From 72002b401dcc0f9fb14ebf2d8d255f051bfdc513 Mon Sep 17 00:00:00 2001
From: Dawer <7803845+iDawer@users.noreply.github.com>
Date: Wed, 9 Jun 2021 11:49:12 +0500
Subject: [PATCH] internal: allow ambiguous unsize coercion.

---
 crates/hir_ty/src/infer/coerce.rs | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/crates/hir_ty/src/infer/coerce.rs b/crates/hir_ty/src/infer/coerce.rs
index d2e999a730f..0e6b474486a 100644
--- a/crates/hir_ty/src/infer/coerce.rs
+++ b/crates/hir_ty/src/infer/coerce.rs
@@ -17,8 +17,8 @@
         Adjust, Adjustment, AutoBorrow, InferOk, InferResult, InferenceContext, OverloadedDeref,
         PointerCast, TypeError, TypeMismatch,
     },
-    static_lifetime, Canonical, DomainGoal, FnPointer, FnSig, InEnvironment, Interner, Solution,
-    Substitution, Ty, TyBuilder, TyExt, TyKind,
+    static_lifetime, Canonical, DomainGoal, FnPointer, FnSig, Guidance, InEnvironment, Interner,
+    Solution, Substitution, Ty, TyBuilder, TyExt, TyKind,
 };
 
 pub(crate) type CoerceResult = Result<InferOk<(Vec<Adjustment>, Ty)>, TypeError>;
@@ -541,7 +541,7 @@ fn try_coerce_unsized(&mut self, from_ty: &Ty, to_ty: &Ty) -> CoerceResult {
             _ => return Err(TypeError),
         };
 
-        let trait_ref = {
+        let coerce_unsized_tref = {
             let b = TyBuilder::trait_ref(self.db, coerce_unsized_trait);
             if b.remaining() != 2 {
                 // The CoerceUnsized trait should have two generic params: Self and T.
@@ -551,7 +551,7 @@ fn try_coerce_unsized(&mut self, from_ty: &Ty, to_ty: &Ty) -> CoerceResult {
         };
 
         let goal: InEnvironment<DomainGoal> =
-            InEnvironment::new(&self.trait_env.env, trait_ref.cast(&Interner));
+            InEnvironment::new(&self.trait_env.env, coerce_unsized_tref.cast(&Interner));
 
         let canonicalized = self.canonicalize(goal);
 
@@ -575,8 +575,11 @@ fn try_coerce_unsized(&mut self, from_ty: &Ty, to_ty: &Ty) -> CoerceResult {
                     },
                 );
             }
-            // FIXME: should we accept ambiguous results here?
-            _ => return Err(TypeError),
+            Solution::Ambig(guidance) => {
+                if let Guidance::Definite(subst) = guidance {
+                    canonicalized.apply_solution(&mut self.table, subst);
+                }
+            }
         };
         let unsize =
             Adjustment { kind: Adjust::Pointer(PointerCast::Unsize), target: to_ty.clone() };