Co-Authored-By: Mazdak Farrokhzad <twingoow@gmail.com>

This test was actually about the unreachable_code flag, not dead_code,
so I renamed it for clarity (to prepare for the next commit, where I
plan to move a bunch of the dead_code tests to a single folder)

This helps organize the tests better.  I also renamed several of the tests to remove redundant dead-code in the path, and better match what they're testing

This is leftover from a restructuring of lint registration for drivers;
it should now happen via the register_lints field on Config rather than
this function.

This allows us to directly pass in a lint buffer

According to @estebank, def_span scans forward on the line until it finds a {,
and if it can't find one, fallse back to the span for the whole item.  This
was apparently written before the identifier span was explicitly tracked on
each node.

This means that if an unused function signature spans multiple lines, the
entire function (potentially hundreds of lines) gets flagged as dead code.
This could, for example, cause IDEs to add error squiggly's to the whole
function.

By using the span from the ident instead, we narrow the scope of this in
most cases.  In a wider sense, it's probably safe to use ident.span
instead of def_span in most locations throughout the whole code base,
but since this is my first contribution, I kept it small.

Some interesting points that came up while I was working on this:
 - I reorganized the tests a bit to bring some of the dead code ones all
   into the same location
 - A few tests were for things unrelated to dead code (like the
   path-lookahead for parens), so I added #![allow(dead_code)] and
   cleaned up the stderr file to reduce noise in the future
 - The same fix doesn't apply to const and static declarations.  I tried
   adding these cases to the match expression, but that created a much
   wider change to tests and error messages, so I left it off until I
   could get some code review to validate the approach.

…omatsakis

Point at associated type for some obligations

Partially address rust-lang#57663.

…nikomatsakis

rustc: add `Span`s to `inferred_outlives_of` predicates.

This would simplify rust-lang#59789, and I suspect it has some potential in diagnostics (although we don't seem to use the predicate `Span`s much atm).

…low-fundamental-local, r=nikomatsakis

Coherence should allow fundamental types to impl traits when they are local

After rust-lang#64414, `impl<T> Remote for Box<T> { }` is disallowed, but it is also disallowed in liballoc, where `Box` is a local type!

Enabling `#![feature(re_rebalance_coherence)]` in `liballoc` results in:
```
error[E0210]: type parameter `F` must be used as the type parameter for some local type (e.g., `MyStruct<F>`)
    --> src\liballoc\boxed.rs:1098:1
     |
1098 | impl<F: ?Sized + Future + Unpin> Future for Box<F> {
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type parameter `F` must be used as the type parameter for some local type
```

This PR relaxes `uncover_fundamental_ty` to skip local fundamental types.
I didn't add a test since `liballoc` already fails to compile, but I can add one if needed.

r? @nikomatsakis

cc rust-lang#63599

…nsion, r=davidtwco

Don't ICE for completely unexpandable `impl Trait` types

Save the resolution of these types (to themselves) to the typeck tables so that they will eventually reach E0720.

closes rust-lang#65561

Use ident.span instead of def_span in dead-code pass

Hello! First time contributor! :)

This should fix rust-lang#58729.

According to @estebank in the duplicate rust-lang#63064, def_span scans forward on the line until it finds a {,
and if it can't find one, falls back to the span for the whole item. This
was apparently written before the identifier span was explicitly tracked on
each node.

This means that if an unused function signature spans multiple lines, the
entire function (potentially hundreds of lines) gets flagged as dead code.
This could, for example, cause IDEs to add error squiggly's to the whole
function.

By using the span from the ident instead, we narrow the scope of this in
most cases. In a wider sense, it's probably safe to use ident.span
instead of def_span in most locations throughout the whole code base,
but since this is my first contribution, I kept it small.

Some interesting points that came up while I was working on this:
- I reorganized the tests a bit to bring some of the dead code ones all
into the same location
- A few tests were for things unrelated to dead code (like the
path-lookahead for parens), so I added #![allow(dead_code)] and
cleaned up the stderr file to reduce noise in the future
- The same fix doesn't apply to const and static declarations. I tried
adding these cases to the match expression, but that created a much
wider change to tests and error messages, so I left it off until I
could get some code review to validate the approach.

…omatsakis

Remove lint callback from driver

This is leftover from a restructuring of lint registration for drivers; it should now happen via the register_lints field on Config rather than this function.

This is not used by anyone to my knowledge (including the compiler itself); it was introduced in an abandoned refactor in rust-lang#65193.

…, r=nikomatsakis

Remove LintBuffer from Session

This moves the `LintBuffer` from `Session` into the `Resolver`, where it is used until lowering is done and then consumed by early lint passes. This also happily removes the failure mode of buffering lints too late where it would have previously lead to ICEs; it is statically no longer possible to do so.

I suspect that with a bit more work a similar move could be done for the lint buffer inside `ParseSess`, but this PR doesn't touch it (in part to keep itself small).

The last commit is the "interesting" commit -- the ones before it don't work (though they compile) as they sort of prepare the various crates for the lint buffer to be passed in rather than accessed through Session.