From ac5aa8c1a4370b34674d18ef20b642645d2172e0 Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Mon, 25 Sep 2023 18:57:12 +0000
Subject: [PATCH] Don't suggest nonsense suggestions for unconstrained type
 vars in note_source_of_type_mismatch_constraint

---
 compiler/rustc_hir_typeck/src/demand.rs        | 10 ++++++++--
 .../point-at-inference-issue-116155.rs         | 17 +++++++++++++++++
 .../point-at-inference-issue-116155.stderr     | 18 ++++++++++++++++++
 3 files changed, 43 insertions(+), 2 deletions(-)
 create mode 100644 tests/ui/type/type-check/point-at-inference-issue-116155.rs
 create mode 100644 tests/ui/type/type-check/point-at-inference-issue-116155.stderr

diff --git a/compiler/rustc_hir_typeck/src/demand.rs b/compiler/rustc_hir_typeck/src/demand.rs
index d97691369c958..5c3f2b85966a8 100644
--- a/compiler/rustc_hir_typeck/src/demand.rs
+++ b/compiler/rustc_hir_typeck/src/demand.rs
@@ -14,7 +14,7 @@ use rustc_middle::ty::adjustment::AllowTwoPhase;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
 use rustc_middle::ty::fold::BottomUpFolder;
 use rustc_middle::ty::print::with_no_trimmed_paths;
-use rustc_middle::ty::{self, Article, AssocItem, Ty, TypeAndMut, TypeFoldable};
+use rustc_middle::ty::{self, Article, AssocItem, Ty, TypeAndMut, TypeFoldable, TypeVisitableExt};
 use rustc_span::symbol::sym;
 use rustc_span::{BytePos, Span, DUMMY_SP};
 use rustc_trait_selection::infer::InferCtxtExt as _;
@@ -504,12 +504,18 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     // incompatible fix at the original mismatch site.
                     if matches!(source, TypeMismatchSource::Ty(_))
                         && let Some(ideal_method) = ideal_method
+                        && let ideal_arg_ty = self.resolve_vars_if_possible(ideal_method.sig.inputs()[idx + 1])
+                        // HACK(compiler-errors): We don't actually consider the implications
+                        // of our inference guesses in `emit_type_mismatch_suggestions`, so
+                        // only suggest things when we know our type error is precisely due to
+                        // a type mismatch, and not via some projection or something. See #116155.
+                        && !ideal_arg_ty.has_non_region_infer()
                     {
                         self.emit_type_mismatch_suggestions(
                             err,
                             arg_expr,
                             arg_ty,
-                            self.resolve_vars_if_possible(ideal_method.sig.inputs()[idx + 1]),
+                            ideal_arg_ty,
                             None,
                             None,
                         );
diff --git a/tests/ui/type/type-check/point-at-inference-issue-116155.rs b/tests/ui/type/type-check/point-at-inference-issue-116155.rs
new file mode 100644
index 0000000000000..1e9942d42e867
--- /dev/null
+++ b/tests/ui/type/type-check/point-at-inference-issue-116155.rs
@@ -0,0 +1,17 @@
+struct S<T>(T);
+
+impl<T> S<T> {
+    fn new() -> Self {
+        loop {}
+    }
+
+    fn constrain<F: Fn() -> T>(&self, _f: F) {}
+}
+
+fn main() {
+    let s = S::new();
+    let c = || true;
+    s.constrain(c);
+    let _: S<usize> = s;
+    //~^ ERROR mismatched types
+}
diff --git a/tests/ui/type/type-check/point-at-inference-issue-116155.stderr b/tests/ui/type/type-check/point-at-inference-issue-116155.stderr
new file mode 100644
index 0000000000000..c8c01603cb848
--- /dev/null
+++ b/tests/ui/type/type-check/point-at-inference-issue-116155.stderr
@@ -0,0 +1,18 @@
+error[E0308]: mismatched types
+  --> $DIR/point-at-inference-issue-116155.rs:15:23
+   |
+LL |     s.constrain(c);
+   |     -           - this argument has type `{closure@$DIR/point-at-inference-issue-116155.rs:13:13: 13:15}`...
+   |     |
+   |     ... which causes `s` to have type `S<bool>`
+LL |     let _: S<usize> = s;
+   |            --------   ^ expected `S<usize>`, found `S<bool>`
+   |            |
+   |            expected due to this
+   |
+   = note: expected struct `S<usize>`
+              found struct `S<bool>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.