Previously, on a type mismatch (and if this wasn't preëmpted by a
higher-priority suggestion), we would look for argumentless methods
returning the expected type, and list them in a `help` note.

This had two major shortcomings. Firstly, a lot of the suggestions didn't
really make sense (if you used a &str where a String was expected,
`.to_ascii_uppercase()` is probably not the solution you were hoping
for). Secondly, we weren't generating suggestions from the most useful
traits!

We address the first problem with an internal
`#[rustc_conversion_suggestion]` attribute meant to mark methods that keep
the "same value" in the relevant sense, just converting the type. We
address the second problem by making `FnCtxt.probe_for_return_type` pass
the `ProbeScope::AllTraits` to `probe_op`: this would seem to be safe
because grep reveals no other callers of `probe_for_return_type`.

Also, structured suggestions are preferred (because they're pretty, but
also for RLS and friends).

Also also, we make the E0055 autoderef recursion limit error use the
one-time-diagnostics set, because we can potentially hit the limit a lot
during probing. (Without this,
test/ui/did_you_mean/recursion_limit_deref.rs would report "aborting due to
51 errors").

Unfortunately, the trait probing is still not all one would hope for: at a
minimum, we don't know how to rule out `into()` in cases where it wouldn't
actually work, and we don't know how to rule in `.to_owned()` where it
would. Issues #46459 and #46460 have been filed and are ref'd in a FIXME.

This is hoped to resolve #42929, #44672, and #45777.

The `#[simd]` attribute has been deprecated since c8b6d5b back in 2015. Any nightly crates using it have had ample time to switch to `#[repr(simd)]`, and if they didn't they're likely broken by now anyway.

 - Recover from struct parse error on match and point out missing match
   body.
 - Point at struct when finding non-identifier while parsing its fields.
 - Add label to "expected identifier, found {}" error.

…ble length integer encodings.

Re-implement

```bash
CFG_LIBDIR_RELATIVE=`echo ${CFG_LIBDIR} | cut -c$((${#CFG_PREFIX}+${CAT_INC}))-`
```

from old `configure` script.

…ariable.

…e, r=estebank

 type error method suggestions use whitelisted identity-like conversions

![method_jamboree_summit](https://user-images.githubusercontent.com/1076988/33523646-e5c43184-d7c0-11e7-98e5-1bff426ade86.png)

Previously, on a type mismatch (and if this wasn't preëmpted by a
higher-priority suggestion), we would look for argumentless methods
returning the expected type, and list them in a `help` note. This had two
major shortcomings: firstly, a lot of the suggestions didn't really make
sense (if you used a &str where a String was expected,
`.to_ascii_uppercase()` is probably not the solution you were hoping
for). Secondly, we weren't generating suggestions from the most useful
traits! We address the first problem with an internal
`#[rustc_conversion_suggestion]` attribute meant to mark methods that keep
the "same value" in the relevant sense, just converting the type. We
address the second problem by making `FnCtxt.probe_for_return_type` pass
the `ProbeScope::AllTraits` to `probe_op`: this would seem to be safe
because grep reveals no other callers of `probe_for_return_type`.

Also, structured suggestions are pretty and good for RLS and friends.

Unfortunately, the trait probing is still not all one would hope for: at a
minimum, we don't know how to rule out `into()` in cases where it wouldn't
actually work, and we don't know how to rule in `.to_owned()` where it
would. Issues #46459 and #46460 have been filed and are ref'd in a FIXME.

This is hoped to resolve #42929, #44672, and #45777.

Fix 45345

There is a fix for #45345

It re-introduces `CFG_LIBDIR_RELATIVE` which was broken when migration from `configure` script to `x.py`.

Other commits fix errors which happen after rustbuild cleanups.

Use DefIndex encoding that works better with on-disk variable length integer representations.

Use the least instead of the most significant bit for representing the address space.

r? @eddyb

`struct` pattern parsing and diagnostic tweaks

 - Recover from struct parse error on match and point out missing match
   body.
 - Point at struct when finding non-identifier while parsing its fields.
 - Add label to "expected identifier, found {}" error.

Fix #15980.

Remove deprecated unstable attribute #[simd]

The `#[simd]` attribute has been deprecated since c8b6d5b back in 2015. Any nightly crates using it have had ample time to switch to `#[repr(simd)]`, and if they didn't they're likely broken by now anyway.

r? @eddyb