Suggest coercion of Result using ?

compiler/rustc_hir_typeck/src/coercion.rs

@@ -1619,6 +1619,7 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
             }
         }
     }
+
     fn note_unreachable_loop_return(
         &self,
         err: &mut Diagnostic,

compiler/rustc_hir_typeck/src/demand.rs

@@ -59,7 +59,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             || self.suggest_copied_or_cloned(err, expr, expr_ty, expected)
             || self.suggest_clone_for_ref(err, expr, expr_ty, expected)
             || self.suggest_into(err, expr, expr_ty, expected)
-            || self.suggest_floating_point_literal(err, expr, expected);
+            || self.suggest_floating_point_literal(err, expr, expected)
+            || self.note_result_coercion(err, expr, expected, expr_ty);
         if !suggested {
             self.point_at_expr_source_of_inferred_type(err, expr, expr_ty, expected);
         }
@@ -697,6 +698,56 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         );
     }
 
+    pub(crate) fn note_result_coercion(
+        &self,
+        err: &mut Diagnostic,
+        expr: &hir::Expr<'tcx>,
+        expected: Ty<'tcx>,
+        found: Ty<'tcx>,
+    ) -> bool {
+        let ty::Adt(e, substs_e) = expected.kind() else { return false; };
+        let ty::Adt(f, substs_f) = found.kind() else { return false; };
+        if e.did() != f.did() {
+            return false;
+        }
+        if Some(e.did()) != self.tcx.get_diagnostic_item(sym::Result) {
+            return false;
+        }
+        let map = self.tcx.hir();
+        if let Some(hir::Node::Expr(expr)) = map.find_parent(expr.hir_id)
+            && let hir::ExprKind::Ret(_) = expr.kind
+        {
+            // `return foo;`
+        } else if map.get_return_block(expr.hir_id).is_some() {
+            // Function's tail expression.
+        } else {
+            return false;
+        }
+        let e = substs_e.type_at(1);
+        let f = substs_f.type_at(1);
+        if self
+            .infcx
+            .type_implements_trait(
+                self.tcx.get_diagnostic_item(sym::Into).unwrap(),
+                [f, e],
+                self.param_env,
+            )
+            .must_apply_modulo_regions()
+        {
+            err.multipart_suggestion(
+                "use `?` to coerce and return an appropriate `Err`, and wrap the resulting value \
+                 in `Ok` so the expression remains of type `Result`",
+                vec![
+                    (expr.span.shrink_to_lo(), "Ok(".to_string()),
+                    (expr.span.shrink_to_hi(), "?)".to_string()),
+                ],
+                Applicability::MaybeIncorrect,
+            );
+            return true;
+        }
+        false
+    }
+
     /// If the expected type is an enum (Issue #55250) with any variants whose
     /// sole field is of the found type, suggest such variants. (Issue #42764)
     fn suggest_compatible_variants(

tests/ui/type/type-check/coerce-result-return-value-2.rs

@@ -0,0 +1,24 @@
+struct A;
+struct B;
+impl From<A> for B {
+    fn from(_: A) -> Self { B }
+}
+fn foo4(x: Result<(), A>) -> Result<(), B> {
+    match true {
+        true => x, //~ ERROR mismatched types
+        false => x,
+    }
+}
+fn foo5(x: Result<(), A>) -> Result<(), B> {
+    match true {
+        true => return x, //~ ERROR mismatched types
+        false => return x,
+    }
+}
+fn main() {
+    let _ = foo4(Ok(()));
+    let _ = foo5(Ok(()));
+    let _: Result<(), B> = { //~ ERROR mismatched types
+        Err(A);
+    };
+}

tests/ui/type/type-check/coerce-result-return-value-2.stderr

@@ -0,0 +1,47 @@
+error[E0308]: mismatched types
+  --> $DIR/coerce-result-return-value-2.rs:8:17
+   |
+LL | fn foo4(x: Result<(), A>) -> Result<(), B> {
+   |                              ------------- expected `Result<(), B>` because of return type
+LL |     match true {
+LL |         true => x,
+   |                 ^ expected struct `B`, found struct `A`
+   |
+   = note: expected enum `Result<_, B>`
+              found enum `Result<_, A>`
+help: use `?` to coerce and return an appropriate `Err`, and wrap the resulting value in `Ok` so the expression remains of type `Result`
+   |
+LL |         true => Ok(x?),
+   |                 +++ ++
+
+error[E0308]: mismatched types
+  --> $DIR/coerce-result-return-value-2.rs:14:24
+   |
+LL | fn foo5(x: Result<(), A>) -> Result<(), B> {
+   |                              ------------- expected `Result<(), B>` because of return type
+LL |     match true {
+LL |         true => return x,
+   |                        ^ expected struct `B`, found struct `A`
+   |
+   = note: expected enum `Result<_, B>`
+              found enum `Result<_, A>`
+help: use `?` to coerce and return an appropriate `Err`, and wrap the resulting value in `Ok` so the expression remains of type `Result`
+   |
+LL |         true => return Ok(x?),
+   |                        +++ ++
+
+error[E0308]: mismatched types
+  --> $DIR/coerce-result-return-value-2.rs:21:28
+   |
+LL |       let _: Result<(), B> = {
+   |  ____________________________^
+LL | |         Err(A);
+LL | |     };
+   | |_____^ expected enum `Result`, found `()`
+   |
+   = note:   expected enum `Result<(), B>`
+           found unit type `()`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/type/type-check/coerce-result-return-value.fixed

@@ -0,0 +1,24 @@
+// run-rustfix
+struct A;
+struct B;
+impl From<A> for B {
+    fn from(_: A) -> Self { B }
+}
+fn foo1(x: Result<(), A>) -> Result<(), B> {
+    Ok(x?) //~ ERROR mismatched types
+}
+fn foo2(x: Result<(), A>) -> Result<(), B> {
+    return Ok(x?); //~ ERROR mismatched types
+}
+fn foo3(x: Result<(), A>) -> Result<(), B> {
+    if true {
+        Ok(x?) //~ ERROR mismatched types
+    } else {
+        Ok(x?) //~ ERROR mismatched types
+    }
+}
+fn main() {
+    let _ = foo1(Ok(()));
+    let _ = foo2(Ok(()));
+    let _ = foo3(Ok(()));
+}

tests/ui/type/type-check/coerce-result-return-value.rs

@@ -0,0 +1,24 @@
+// run-rustfix
+struct A;
+struct B;
+impl From<A> for B {
+    fn from(_: A) -> Self { B }
+}
+fn foo1(x: Result<(), A>) -> Result<(), B> {
+    x //~ ERROR mismatched types
+}
+fn foo2(x: Result<(), A>) -> Result<(), B> {
+    return x; //~ ERROR mismatched types
+}
+fn foo3(x: Result<(), A>) -> Result<(), B> {
+    if true {
+        x //~ ERROR mismatched types
+    } else {
+        x //~ ERROR mismatched types
+    }
+}
+fn main() {
+    let _ = foo1(Ok(()));
+    let _ = foo2(Ok(()));
+    let _ = foo3(Ok(()));
+}

tests/ui/type/type-check/coerce-result-return-value.stderr

@@ -0,0 +1,65 @@
+error[E0308]: mismatched types
+  --> $DIR/coerce-result-return-value.rs:8:5
+   |
+LL | fn foo1(x: Result<(), A>) -> Result<(), B> {
+   |                              ------------- expected `Result<(), B>` because of return type
+LL |     x
+   |     ^ expected struct `B`, found struct `A`
+   |
+   = note: expected enum `Result<_, B>`
+              found enum `Result<_, A>`
+help: use `?` to coerce and return an appropriate `Err`, and wrap the resulting value in `Ok` so the expression remains of type `Result`
+   |
+LL |     Ok(x?)
+   |     +++ ++
+
+error[E0308]: mismatched types
+  --> $DIR/coerce-result-return-value.rs:11:12
+   |
+LL | fn foo2(x: Result<(), A>) -> Result<(), B> {
+   |                              ------------- expected `Result<(), B>` because of return type
+LL |     return x;
+   |            ^ expected struct `B`, found struct `A`
+   |
+   = note: expected enum `Result<_, B>`
+              found enum `Result<_, A>`
+help: use `?` to coerce and return an appropriate `Err`, and wrap the resulting value in `Ok` so the expression remains of type `Result`
+   |
+LL |     return Ok(x?);
+   |            +++ ++
+
+error[E0308]: mismatched types
+  --> $DIR/coerce-result-return-value.rs:15:9
+   |
+LL | fn foo3(x: Result<(), A>) -> Result<(), B> {
+   |                              ------------- expected `Result<(), B>` because of return type
+LL |     if true {
+LL |         x
+   |         ^ expected struct `B`, found struct `A`
+   |
+   = note: expected enum `Result<_, B>`
+              found enum `Result<_, A>`
+help: use `?` to coerce and return an appropriate `Err`, and wrap the resulting value in `Ok` so the expression remains of type `Result`
+   |
+LL |         Ok(x?)
+   |         +++ ++
+
+error[E0308]: mismatched types
+  --> $DIR/coerce-result-return-value.rs:17:9
+   |
+LL | fn foo3(x: Result<(), A>) -> Result<(), B> {
+   |                              ------------- expected `Result<(), B>` because of return type
+...
+LL |         x
+   |         ^ expected struct `B`, found struct `A`
+   |
+   = note: expected enum `Result<_, B>`
+              found enum `Result<_, A>`
+help: use `?` to coerce and return an appropriate `Err`, and wrap the resulting value in `Ok` so the expression remains of type `Result`
+   |
+LL |         Ok(x?)
+   |         +++ ++
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0308`.