Suggest adding a lifetime constraint for opaque type

Fixes #67577, where code like this:

```
struct List {
  data: Vec<String>,
}
impl List {
  fn started_with<'a>(&'a self, prefix: &'a str) -> impl Iterator<Item=&'a str> {
    self.data.iter().filter(|s| s.starts_with(prefix)).map(|s| s.as_ref())
  }
}
```

will show this error:
```
   Compiling playground v0.0.1 (/playground)
error[E0597]: `prefix` does not live long enough
 --> src/lib.rs:6:47
  |
5 |   fn started_with<'a>(&'a self, prefix: &'a str) -> impl Iterator<Item=&'a str> {
  |                   -- lifetime `'a` defined here     --------------------------- opaque type requires that `prefix` is borrowed for `'a`
...
```

but without suggesting the lovely `help: you can add a constraint..`.

r? @estebank

Allow for setting a ThinLTO import limit during bootstrap

The benchmarks in https://github.com/rust-lang/rust/pull/66625 have shown that a lower ThinLTO import limit can be a net win for bootstrap times. This PR:
- exposes the setting to `config.toml`,
- defaults to a lower limit if `incremental = true` in `config.toml`, and
- sets a lower limit for `x86_64-gnu-llvm-7` CI image in order to make the jobs complete more quickly (which remains to be tested).

This setting will affect how the compiler and it's tools are compiled. It will not affect the settings the compiler uses when compiling user code.

r? @pietroalbini
cc @rust-lang/infra

Add Iterator::try_find

I found a need for this fn, and created this PR.

Tracking issue: #63178

I did a fair amount of thinking about the function name, and settled on the current one.
I don't see other anything else that's non-trivial here, but I'm open for debate. I just want this functionality to be there.
It couples with the `collect` trick for collecting `Result<Vec<T>, E>` from `Iterator<Item = Result<T, E>>`.

UPD:

I've already looked at `fallible_iterator` crate, but I don't think it supports my use case.
The main problem is that I can't construct a failable iterator. I have a regular iterator, and I just need to apply a predicate that can fail via `find` method.

UPD: `fallible_iterator` would work, but it's not elegant cause I'd have to make a failable iterator by mapping iterator with `Result::Ok` first.

Update the barrier cache during ARM EHABI unwinding

Fixes #67242

r? @alexcrichton

Move stability queries to librustc_passes.

Contains:
- check_mod_unstable_api_usage query;
- stability_index query;
- check_unused_or_stable features pass.

Split out of #67688

r? @Zoxc

Update rustc-guide

r? @ehuss

cc @rust-lang/wg-learning

Lint overflowing integer casts in const prop

This extends the invalid cases we catch in const prop to include
overflowing integer casts using the same machinery as the overflowing
binary and unary operation logic.

r? @oli-obk

Bump smallvec

Contains:
- check_mod_unstable_api_usage query;
- stability_index query;
- check_unused_or_stable features pass.

Revert parts of #66405.

Because PR #66405 caused major performance regressions in some cases.

That PR had five commits, two of which affected performance, and three
of which were refactorings. This change undoes the performance-affecting
changes, while keeping the refactorings in place.

Fixes #67454.

r? @nikomatsakis

Rename `syntax_pos` to `rustc_span` in source code

Follow-up to https://github.com/rust-lang/rust/pull/67707.

r? @Centril

Because it caused major performance regressions in some cases.

That PR had five commits, two of which affected performance, and three
of which were refactorings. This change undoes the performance-affecting
changes, while keeping the refactorings in place.

Fixes #67454.

Update Clippy

Fixes #67716
r? @oli-obk

Rollup of 6 pull requests

Successful merges:

 - #67574 (Extract `rustc_ast_lowering` crate from `rustc`)
 - #67685 (Constify Result)
 - #67702 (Add symbol normalization for proc_macro_server.)
 - #67730 (Cleanup pattern type checking, fix diagnostics bugs (+ improvements))
 - #67744 (parser: reduce diversity in error handling mechanisms)
 - #67748 (Use function attribute "frame-pointer" instead of "no-frame-pointer-elim")

Failed merges:

r? @ghost

Use function attribute "frame-pointer" instead of "no-frame-pointer-elim"

LLVM 8 ([D56351](http://reviews.llvm.org/D56351)) introduced "frame-pointer". In LLVM 10 (D71863),
"no-frame-pointer-elim"/"no-frame-pointer-elim-non-leaf" will be
ignored.

-----

In the LLVM monorepo, run `git show origin/release/8.x:llvm/lib/CodeGen/TargetOptionsImpl.cpp` to see that `"frame-pointer"` is available since LLVM 8.

parser: reduce diversity in error handling mechanisms

Instead of having e.g. `span_err`, `fatal`, etc., we prefer to move towards uniformly using `struct_span_err` thus making it harder to emit fatal and/or unstructured diagnostics.

This PR also de-fatalizes some diagnostics.

r? @estebank

Cleanup pattern type checking, fix diagnostics bugs (+ improvements)

r? @estebank

Add symbol normalization for proc_macro_server.

Follow up for #66670, finishing the first bullet point in #55467.

r? @petrochenkov

Constify Result

r? @oli-obk

This is just the `Result`-part of #67494 which I'll resubmit once #66254 has landed.

Extract `rustc_ast_lowering` crate from `rustc`

Working towards https://github.com/rust-lang/rust/issues/65031.

This PR moves `src/librustc/hir/lowering{/, .rs}` to its own crate (`librustc_ast_lowering`) which is very self-contained (only `fn lower_crate` and `trait Resolver` are exposed).

r? @Mark-Simulacrum

Move resolve_lifetimes query in librustc_resolve.

Split out of #67688

r? @Zoxc

Rollup of 5 pull requests

Successful merges:

 - #67430 (doc: minus (U+2212) instead of dash (U+002D) for negative infinity)
 - #67697 (Move the region_scope_tree query to librustc_passes.)
 - #67719 (Add self to .mailmap)
 - #67723 (Add error code explanation for E0477)
 - #67735 (Support `-Z ui-testing=yes/no`)

Failed merges:

r? @ghost

Support `-Z ui-testing=yes/no`

`ui-testing` is now a boolean option (`-Z ui-testing=yes/no`) and can be specified multiple times with later values overriding earlier values (`-Z ui-testing=yes -Z ui-testing=no` == `-Z ui-testing=no`), so it can be set in a hierarchical way, e.g. UI testing infra may enable it by default with specific tests being able to opt-out.

This way we can remove the special opt-out support from `compiletest`.

Inspired by https://github.com/rust-lang/rust/pull/67709.

Add error code explanation for E0477

Part of #61137

Add self to .mailmap

r? @Mark-Simulacrum

Cheers.

Move the region_scope_tree query to librustc_passes.

Split out of #67688.

r? @Zoxc

doc: minus (U+2212) instead of dash (U+002D) for negative infinity

The documentation for [`f32::NEG_INFINITY`](https://doc.rust-lang.org/std/f32/constant.NEG_INFINITY.html) contains “-∞” with a dash instead of a minus sign, “−∞” with a proper minus sign looks better with the used Source Serif Pro font. Similarly for [`f64::NEG_INFINITY`](https://doc.rust-lang.org/std/f64/constant.NEG_INFINITY.html).