Auto merge of #88804 - Mark-Simulacrum:never-algo-v2, r=nikomatsakis,jackh726

[rust.git] / compiler / rustc_typeck / src / check / coercion.rs

diff --git a/compiler/rustc_typeck/src/check/coercion.rs b/compiler/rustc_typeck/src/check/coercion.rs
index 013aecae586ca93369917a4e7008377de62e4dbb..5b9481ce4e1b9d0cd92efa4d1014ce0f4130a83f 100644 (file)
--- a/compiler/rustc_typeck/src/check/coercion.rs
+++ b/compiler/rustc_typeck/src/check/coercion.rs
@@ -159,24 +159,7 @@ fn coerce(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
 
         // Coercing from `!` to any type is allowed:
         if a.is_never() {
-            // Subtle: If we are coercing from `!` to `?T`, where `?T` is an unbound
-            // type variable, we want `?T` to fallback to `!` if not
-            // otherwise constrained. An example where this arises:
-            //
-            //     let _: Option<?T> = Some({ return; });
-            //
-            // here, we would coerce from `!` to `?T`.
-            return if b.is_ty_var() {
-                // Micro-optimization: no need for this if `b` is
-                // already resolved in some way.
-                let diverging_ty = self.next_diverging_ty_var(TypeVariableOrigin {
-                    kind: TypeVariableOriginKind::AdjustmentType,
-                    span: self.cause.span,
-                });
-                self.coerce_from_inference_variable(diverging_ty, b, simple(Adjust::NeverToAny))
-            } else {
-                success(simple(Adjust::NeverToAny)(b), b, vec![])
-            };
+            return success(simple(Adjust::NeverToAny)(b), b, vec![]);
         }
 
         // Coercing *from* an unresolved inference variable means that
@@ -935,11 +918,13 @@ pub fn try_coerce(
         expr_ty: Ty<'tcx>,
         target: Ty<'tcx>,
         allow_two_phase: AllowTwoPhase,
+        cause: Option<ObligationCause<'tcx>>,
     ) -> RelateResult<'tcx, Ty<'tcx>> {
         let source = self.resolve_vars_with_obligations(expr_ty);
         debug!("coercion::try({:?}: {:?} -> {:?})", expr, source, target);
 
-        let cause = self.cause(expr.span, ObligationCauseCode::ExprAssignable);
+        let cause =
+            cause.unwrap_or_else(|| self.cause(expr.span, ObligationCauseCode::ExprAssignable));
         let coerce = Coerce::new(self, cause, allow_two_phase);
         let ok = self.commit_if_ok(|_| coerce.coerce(source, target))?;
 
@@ -1363,7 +1348,13 @@ pub fn coerce_forced_unit<'a>(
                 // Special-case the first expression we are coercing.
                 // To be honest, I'm not entirely sure why we do this.
                 // We don't allow two-phase borrows, see comment in try_find_coercion_lub for why
-                fcx.try_coerce(expression, expression_ty, self.expected_ty, AllowTwoPhase::No)
+                fcx.try_coerce(
+                    expression,
+                    expression_ty,
+                    self.expected_ty,
+                    AllowTwoPhase::No,
+                    Some(cause.clone()),
+                )
             } else {
                 match self.expressions {
                     Expressions::Dynamic(ref exprs) => fcx.try_find_coercion_lub(