Account for missing lifetime in opaque and trait object return types #72543

When encountering an opaque closure return type that needs to bound a lifetime to the function's arguments, including borrows and type params, provide appropriate suggestions that lead to working code. Get the user from ```rust fn foo<G, T>(g: G, dest: &mut T) -> impl FnOnce() where G: Get<T> { move || { *dest = g.get(); } } ``` to ```rust fn foo<'a, G: 'a, T>(g: G, dest: &'a mut T) -> impl FnOnce() +'a where G: Get<T> { move || { *dest = g.get(); } } ```

…ing opaque return type Go from ``` error[E0495]: cannot infer an appropriate lifetime due to conflicting requirements --> file8.rs:22:5 | 22 | / move || { 23 | | *dest = g.get(); 24 | | } | |_____^ | note: first, the lifetime cannot outlive the anonymous lifetime rust-lang#1 defined on the function body at 18:1... --> file8.rs:18:1 | 18 | / fn bat<'a, G: 'a, T>(g: G, dest: &mut T) -> impl FnOnce() + '_ + 'a 19 | | where 20 | | G: Get<T> 21 | | { ... | 24 | | } 25 | | } | |_^ note: ...so that the types are compatible --> file8.rs:22:5 | 22 | / move || { //~ ERROR cannot infer an appropriate lifetime 23 | | *dest = g.get(); 24 | | } | |_____^ = note: expected `&mut T` found `&mut T` note: but, the lifetime must be valid for the lifetime `'a` as defined on the function body at 18:8... --> file8.rs:18:8 | 18 | fn bat<'a, G: 'a, T>(g: G, dest: &mut T) -> impl FnOnce() + '_ + 'a | ^^ note: ...so that return value is valid for the call --> file8.rs:18:45 | 18 | fn bat<'a, G: 'a, T>(g: G, dest: &mut T) -> impl FnOnce() + '_ + 'a | ^^^^^^^^^^^^^^^^^^^^^^^ ``` to ``` error[E0621]: explicit lifetime required in the type of `dest` --> file8.rs:18:45 | 18 | fn bat<'a, G: 'a, T>(g: G, dest: &mut T) -> impl FnOnce() + '_ + 'a | ------ ^^^^^^^^^^^^^^^^^^^^^^^ lifetime `'a` required | | | help: add explicit lifetime `'a` to the type of `dest`: `&'a mut T` ```

Provide a suggestion for `dyn Trait + '_` when possible.

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Account for missing lifetime in opaque and trait object return types #72543

Account for missing lifetime in opaque and trait object return types #72543

Commits on May 30, 2020