From de074fe6368aca7435256925ee749f5664d49b69 Mon Sep 17 00:00:00 2001
From: Dawer <7803845+iDawer@users.noreply.github.com>
Date: Wed, 11 Aug 2021 22:09:32 +0500
Subject: [PATCH] Accept ambiguous unsize coercion only if it has definite
 guidance.

---
 crates/hir_ty/src/infer/coerce.rs   |  8 ++++----
 crates/hir_ty/src/tests/coercion.rs | 13 +++++++++++++
 2 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/crates/hir_ty/src/infer/coerce.rs b/crates/hir_ty/src/infer/coerce.rs
index 0e6b474486..9543404114 100644
--- a/crates/hir_ty/src/infer/coerce.rs
+++ b/crates/hir_ty/src/infer/coerce.rs
@@ -575,11 +575,11 @@ impl<'a> InferenceContext<'a> {
                     },
                 );
             }
-            Solution::Ambig(guidance) => {
-                if let Guidance::Definite(subst) = guidance {
-                    canonicalized.apply_solution(&mut self.table, subst);
-                }
+            Solution::Ambig(Guidance::Definite(subst)) => {
+                canonicalized.apply_solution(&mut self.table, subst)
             }
+            // FIXME: should we accept ambiguous results here?
+            _ => return Err(TypeError),
         };
         let unsize =
             Adjustment { kind: Adjust::Pointer(PointerCast::Unsize), target: to_ty.clone() };
diff --git a/crates/hir_ty/src/tests/coercion.rs b/crates/hir_ty/src/tests/coercion.rs
index cef1ffe9c9..4e761a6b47 100644
--- a/crates/hir_ty/src/tests/coercion.rs
+++ b/crates/hir_ty/src/tests/coercion.rs
@@ -559,3 +559,16 @@ fn test() {
         "#,
     );
 }
+
+#[test]
+fn coerce_type_var() {
+    check_types(
+        r#"
+//- minicore: from, coerce_unsized
+fn test() {
+    let x = ();
+    let _: &() = &x.into();
+}               //^^^^^^^^ ()
+"#,
+    )
+}