When passing too many lifetimes to a struct, there are tons of unrelated errors about needing explicit lifetime annotations

[RalfJung]

To reproduce, check out commit cdc509f of rustc and then edit this line:

rust/src/tools/miri/src/borrow_tracker/stacked_borrows/mod.rs

Line 278 in d50c2b0

dcx: &mut DiagnosticCx<'_, '_, '_, 'tcx>,

Specifically, add another '_, before the 'tcx. Then run ./x.py check miri. This results in some surprising errors:

error[E0107]: struct takes 4 lifetime arguments but 5 lifetime arguments were supplied
   --> src/tools/miri/src/borrow_tracker/stacked_borrows/mod.rs:278:19
    |
278 |         dcx: &mut DiagnosticCx<'_, '_, '_, '_,'tcx>,
    |                   ^^^^^^^^^^^^                ---- help: remove this lifetime argument
    |                   |
    |                   expected 4 lifetime arguments
    |
note: struct defined here, with 4 lifetime parameters: `'history`, `'ecx`, `'mir`, `'tcx`
   --> src/tools/miri/src/borrow_tracker/stacked_borrows/diagnostics.rs:123:12
    |
123 | pub struct DiagnosticCx<'history, 'ecx, 'mir, 'tcx> {
    |            ^^^^^^^^^^^^ --------  ----  ----  ----

error[E0621]: explicit lifetime required in the type of `dcx`
   --> src/tools/miri/src/borrow_tracker/stacked_borrows/mod.rs:285:13
    |
278 |         dcx: &mut DiagnosticCx<'_, '_, '_, '_,'tcx>,
    |              -------------------------------------- help: add explicit lifetime `'tcx` to the type of `dcx`: `&mut DiagnosticCx<'_, '_, '_, 'tcx>`
...
285 |             self.find_granting(access, tag, exposed_tags).map_err(|()| dcx.access_error(self))?;
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime `'tcx` required

error[E0621]: explicit lifetime required in the type of `dcx`
   --> src/tools/miri/src/borrow_tracker/stacked_borrows/mod.rs:305:17
    |
278 |         dcx: &mut DiagnosticCx<'_, '_, '_, '_,'tcx>,
    |              -------------------------------------- help: add explicit lifetime `'tcx` to the type of `dcx`: `&mut DiagnosticCx<'_, 'tcx, '_, '_>`
...
305 |                 dcx.log_invalidation(item.tag());
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime `'tcx` required

The first error is what I expected, but the other two are not useful: they are referring to an argument type that's not even a valid type, so they should not be shown. When doing refactoring, it can be quite hard to spot the actual errors in this sea of false errors.

So far I have not managed to reproduce this with a smaller example.

Cc @oli-obk is this a consequence of your recent work to keep compiling in more cases?

[oli-obk]

is this a consequence of your recent work to keep compiling in more cases?

probably ^^

I'm gonna guess due to the error all the lifetimes become something else than their default, instead of just continueing as if the extra argument was removed and the rest was normal

[oli-obk]

Not minimal, but small:

struct Foo<'a, 'b, 'c, 'd>(&'a(), &'b(), &'c(), &'d());

impl<'a, 'b, 'c, 'd>  Foo<'a, 'b, 'c, 'd> {
    fn acc(&mut self, _bar: &Bar) -> &'d() {
        todo!()
    }
}

struct Bar;

impl<'a> Bar {
    fn boom(&self, foo: &mut Foo<'_, '_, '_, '_, 'a>) -> Result<(), &'a ()> {
        self.bar().map_err(|()| foo.acc(self))?;
        Ok(())
    }
    fn bar(&self) -> Result<(), &'a ()> { todo!() }
}

fn main() {}