Rollup merge of #141236 - jagunter:issue-140823, r=compiler-errors

Resolved issue with mismatched types triggering ICE in certain scenarios

## Background

The function `annotate_mut_binding_to_immutable_binding` called in `emit_coerce_suggestions` performs a type comparison between the `expected` and `found` types from `ExpectedFound` in the `TypeError`. This can fail if the `found` type contains a region variable that's been rolled back.

## What is being changed?

This updates `annotate_mut_binding_to_immutable_binding` to use `expr_ty` and `expected` from the parent function instead of the types from the `TypeError`. This sidesteps the issue of using `found` from `TypeError` which may leak lingering inference region variables.

This does change the diagnostic behavior to _only_ support cases where the expected outermost type is `&T`, but that seems to be the intended functionality.

Also fixed the example in the `annotate_mut_binding_to_immutable_binding` rustdocs.

r? rust-lang/types

Fixes #140823

Author: matthiaskrgr

compiler/rustc_hir_typeck/src/demand.rs

@@ -84,7 +84,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         self.annotate_expected_due_to_let_ty(err, expr, error);
         self.annotate_loop_expected_due_to_inference(err, expr, error);
-        if self.annotate_mut_binding_to_immutable_binding(err, expr, error) {
+        if self.annotate_mut_binding_to_immutable_binding(err, expr, expr_ty, expected, error) {
             return;
         }
 
@@ -799,17 +799,17 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// Detect the following case
     ///
     /// ```text
-    /// fn change_object(mut a: &Ty) {
+    /// fn change_object(mut b: &Ty) {
     ///     let a = Ty::new();
     ///     b = a;
     /// }
     /// ```
     ///
-    /// where the user likely meant to modify the value behind there reference, use `a` as an out
+    /// where the user likely meant to modify the value behind there reference, use `b` as an out
     /// parameter, instead of mutating the local binding. When encountering this we suggest:
     ///
     /// ```text
-    /// fn change_object(a: &'_ mut Ty) {
+    /// fn change_object(b: &'_ mut Ty) {
     ///     let a = Ty::new();
     ///     *b = a;
     /// }
@@ -818,13 +818,15 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         &self,
         err: &mut Diag<'_>,
         expr: &hir::Expr<'_>,
+        expr_ty: Ty<'tcx>,
+        expected: Ty<'tcx>,
         error: Option<TypeError<'tcx>>,
     ) -> bool {
-        if let Some(TypeError::Sorts(ExpectedFound { expected, found })) = error
+        if let Some(TypeError::Sorts(ExpectedFound { .. })) = error
             && let ty::Ref(_, inner, hir::Mutability::Not) = expected.kind()
 
             // The difference between the expected and found values is one level of borrowing.
-            && self.can_eq(self.param_env, *inner, found)
+            && self.can_eq(self.param_env, *inner, expr_ty)
 
             // We have an `ident = expr;` assignment.
             && let hir::Node::Expr(hir::Expr { kind: hir::ExprKind::Assign(lhs, rhs, _), .. }) =

tests/crashes/140823.rs

@@ -0,0 +1,26 @@
+//! Functions with a mismatch between the expected and found type where the difference is a
+//! reference may trigger analysis for additional help. In this test the expected type will be
+//! &'a Container<&'a u8> and the found type will be Container<&'?0 u8>.
+//!
+//! This test exercises a scenario where the found type being analyzed contains an inference region
+//! variable ('?0). This cannot be used in comparisons because the variable no longer exists by the
+//! time the later analysis is performed.
+//!
+//! This is a regression test of #140823
+
+trait MyFn<P> {}
+
+struct Container<T> {
+    data: T,
+}
+
+struct Desugared {
+    callback: Box<dyn for<'a> MyFn<&'a Container<&'a u8>>>,
+}
+
+fn test(callback: Box<dyn for<'a> MyFn<Container<&'a u8>>>) -> Desugared {
+    Desugared { callback }
+    //~^ ERROR mismatched types
+}
+
+fn main() {}

tests/ui/fn/coerce-suggestion-infer-region.rs

@@ -0,0 +1,12 @@
+error[E0308]: mismatched types
+  --> $DIR/coerce-suggestion-infer-region.rs:22:17
+   |
+LL |     Desugared { callback }
+   |                 ^^^^^^^^ expected `Box<dyn MyFn<&Container<&u8>>>`, found `Box<dyn MyFn<Container<&u8>>>`
+   |
+   = note: expected struct `Box<(dyn for<'a> MyFn<&'a Container<&'a u8>> + 'static)>`
+              found struct `Box<(dyn for<'a> MyFn<Container<&'a u8>> + 'static)>`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.