add a try_structurally_resolve_type in coerce

Author: lcnr

compiler/rustc_hir_typeck/src/coercion.rs

@@ -1005,7 +1005,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         allow_two_phase: AllowTwoPhase,
         cause: Option<ObligationCause<'tcx>>,
     ) -> RelateResult<'tcx, Ty<'tcx>> {
-        let source = self.resolve_vars_with_obligations(expr_ty);
+        let source = self.try_structurally_resolve_type(expr.span, expr_ty);
         debug!("coercion::try({:?}: {:?} -> {:?})", expr, source, target);
 
         let cause =

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -1465,16 +1465,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
     }
 
-    /// Resolves `typ` by a single level if `typ` is a type variable.
+    /// Try to resolve `ty` to a structural type, normalizing aliases.
     ///
-    /// When the new solver is enabled, this will also attempt to normalize
-    /// the type if it's a projection (note that it will not deeply normalize
-    /// projections within the type, just the outermost layer of the type).
-    ///
-    /// If no resolution is possible, then an error is reported.
-    /// Numeric inference variables may be left unresolved.
-    pub fn structurally_resolve_type(&self, sp: Span, ty: Ty<'tcx>) -> Ty<'tcx> {
-        let mut ty = self.resolve_vars_with_obligations(ty);
+    /// In case there is still ambiguity, the returned type may be an inference
+    /// variable. This is different from `structurally_resolve_type` which errors
+    /// in this case.
+    pub fn try_structurally_resolve_type(&self, sp: Span, ty: Ty<'tcx>) -> Ty<'tcx> {
+        let ty = self.resolve_vars_with_obligations(ty);
 
         if self.next_trait_solver()
             && let ty::Alias(ty::Projection, _) = ty.kind()
@@ -1483,15 +1480,27 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 .at(&self.misc(sp), self.param_env)
                 .structurally_normalize(ty, &mut **self.fulfillment_cx.borrow_mut())
             {
-                Ok(normalized_ty) => {
-                    ty = normalized_ty;
-                },
+                Ok(normalized_ty) => normalized_ty,
                 Err(errors) => {
                     let guar = self.err_ctxt().report_fulfillment_errors(&errors);
                     return self.tcx.ty_error(guar);
                 }
             }
-        }
+        } else {
+            ty
+       }
+    }
+
+    /// Resolves `ty` by a single level if `ty` is a type variable.
+    ///
+    /// When the new solver is enabled, this will also attempt to normalize
+    /// the type if it's a projection (note that it will not deeply normalize
+    /// projections within the type, just the outermost layer of the type).
+    ///
+    /// If no resolution is possible, then an error is reported.
+    /// Numeric inference variables may be left unresolved.
+    pub fn structurally_resolve_type(&self, sp: Span, ty: Ty<'tcx>) -> Ty<'tcx> {
+        let ty = self.try_structurally_resolve_type(sp, ty);
 
         if !ty.is_ty_var() {
             ty