Accept ambiguous unsize coercion only if it has definite guidance.

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() };

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();
+}               //^^^^^^^^ ()
+"#,
+    )
+}