Broken help message for missing lifetime when using <_ as Trait> syntax in type alias

[JanBeh]

I tried this code:

use std::marker::PhantomData;
use std::rc::Rc;

trait Machine<'a> {
    type Datum;
}

#[derive(Default)]
struct LuaMachine<'a> {
    _phantom: PhantomData<fn(&'a ()) -> &'a ()>,
}

struct LuaDatum<'a> {
    _machine: Rc<LuaMachine<'a>>,
}

impl<'a> Machine<'a> for LuaMachine<'a> {
    type Datum = LuaDatum<'a>;
}

type M<'a> = LuaMachine<'a>;
type D<'a> = <M<'a> as Machine>::Datum;

fn main() {}

(Playground)

I expected to see this happen (which works correctly with rustc 1.57.0 stable):

help: consider using the `'a` lifetime
   |
22 | type D<'a> = <M<'a> as Machine<'a>>::Datum;
   |                        ~~~~~~~~~~~

But with rustc 1.59.0-nightly (f8abed9ed 2021-12-26), I get:

help: consider using the `'a` lifetime
   |
22 | type D<'a> = <M<'a> as Machine>::Datum<'a>;
   |              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Meta

rustc --version --verbose:

rustc 1.59.0-nightly (f8abed9ed 2021-12-26)
binary: rustc
commit-hash: f8abed9ed48bace6be0087bcd44ed534e239b8d8
commit-date: 2021-12-26
host: x86_64-unknown-freebsd
release: 1.59.0-nightly
LLVM version: 13.0.0

[JanBeh]

Also consider the following code, which creates an error message that could be more precise. I wouldn't call the following example a bug, but maybe these two issues are related:

#![feature(generic_associated_types)]

use std::marker::PhantomData;

trait Machine<'a> {
    type Datum<'b>
    where
        Self: 'b;
}

#[derive(Default)]
struct LuaMachine<'a> {
    _phantom: PhantomData<fn(&'a ()) -> &'a ()>,
}

struct LuaDatum<'a, 'b> {
    _machine: &'b LuaMachine<'a>,
}

impl<'a> Machine<'a> for LuaMachine<'a> {
    type Datum<'b>
    where
        Self: 'b,
    = LuaDatum<'a, 'b>;
}

type M<'a> = LuaMachine<'a>;
// This won't work:
type D<'a, 'b> = <M<'a> as Machine>::Datum<'b>;

fn main() {
    let _m: M = Default::default();
    // But this works:
    let _d: <M as Machine>::Datum<'_> = LuaDatum { _machine: &_m };
}

(Playground)

Error output:

   Compiling playground v0.0.1 (/playground)
error[E0106]: missing lifetime specifier
  --> src/main.rs:29:18
   |
29 | type D<'a, 'b> = <M<'a> as Machine>::Datum<'b>;
   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected named lifetime parameter
   |
note: these named lifetimes are available to use
  --> src/main.rs:29:8
   |
29 | type D<'a, 'b> = <M<'a> as Machine>::Datum<'b>;
   |        ^^  ^^

For more information about this error, try `rustc --explain E0106`.
error: could not compile `playground` due to previous error

[compiler-errors]

I think this is due to #90446 cc: @cjgillot, let me see if I can fix this real quick.

[compiler-errors]

@rustbot claim

[BGR360]

@rustbot label +regression-from-stable-to-nightly +A-diagnostics

[rustbot]

Error: Label P-low can only be set by Rust team members

Please let @rust-lang/release know if you're having trouble with this bot.

[apiraino]

Assigning priority as discussed in the Zulip thread of the Prioritization Working Group.

@rustbot label -I-prioritize +P-medium