #121941
---
 .../src/traits/select/candidate_assembly.rs              | 7 +++++++
 ...r-ty-with-calller-supplied-obligation-issue-121941.rs | 5 +++++
 ...-with-calller-supplied-obligation-issue-121941.stderr | 9 +++++++++
 3 files changed, 21 insertions(+)
 create mode 100644 tests/ui/traits/dont-match-error-ty-with-calller-supplied-obligation-issue-121941.rs
 create mode 100644 tests/ui/traits/dont-match-error-ty-with-calller-supplied-obligation-issue-121941.stderr

diff --git a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
index 66f740b761d32..89654ed61aeb9 100644
--- a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
@@ -219,6 +219,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     ) -> Result<(), SelectionError<'tcx>> {
         debug!(?stack.obligation);
 
+        // An error type will unify with anything. So, avoid
+        // matching an error type with `ParamCandidate`.
+        // This helps us avoid spurious errors like issue #121941.
+        if stack.obligation.predicate.references_error() {
+            return Ok(());
+        }
+
         let all_bounds = stack
             .obligation
             .param_env
diff --git a/tests/ui/traits/dont-match-error-ty-with-calller-supplied-obligation-issue-121941.rs b/tests/ui/traits/dont-match-error-ty-with-calller-supplied-obligation-issue-121941.rs
new file mode 100644
index 0000000000000..a08407683d8fe
--- /dev/null
+++ b/tests/ui/traits/dont-match-error-ty-with-calller-supplied-obligation-issue-121941.rs
@@ -0,0 +1,5 @@
+fn function<T: PartialEq>() {
+    foo == 2; //~ ERROR cannot find value `foo` in this scope [E0425]
+}
+
+fn main() {}
diff --git a/tests/ui/traits/dont-match-error-ty-with-calller-supplied-obligation-issue-121941.stderr b/tests/ui/traits/dont-match-error-ty-with-calller-supplied-obligation-issue-121941.stderr
new file mode 100644
index 0000000000000..2da731dcc4b14
--- /dev/null
+++ b/tests/ui/traits/dont-match-error-ty-with-calller-supplied-obligation-issue-121941.stderr
@@ -0,0 +1,9 @@
+error[E0425]: cannot find value `foo` in this scope
+  --> $DIR/dont-match-error-ty-with-calller-supplied-obligation-issue-121941.rs:2:5
+   |
+LL |     foo == 2;
+   |     ^^^ not found in this scope
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0425`.