Hint for missing lifetime bound on trait object is missing if type alias is used

[leo60228]

Given the following code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=a0979812dc1acbc732481dd2d3185637

trait Greeter {
    fn greet(&self);
}

type BoxedGreeter = Box<dyn Greeter>;

struct FixedGreeter<'a>(pub &'a str);

impl Greeter for FixedGreeter<'_> {
    fn greet(&self) {
        println!("{}", self.0)
    }
}

struct Greetings(pub Vec<String>);

impl Greetings {
    pub fn get(&self, i: usize) -> BoxedGreeter {
        Box::new(FixedGreeter(&self.0[i]))
    }
}

The current output is:

error: lifetime may not live long enough
  --> src/lib.rs:19:9
   |
18 |     pub fn get(&self, i: usize) -> BoxedGreeter {
   |                - let's call the lifetime of this reference `'1`
19 |         Box::new(FixedGreeter(&self.0[i]))
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ returning this value requires that `'1` must outlive `'static`

However, by removing the type alias: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=bfde1cf5c8a72349b117aa21bbd61c3d

trait Greeter {
    fn greet(&self);
}

struct FixedGreeter<'a>(pub &'a str);

impl Greeter for FixedGreeter<'_> {
    fn greet(&self) {
        println!("{}", self.0)
    }
}

struct Greetings(pub Vec<String>);

impl Greetings {
    pub fn get(&self, i: usize) -> Box<dyn Greeter> {
        Box::new(FixedGreeter(&self.0[i]))
    }
}

The output looks like this, which is much more helpful:

error: lifetime may not live long enough
  --> src/lib.rs:17:9
   |
16 |     pub fn get(&self, i: usize) -> Box<dyn Greeter> {
   |                - let's call the lifetime of this reference `'1`
17 |         Box::new(FixedGreeter(&self.0[i]))
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ returning this value requires that `'1` must outlive `'static`
   |
help: to declare that the trait object captures data from argument `self`, you can add an explicit `'_` lifetime bound
   |
16 |     pub fn get(&self, i: usize) -> Box<dyn Greeter + '_> {
   |                                                    ++++

Without the hint, this error isn't very helpful for beginners, as it doesn't do anything to explain why '1 must outlive 'static. It would be helpful if a similar hint could be provided when a type alias is used.

[leo60228]

A similar issue occurs for fields of a struct, but that might be harder to fix.

[yanchen4791]

@rustbot claim

[chylex]

I've run into the same confusing error, but with a function call:

fn create_confirmation_dialog<'a>() -> Option<MessageDialogLayer<'a>> {
	Some(MessageDialogLayer::new(Color::LightRed, "Error", "Test", Box::new(MessageDialogActionMap::ok())))
}

After searching for a while, I found it was because a Box<dyn MessageDialogActions<'a> parameter in my new function was implicitly converted into Box<dyn ... + 'static>. Very confusing for a beginner, especially with the additional compiler notes about variance that don't seem to even be relevant here?

pub fn new(..., actions: Box<dyn MessageDialogActions<'a>>) -> Self      // error
pub fn new(..., actions: Box<dyn MessageDialogActions<'a> + 'a>) -> Self // correct

This is the closest existing issue I found, is this a similar enough case or should I open a new issue?

[fmease]

Re. the snippet from the issue description, ideally the compiler would suggest:

- type BoxedGreeter = Box<dyn Greeter>;
+ type BoxedGreeter<'a> = Box<dyn Greeter + 'a>;

This makes the snippet compile (unless the lint elided_lifetime_in_paths was denied) because BoxedGreeter in the return type means BoxGreeter<'_> which "picks up" the anonymous lifetime of the self parameter.