GAT lifetime error is unhelpful

[vorot93]

Given the following code:

#![feature(generic_associated_types)]

use std::fmt::Debug;
use std::marker::PhantomData;

#[derive(Debug)]
pub struct TransactionImpl<'db> {
    _marker: PhantomData<&'db ()>,
}

#[derive(Debug)]
pub struct CursorImpl<'txn> {
    _marker: PhantomData<&'txn ()>,
}

pub trait Cursor<'txn> {}

pub trait Transaction<'db>: Send + Sync + Debug + Sized {
    type Cursor<'tx>: Cursor<'tx>
    where
        'db: 'tx,
        Self: 'tx;

    fn cursor<'tx>(&'tx self) -> Result<Self::Cursor<'tx>, ()>
    where
        'db: 'tx;
}

impl<'tx> Cursor<'tx> for CursorImpl<'tx> {}

impl<'db> Transaction<'db> for TransactionImpl<'db> {
    type Cursor<'tx> = CursorImpl<'tx>;

    fn cursor<'tx>(&'tx self) -> Result<Self::Cursor<'tx>, ()>
    where
        'db: 'tx,
    {
        loop {}
    }
}

The current output is:

error[E0478]: lifetime bound not satisfied
  --> src/lib.rs:32:24
   |
32 |     type Cursor<'tx> = CursorImpl<'tx>;
   |                        ^^^^^^^^^^^^^^^
   |
note: lifetime parameter instantiated with the lifetime `'db` as defined here
  --> src/lib.rs:31:6
   |
31 | impl<'db> Transaction<'db> for TransactionImpl<'db> {
   |      ^^^
note: but lifetime parameter must outlive the lifetime `'tx` as defined here
  --> src/lib.rs:32:17
   |
32 |     type Cursor<'tx> = CursorImpl<'tx>;

Error is fixed by adding a where clause to associated type in impl, which is not immediately clear:

impl<'db> Transaction<'db> for TransactionImpl<'db> {
    type Cursor<'tx>
    where
        'db: 'tx,
    = CursorImpl<'tx>;
}

[jackh726]

GATs issue triage: not blocking. Poor diagnostic, but not a miscompile.

[BGR360]

@rustbot label +D-papercut +D-terse

[jackh726]

Current output is

help: try copying these clauses from the trait: `where 'db: 'tx, Self: 'tx`

I think that's pretty helpful. Going to mark as needs-test