Rollup merge of #98277 - compiler-errors:issue-93596, r=estebank

Fix trait object reborrow suggestion

Fixes #93596

Slightly generalizes the logic we use to suggest fix first implemented in #95609, specifically when we have a `Sized` obligation that comes from a struct's unsized tail.

compiler/rustc_middle/src/traits/mod.rs

@@ -253,7 +253,7 @@ pub enum ObligationCauseCode<'tcx> {
     ObjectTypeBound(Ty<'tcx>, ty::Region<'tcx>),
 
     /// Obligation incurred due to an object cast.
-    ObjectCastObligation(/* Object type */ Ty<'tcx>),
+    ObjectCastObligation(/* Concrete type */ Ty<'tcx>, /* Object type */ Ty<'tcx>),
 
     /// Obligation incurred due to a coercion.
     Coercion {

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

@@ -484,10 +484,9 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                             err.span_label(span, explanation);
                         }
 
-                        if let ObligationCauseCode::ObjectCastObligation(obj_ty) = obligation.cause.code().peel_derives() &&
-                           let Some(self_ty) = trait_predicate.self_ty().no_bound_vars() &&
+                        if let ObligationCauseCode::ObjectCastObligation(concrete_ty, obj_ty) = obligation.cause.code().peel_derives() &&
                            Some(trait_ref.def_id()) == self.tcx.lang_items().sized_trait() {
-                            self.suggest_borrowing_for_object_cast(&mut err, &obligation, self_ty, *obj_ty);
+                            self.suggest_borrowing_for_object_cast(&mut err, &root_obligation, *concrete_ty, *obj_ty);
                         }
 
                         if trait_predicate.is_const_if_const() && obligation.param_env.is_const() {
@@ -1560,7 +1559,7 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                     obligation.cause.code().peel_derives(),
                     ObligationCauseCode::ItemObligation(_)
                         | ObligationCauseCode::BindingObligation(_, _)
-                        | ObligationCauseCode::ObjectCastObligation(_)
+                        | ObligationCauseCode::ObjectCastObligation(..)
                         | ObligationCauseCode::OpaqueType
                 );
                 if let Err(error) = self.at(&obligation.cause, obligation.param_env).eq_exp(

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -2217,9 +2217,10 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                     err.span_note(tcx.def_span(item_def_id), &descr);
                 }
             }
-            ObligationCauseCode::ObjectCastObligation(object_ty) => {
+            ObligationCauseCode::ObjectCastObligation(concrete_ty, object_ty) => {
                 err.note(&format!(
-                    "required for the cast to the object type `{}`",
+                    "required for the cast from `{}` to the object type `{}`",
+                    self.ty_to_string(concrete_ty),
                     self.ty_to_string(object_ty)
                 ));
             }

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -813,7 +813,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 let cause = ObligationCause::new(
                     obligation.cause.span,
                     obligation.cause.body_id,
-                    ObjectCastObligation(target),
+                    ObjectCastObligation(source, target),
                 );
                 let outlives = ty::OutlivesPredicate(r_a, r_b);
                 nested.push(Obligation::with_depth(
@@ -910,7 +910,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 let cause = ObligationCause::new(
                     obligation.cause.span,
                     obligation.cause.body_id,
-                    ObjectCastObligation(target),
+                    ObjectCastObligation(source, target),
                 );
                 let outlives = ty::OutlivesPredicate(r_a, r_b);
                 nested.push(Obligation::with_depth(
@@ -931,7 +931,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 let cause = ObligationCause::new(
                     obligation.cause.span,
                     obligation.cause.body_id,
-                    ObjectCastObligation(target),
+                    ObjectCastObligation(source, target),
                 );
 
                 let predicate_to_obligation = |predicate| {

src/test/ui/associated-type-bounds/assoc-type-eq-with-dyn-atb-fail.stderr

@@ -4,7 +4,7 @@ error[E0277]: the trait bound `String: Copy` is not satisfied
 LL |         Box::new(AssocNoCopy)
    |         ^^^^^^^^^^^^^^^^^^^^^ the trait `Copy` is not implemented for `String`
    |
-   = note: required for the cast to the object type `dyn Bar<Assoc = <AssocNoCopy as Thing>::Out::{opaque#0}>`
+   = note: required for the cast from `AssocNoCopy` to the object type `dyn Bar<Assoc = <AssocNoCopy as Thing>::Out::{opaque#0}>`
 
 error: aborting due to previous error
 

src/test/ui/associated-types/associated-types-eq-3.stderr

@@ -41,7 +41,7 @@ note: expected this to be `Bar`
    |
 LL |     type A = usize;
    |              ^^^^^
-   = note: required for the cast to the object type `dyn Foo<A = Bar>`
+   = note: required for the cast from `isize` to the object type `dyn Foo<A = Bar>`
 
 error: aborting due to 3 previous errors
 

src/test/ui/associated-types/associated-types-overridden-binding-2.stderr

@@ -4,7 +4,7 @@ error[E0271]: type mismatch resolving `<std::vec::IntoIter<u32> as Iterator>::It
 LL |     let _: &dyn I32Iterator<Item = u32> = &vec![42].into_iter();
    |                                           ^^^^^^^^^^^^^^^^^^^^^ expected `i32`, found `u32`
    |
-   = note: required for the cast to the object type `dyn Iterator<Item = u32, Item = i32>`
+   = note: required for the cast from `std::vec::IntoIter<u32>` to the object type `dyn Iterator<Item = u32, Item = i32>`
 
 error: aborting due to previous error
 

src/test/ui/associated-types/issue-65774-1.stderr

@@ -23,7 +23,7 @@ note: required because of the requirements on the impl of `MyDisplay` for `&mut
    |
 LL | impl<'a, T: MyDisplay> MyDisplay for &'a mut T { }
    |                        ^^^^^^^^^     ^^^^^^^^^
-   = note: required for the cast to the object type `dyn MyDisplay`
+   = note: required for the cast from `&mut T` to the object type `dyn MyDisplay`
 
 error: aborting due to 2 previous errors
 

src/test/ui/associated-types/issue-65774-2.stderr

@@ -18,7 +18,7 @@ LL |         writer.my_write(valref)
    |                         ^^^^^^ the trait `MyDisplay` is not implemented for `T`
    |
    = help: the trait `MyDisplay` is implemented for `&'a mut T`
-   = note: required for the cast to the object type `dyn MyDisplay`
+   = note: required for the cast from `T` to the object type `dyn MyDisplay`
 
 error: aborting due to 2 previous errors
 

src/test/ui/async-await/async-block-control-flow-static-semantics.stderr

@@ -37,7 +37,7 @@ error[E0271]: type mismatch resolving `<impl Future<Output = u8> as Future>::Out
 LL |     let _: &dyn Future<Output = ()> = &block;
    |                                       ^^^^^^ expected `()`, found `u8`
    |
-   = note: required for the cast to the object type `dyn Future<Output = ()>`
+   = note: required for the cast from `impl Future<Output = u8>` to the object type `dyn Future<Output = ()>`
 
 error[E0308]: mismatched types
   --> $DIR/async-block-control-flow-static-semantics.rs:12:43
@@ -53,7 +53,7 @@ error[E0271]: type mismatch resolving `<impl Future<Output = u8> as Future>::Out
 LL |     let _: &dyn Future<Output = ()> = &block;
    |                                       ^^^^^^ expected `()`, found `u8`
    |
-   = note: required for the cast to the object type `dyn Future<Output = ()>`
+   = note: required for the cast from `impl Future<Output = u8>` to the object type `dyn Future<Output = ()>`
 
 error[E0308]: mismatched types
   --> $DIR/async-block-control-flow-static-semantics.rs:47:44

src/test/ui/async-await/issue-86507.stderr

@@ -13,7 +13,7 @@ note: captured value is not `Send` because `&` references cannot be sent unless
    |
 LL |                     let x = x;
    |                             ^ has type `&T` which is not `Send`, because `T` is not `Sync`
-   = note: required for the cast to the object type `dyn Future<Output = ()> + Send`
+   = note: required for the cast from `impl Future<Output = ()>` to the object type `dyn Future<Output = ()> + Send`
 help: consider further restricting this bound
    |
 LL |     fn bar<'me, 'async_trait, T: Send + std::marker::Sync>(x: &'me T)

src/test/ui/coercion/coerce-issue-49593-box-never-windows.nofallback.stderr

@@ -4,15 +4,15 @@ error[E0277]: the trait bound `(): std::error::Error` is not satisfied
 LL |     /* *mut $0 is coerced to Box<dyn Error> here */ Box::<_ /* ! */>::new(x)
    |                                                     ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::error::Error` is not implemented for `()`
    |
-   = note: required for the cast to the object type `dyn std::error::Error`
+   = note: required for the cast from `()` to the object type `dyn std::error::Error`
 
 error[E0277]: the trait bound `(): std::error::Error` is not satisfied
   --> $DIR/coerce-issue-49593-box-never-windows.rs:23:49
    |
 LL |     /* *mut $0 is coerced to *mut Error here */ raw_ptr_box::<_ /* ! */>(x)
    |                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::error::Error` is not implemented for `()`
    |
-   = note: required for the cast to the object type `(dyn std::error::Error + 'static)`
+   = note: required for the cast from `()` to the object type `(dyn std::error::Error + 'static)`
 
 error: aborting due to 2 previous errors
 

src/test/ui/coercion/coerce-issue-49593-box-never.nofallback.stderr

@@ -4,15 +4,15 @@ error[E0277]: the trait bound `(): std::error::Error` is not satisfied
 LL |     /* *mut $0 is coerced to Box<dyn Error> here */ Box::<_ /* ! */>::new(x)
    |                                                     ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::error::Error` is not implemented for `()`
    |
-   = note: required for the cast to the object type `dyn std::error::Error`
+   = note: required for the cast from `()` to the object type `dyn std::error::Error`
 
 error[E0277]: the trait bound `(): std::error::Error` is not satisfied
   --> $DIR/coerce-issue-49593-box-never.rs:23:49
    |
 LL |     /* *mut $0 is coerced to *mut Error here */ raw_ptr_box::<_ /* ! */>(x)
    |                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::error::Error` is not implemented for `()`
    |
-   = note: required for the cast to the object type `(dyn std::error::Error + 'static)`
+   = note: required for the cast from `()` to the object type `(dyn std::error::Error + 'static)`
 
 error: aborting due to 2 previous errors
 

src/test/ui/const-generics/defaults/trait_objects_fail.stderr

@@ -7,7 +7,7 @@ LL |     foo(&10_u32);
    |     required by a bound introduced by this call
    |
    = help: the trait `Trait<2_u8>` is implemented for `u32`
-   = note: required for the cast to the object type `dyn Trait`
+   = note: required for the cast from `u32` to the object type `dyn Trait`
 
 error[E0277]: the trait bound `bool: Traitor<{_: u8}>` is not satisfied
   --> $DIR/trait_objects_fail.rs:28:9
@@ -18,7 +18,7 @@ LL |     bar(&true);
    |     required by a bound introduced by this call
    |
    = help: the trait `Traitor<2_u8, 3_u8>` is implemented for `bool`
-   = note: required for the cast to the object type `dyn Traitor<{_: u8}>`
+   = note: required for the cast from `bool` to the object type `dyn Traitor<{_: u8}>`
 
 error: aborting due to 2 previous errors
 

src/test/ui/custom_test_frameworks/mismatch.stderr

@@ -6,7 +6,7 @@ LL | #[test]
 LL | fn wrong_kind(){}
    | ^^^^^^^^^^^^^^^^^ the trait `Testable` is not implemented for `TestDescAndFn`
    |
-   = note: required for the cast to the object type `dyn Testable`
+   = note: required for the cast from `TestDescAndFn` to the object type `dyn Testable`
    = note: this error originates in the attribute macro `test` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to previous error

src/test/ui/dst/dst-bad-coerce1.stderr

@@ -15,7 +15,7 @@ error[E0277]: the trait bound `Foo: Bar` is not satisfied
 LL |     let f3: &Fat<dyn Bar> = f2;
    |                             ^^ the trait `Bar` is not implemented for `Foo`
    |
-   = note: required for the cast to the object type `dyn Bar`
+   = note: required for the cast from `Foo` to the object type `dyn Bar`
 
 error[E0308]: mismatched types
   --> $DIR/dst-bad-coerce1.rs:28:27
@@ -34,7 +34,7 @@ error[E0277]: the trait bound `Foo: Bar` is not satisfied
 LL |     let f3: &(dyn Bar,) = f2;
    |                           ^^ the trait `Bar` is not implemented for `Foo`
    |
-   = note: required for the cast to the object type `dyn Bar`
+   = note: required for the cast from `Foo` to the object type `dyn Bar`
 
 error: aborting due to 4 previous errors
 

src/test/ui/dst/dst-object-from-unsized-type.stderr

@@ -6,7 +6,7 @@ LL | fn test1<T: ?Sized + Foo>(t: &T) {
 LL |     let u: &dyn Foo = t;
    |                       ^ doesn't have a size known at compile-time
    |
-   = note: required for the cast to the object type `dyn Foo`
+   = note: required for the cast from `T` to the object type `dyn Foo`
 help: consider removing the `?Sized` bound to make the type parameter `Sized`
    |
 LL - fn test1<T: ?Sized + Foo>(t: &T) {
@@ -21,7 +21,7 @@ LL | fn test2<T: ?Sized + Foo>(t: &T) {
 LL |     let v: &dyn Foo = t as &dyn Foo;
    |                       ^ doesn't have a size known at compile-time
    |
-   = note: required for the cast to the object type `dyn Foo`
+   = note: required for the cast from `T` to the object type `dyn Foo`
 help: consider removing the `?Sized` bound to make the type parameter `Sized`
    |
 LL - fn test2<T: ?Sized + Foo>(t: &T) {
@@ -35,7 +35,7 @@ LL |     let _: &[&dyn Foo] = &["hi"];
    |                            ^^^^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `str`
-   = note: required for the cast to the object type `dyn Foo`
+   = note: required for the cast from `str` to the object type `dyn Foo`
 
 error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
   --> $DIR/dst-object-from-unsized-type.rs:23:23
@@ -44,7 +44,7 @@ LL |     let _: &dyn Foo = x as &dyn Foo;
    |                       ^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `[u8]`
-   = note: required for the cast to the object type `dyn Foo`
+   = note: required for the cast from `[u8]` to the object type `dyn Foo`
 
 error: aborting due to 4 previous errors
 

src/test/ui/generic-associated-types/issue-79422.extended.stderr

@@ -27,7 +27,7 @@ LL |     type VRefCont<'a> = &'a V where Self: 'a;
    |                         ^^^^^
    = note: expected trait object `(dyn RefCont<'_, u8> + 'static)`
                  found reference `&u8`
-   = note: required for the cast to the object type `dyn MapLike<u8, u8, VRefCont = (dyn RefCont<'_, u8> + 'static)>`
+   = note: required for the cast from `BTreeMap<u8, u8>` to the object type `dyn MapLike<u8, u8, VRefCont = (dyn RefCont<'_, u8> + 'static)>`
 
 error: aborting due to 2 previous errors
 

src/test/ui/issues/issue-14366.stderr

@@ -5,7 +5,7 @@ LL |     let _x = "test" as &dyn (::std::any::Any);
    |              ^^^^^^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `str`
-   = note: required for the cast to the object type `dyn Any`
+   = note: required for the cast from `str` to the object type `dyn Any`
 help: consider borrowing the value, since `&str` can be coerced into `dyn Any`
    |
 LL |     let _x = &"test" as &dyn (::std::any::Any);

src/test/ui/issues/issue-22034.stderr

@@ -6,7 +6,7 @@ LL |         &mut *(ptr as *mut dyn Fn())
    |
    = help: the trait `Fn<()>` is not implemented for `()`
    = note: wrap the `()` in a closure with no arguments: `|| { /* code */ }`
-   = note: required for the cast to the object type `dyn Fn()`
+   = note: required for the cast from `()` to the object type `dyn Fn()`
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-22872.stderr

@@ -10,7 +10,7 @@ note: required because of the requirements on the impl of `for<'b> Wrap<'b>` for
    |
 LL | impl<'b, P> Wrap<'b> for Wrapper<P>
    |             ^^^^^^^^     ^^^^^^^^^^
-   = note: required for the cast to the object type `dyn for<'b> Wrap<'b>`
+   = note: required for the cast from `Wrapper<P>` to the object type `dyn for<'b> Wrap<'b>`
 help: consider further restricting the associated type
    |
 LL | fn push_process<P>(process: P) where P: Process<'static>, <P as Process<'_>>::Item: Iterator {

src/test/ui/issues/issue-7013.stderr

@@ -11,7 +11,7 @@ note: required because it appears within the type `B`
    |
 LL | struct B {
    |        ^
-   = note: required for the cast to the object type `dyn Foo + Send`
+   = note: required for the cast from `B` to the object type `dyn Foo + Send`
 
 error: aborting due to previous error
 

src/test/ui/kindck/kindck-impl-type-params.stderr

@@ -9,7 +9,7 @@ note: required because of the requirements on the impl of `Gettable<T>` for `S<T
    |
 LL | impl<T: Send + Copy + 'static> Gettable<T> for S<T> {}
    |                                ^^^^^^^^^^^     ^^^^
-   = note: required for the cast to the object type `dyn Gettable<T>`
+   = note: required for the cast from `S<T>` to the object type `dyn Gettable<T>`
 help: consider restricting type parameter `T`
    |
 LL | fn f<T: std::marker::Send>(val: T) {
@@ -26,7 +26,7 @@ note: required because of the requirements on the impl of `Gettable<T>` for `S<T
    |
 LL | impl<T: Send + Copy + 'static> Gettable<T> for S<T> {}
    |                                ^^^^^^^^^^^     ^^^^
-   = note: required for the cast to the object type `dyn Gettable<T>`
+   = note: required for the cast from `S<T>` to the object type `dyn Gettable<T>`
 help: consider restricting type parameter `T`
    |
 LL | fn f<T: std::marker::Copy>(val: T) {
@@ -43,7 +43,7 @@ note: required because of the requirements on the impl of `Gettable<T>` for `S<T
    |
 LL | impl<T: Send + Copy + 'static> Gettable<T> for S<T> {}
    |                                ^^^^^^^^^^^     ^^^^
-   = note: required for the cast to the object type `dyn Gettable<T>`
+   = note: required for the cast from `S<T>` to the object type `dyn Gettable<T>`
 help: consider restricting type parameter `T`
    |
 LL | fn g<T: std::marker::Send>(val: T) {
@@ -60,7 +60,7 @@ note: required because of the requirements on the impl of `Gettable<T>` for `S<T
    |
 LL | impl<T: Send + Copy + 'static> Gettable<T> for S<T> {}
    |                                ^^^^^^^^^^^     ^^^^
-   = note: required for the cast to the object type `dyn Gettable<T>`
+   = note: required for the cast from `S<T>` to the object type `dyn Gettable<T>`
 help: consider restricting type parameter `T`
    |
 LL | fn g<T: std::marker::Copy>(val: T) {
@@ -78,7 +78,7 @@ note: required because of the requirements on the impl of `Gettable<String>` for
    |
 LL | impl<T: Send + Copy + 'static> Gettable<T> for S<T> {}
    |                                ^^^^^^^^^^^     ^^^^
-   = note: required for the cast to the object type `dyn Gettable<String>`
+   = note: required for the cast from `S<String>` to the object type `dyn Gettable<String>`
 
 error[E0277]: the trait bound `Foo: Copy` is not satisfied
   --> $DIR/kindck-impl-type-params.rs:43:37
@@ -92,7 +92,7 @@ note: required because of the requirements on the impl of `Gettable<Foo>` for `S
    |
 LL | impl<T: Send + Copy + 'static> Gettable<T> for S<T> {}
    |                                ^^^^^^^^^^^     ^^^^
-   = note: required for the cast to the object type `dyn Gettable<Foo>`
+   = note: required for the cast from `S<Foo>` to the object type `dyn Gettable<Foo>`
 help: consider annotating `Foo` with `#[derive(Copy)]`
    |
 LL |     #[derive(Copy)]

src/test/ui/mismatched_types/cast-rfc0401.stderr

@@ -220,7 +220,7 @@ LL |     let _ = fat_v as *const dyn Foo;
    |             ^^^^^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `[u8]`
-   = note: required for the cast to the object type `dyn Foo`
+   = note: required for the cast from `[u8]` to the object type `dyn Foo`
 help: consider borrowing the value, since `&[u8]` can be coerced into `dyn Foo`
    |
 LL |     let _ = &fat_v as *const dyn Foo;
@@ -233,7 +233,7 @@ LL |     let _ = a as *const dyn Foo;
    |             ^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `str`
-   = note: required for the cast to the object type `dyn Foo`
+   = note: required for the cast from `str` to the object type `dyn Foo`
 help: consider borrowing the value, since `&str` can be coerced into `dyn Foo`
    |
 LL |     let _ = &a as *const dyn Foo;

src/test/ui/never_type/fallback-closure-wrap.fallback.stderr

@@ -10,7 +10,7 @@ LL | |     }) as Box<dyn FnMut()>);
    |
    = note: expected unit type `()`
                    found type `!`
-   = note: required for the cast to the object type `dyn FnMut()`
+   = note: required for the cast from `[closure@$DIR/fallback-closure-wrap.rs:18:40: 21:6]` to the object type `dyn FnMut()`
 
 error: aborting due to previous error