Do not give incorrect label for return type mismatch

Author: estebank

src/librustc_typeck/check/coercion.rs

@@ -1224,9 +1224,31 @@ impl<'gcx, 'tcx, 'exprs, E> CoerceMany<'gcx, 'tcx, 'exprs, E>
                             cause.span,
                             blk_id,
                         );
-                        if let Some(sp) = fcx.ret_coercion_span.borrow().as_ref() {
-                            if !sp.overlaps(cause.span) {
-                                db.span_label(*sp, reason_label);
+                        // TODO: replace with navigating up the chain until hitting an fn or
+                        // bailing if no "pass-through" Node is found, in order to provide a
+                        // suggestion when encountering something like:
+                        // ```
+                        // fn foo(a: bool) -> impl Debug {
+                        //     if a {
+                        //         bar()?;
+                        //     }
+                        //     {
+                        //         let x = unsafe { bar() };
+                        //         x
+                        //     }
+                        // }
+                        // ```
+                        //
+                        // Verify that this is a tail expression of a function, otherwise the
+                        // label pointing out the cause for the type coercion will be wrong
+                        // as prior return coercions would not be relevant (#57664).
+                        let parent_id = fcx.tcx.hir().get_parent_node(blk_id);
+                        let parent = fcx.tcx.hir().get(fcx.tcx.hir().get_parent_node(parent_id));
+                        if fcx.get_node_fn_decl(parent).is_some() {
+                            if let Some(sp) = fcx.ret_coercion_span.borrow().as_ref() {
+                                if !sp.overlaps(cause.span) {
+                                    db.span_label(*sp, reason_label);
+                                }
                             }
                         }
                     }

src/test/ui/point-to-type-err-cause-on-impl-trait-return-2.rs

@@ -0,0 +1,17 @@
+fn unrelated() -> Result<(), std::string::ParseError> {  // #57664
+    let x = 0;
+
+    match x {
+        1 => {
+            let property_value_as_string = "a".parse()?;
+        }
+        2 => {
+            let value: &bool = unsafe { &42 };
+            //~^ ERROR mismatched types
+        }
+    };
+
+    Ok(())
+}
+
+fn main() {}

src/test/ui/point-to-type-err-cause-on-impl-trait-return-2.stderr

@@ -0,0 +1,12 @@
+error[E0308]: mismatched types
+  --> $DIR/point-to-type-err-cause-on-impl-trait-return-2.rs:9:41
+   |
+LL |             let value: &bool = unsafe { &42 };
+   |                                         ^^^ expected bool, found integer
+   |
+   = note: expected type `&bool`
+              found type `&{integer}`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.

src/test/ui/point-to-type-err-cause-on-impl-trait-return.rs

@@ -0,0 +1,36 @@
+fn foo() -> impl std::fmt::Display {
+    if false {
+        return 0i32;
+    }
+    1u32
+    //~^ ERROR mismatched types
+}
+
+fn bar() -> impl std::fmt::Display {
+    if false {
+        return 0i32;
+    } else {
+        return 1u32;
+        //~^ ERROR mismatched types
+    }
+}
+
+fn baz() -> impl std::fmt::Display {
+    if false {
+    //~^ ERROR mismatched types
+        return 0i32;
+    } else {
+        1u32
+    }
+}
+
+fn qux() -> impl std::fmt::Display {
+    if false {
+        //~^ ERROR if and else have incompatible types
+        0i32
+    } else {
+        1u32
+    }
+}
+
+fn main() {}

src/test/ui/point-to-type-err-cause-on-impl-trait-return.stderr

@@ -0,0 +1,52 @@
+error[E0308]: mismatched types
+  --> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:5:5
+   |
+LL |         return 0i32;
+   |                ---- expected because of this statement
+LL |     }
+LL |     1u32
+   |     ^^^^ expected i32, found u32
+   |
+   = note: expected type `i32`
+              found type `u32`
+
+error[E0308]: mismatched types
+  --> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:13:16
+   |
+LL |         return 1u32;
+   |                ^^^^ expected i32, found u32
+   |
+   = note: expected type `i32`
+              found type `u32`
+
+error[E0308]: mismatched types
+  --> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:19:5
+   |
+LL | /     if false {
+LL | |     //~^ ERROR mismatched types
+LL | |         return 0i32;
+LL | |     } else {
+LL | |         1u32
+LL | |     }
+   | |_____^ expected i32, found u32
+   |
+   = note: expected type `i32`
+              found type `u32`
+
+error[E0308]: if and else have incompatible types
+  --> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:28:5
+   |
+LL | /     if false {
+LL | |         //~^ ERROR if and else have incompatible types
+LL | |         0i32
+LL | |     } else {
+LL | |         1u32
+LL | |     }
+   | |_____^ expected i32, found u32
+   |
+   = note: expected type `i32`
+              found type `u32`
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0308`.