fix type inference in loop and block coercions

Author: adwinwhite

compiler/rustc_hir_typeck/src/expectation.rs

@@ -113,7 +113,12 @@ impl<'a, 'tcx> Expectation<'tcx> {
 
     /// Like `only_has_type`, but instead of returning `None` if no
     /// hard constraint exists, creates a fresh type variable.
+    /// Also avoid using the original type variable as the target, as it may resolve prematurely
+    /// during the first coercion in `CoerceMany`, rather than deferring to the final coerced type.
     pub(super) fn coercion_target_type(self, fcx: &FnCtxt<'a, 'tcx>, span: Span) -> Ty<'tcx> {
-        self.only_has_type(fcx).unwrap_or_else(|| fcx.next_ty_var(span))
+        match self.only_has_type(fcx) {
+            Some(ty) if !ty.is_ty_var() => ty,
+            _ => fcx.next_ty_var(span),
+        }
     }
 }

tests/ui/coercion/coerce-unify.rs

@@ -10,6 +10,20 @@ macro_rules! check {
         let _ = match 0 { $(_ if false => $rest,)+ _ => $last };
 
         let _ = [$($rest,)+ $last];
+
+        let _ = 'a: {
+            $(if false {
+                break 'a $rest;
+            })else+
+            break 'a $last;
+        };
+
+        let _ = loop {
+            $(if false {
+                break $rest;
+            })else+
+            break $last;
+        };
     }
 }