Rollup merge of #122120 - fmease:sugg-assoc-ty-bound-on-eq-bound, r=compiler-errors

Suggest associated type bounds on problematic associated equality bounds

Fixes #105056. TL;DR: Suggest `Trait<Ty: Bound>` on `Trait<Ty = Bound>` in Rust >=2021.

~~Blocked on #122055 (stabilization of `associated_type_bounds`), I'd say.~~ (merged)

tests/ui/associated-type-bounds/suggest-assoc-ty-bound-on-eq-bound.rs

@@ -0,0 +1,30 @@
+// Regression test for issue #105056.
+//@ edition: 2021
+
+fn f(_: impl Trait<T = Copy>) {}
+//~^ ERROR trait objects must include the `dyn` keyword
+//~| HELP add `dyn` keyword before this trait
+//~| HELP you might have meant to write a bound here
+//~| ERROR the trait `Copy` cannot be made into an object
+
+fn g(_: impl Trait<T = std::fmt::Debug + Eq>) {}
+//~^ ERROR trait objects must include the `dyn` keyword
+//~| HELP add `dyn` keyword before this trait
+//~| HELP you might have meant to write a bound here
+//~| ERROR only auto traits can be used as additional traits in a trait object
+//~| HELP consider creating a new trait
+//~| ERROR the trait `Eq` cannot be made into an object
+
+fn h(_: impl Trait<T<> = 'static + for<'a> Fn(&'a ())>) {}
+//~^ ERROR trait objects must include the `dyn` keyword
+//~| HELP add `dyn` keyword before this trait
+//~| HELP you might have meant to write a bound here
+
+// Don't suggest assoc ty bound in trait object types, that's not valid:
+type Obj = dyn Trait<T = Clone>;
+//~^ ERROR trait objects must include the `dyn` keyword
+//~| HELP add `dyn` keyword before this trait
+
+trait Trait { type T; }
+
+fn main() {}

tests/ui/associated-type-bounds/suggest-assoc-ty-bound-on-eq-bound.stderr

@@ -0,0 +1,91 @@
+error[E0038]: the trait `Copy` cannot be made into an object
+  --> $DIR/suggest-assoc-ty-bound-on-eq-bound.rs:4:20
+   |
+LL | fn f(_: impl Trait<T = Copy>) {}
+   |                    ^^^^^^^^ `Copy` cannot be made into an object
+   |
+   = note: the trait cannot be made into an object because it requires `Self: Sized`
+   = note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
+
+error[E0225]: only auto traits can be used as additional traits in a trait object
+  --> $DIR/suggest-assoc-ty-bound-on-eq-bound.rs:10:42
+   |
+LL | fn g(_: impl Trait<T = std::fmt::Debug + Eq>) {}
+   |                        ---------------   ^^ additional non-auto trait
+   |                        |
+   |                        first non-auto trait
+   |
+   = help: consider creating a new trait with all of these as supertraits and using that trait here instead: `trait NewTrait: Debug + Eq {}`
+   = note: auto-traits like `Send` and `Sync` are traits that have special properties; for more information on them, visit <https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits>
+
+error[E0038]: the trait `Eq` cannot be made into an object
+  --> $DIR/suggest-assoc-ty-bound-on-eq-bound.rs:10:24
+   |
+LL | fn g(_: impl Trait<T = std::fmt::Debug + Eq>) {}
+   |                        ^^^^^^^^^^^^^^^^^^^^ `Eq` cannot be made into an object
+   |
+note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
+  --> $SRC_DIR/core/src/cmp.rs:LL:COL
+   |
+   = note: the trait cannot be made into an object because it uses `Self` as a type parameter
+
+error[E0782]: trait objects must include the `dyn` keyword
+  --> $DIR/suggest-assoc-ty-bound-on-eq-bound.rs:4:24
+   |
+LL | fn f(_: impl Trait<T = Copy>) {}
+   |                        ^^^^
+   |
+help: add `dyn` keyword before this trait
+   |
+LL | fn f(_: impl Trait<T = dyn Copy>) {}
+   |                        +++
+help: you might have meant to write a bound here
+   |
+LL | fn f(_: impl Trait<T: Copy>) {}
+   |                     ~
+
+error[E0782]: trait objects must include the `dyn` keyword
+  --> $DIR/suggest-assoc-ty-bound-on-eq-bound.rs:10:24
+   |
+LL | fn g(_: impl Trait<T = std::fmt::Debug + Eq>) {}
+   |                        ^^^^^^^^^^^^^^^^^^^^
+   |
+help: add `dyn` keyword before this trait
+   |
+LL | fn g(_: impl Trait<T = dyn std::fmt::Debug + Eq>) {}
+   |                        +++
+help: you might have meant to write a bound here
+   |
+LL | fn g(_: impl Trait<T: std::fmt::Debug + Eq>) {}
+   |                     ~
+
+error[E0782]: trait objects must include the `dyn` keyword
+  --> $DIR/suggest-assoc-ty-bound-on-eq-bound.rs:18:26
+   |
+LL | fn h(_: impl Trait<T<> = 'static + for<'a> Fn(&'a ())>) {}
+   |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: add `dyn` keyword before this trait
+   |
+LL | fn h(_: impl Trait<T<> = dyn 'static + for<'a> Fn(&'a ())>) {}
+   |                          +++
+help: you might have meant to write a bound here
+   |
+LL | fn h(_: impl Trait<T<>: 'static + for<'a> Fn(&'a ())>) {}
+   |                       ~
+
+error[E0782]: trait objects must include the `dyn` keyword
+  --> $DIR/suggest-assoc-ty-bound-on-eq-bound.rs:24:26
+   |
+LL | type Obj = dyn Trait<T = Clone>;
+   |                          ^^^^^
+   |
+help: add `dyn` keyword before this trait
+   |
+LL | type Obj = dyn Trait<T = dyn Clone>;
+   |                          +++
+
+error: aborting due to 7 previous errors
+
+Some errors have detailed explanations: E0038, E0225, E0782.
+For more information about an error, try `rustc --explain E0038`.

tests/ui/parser/recover/recover-assoc-lifetime-constraint.rs

@@ -1,6 +1,6 @@
 #[cfg(FALSE)]
 fn syntax() {
-    bar::<Item = 'a>(); //~ ERROR associated lifetimes are not supported
+    bar::<Item = 'a>(); //~ ERROR lifetimes are not permitted in this context
 }
 
 fn main() {}

tests/ui/parser/recover/recover-assoc-lifetime-constraint.stderr

@@ -1,12 +1,17 @@
-error: associated lifetimes are not supported
-  --> $DIR/recover-assoc-lifetime-constraint.rs:3:11
+error: lifetimes are not permitted in this context
+  --> $DIR/recover-assoc-lifetime-constraint.rs:3:18
    |
 LL |     bar::<Item = 'a>();
-   |           ^^^^^^^--
-   |                  |
-   |                  the lifetime is given here
+   |           -------^^
+   |           |      |
+   |           |      lifetime is not allowed here
+   |           this introduces an associated item binding
    |
-   = help: if you meant to specify a trait object, write `dyn Trait + 'lifetime`
+   = help: if you meant to specify a trait object, write `dyn /* Trait */ + 'a`
+help: you might have meant to write a bound here
+   |
+LL |     bar::<Item: 'a>();
+   |               ~
 
 error: aborting due to 1 previous error