Remove recursive call from Cow::to_mut

It seems to prevent it from being inlined.

fix #37559: update compiler-rt

Fixes #37559

r? @alexcrichton

Change `Into<Vec<u8>> for String` and `Into<OsString> for PathBuf` to From

Fixes #37561. First contribution, happy with any and all feedback!

Add `-Z hir-stats` for collecting statistics on HIR and AST

The data collected will be printed to the commandline and looks like the following:

```
// stats for libcore

PRE EXPANSION AST STATS

Name                Accumulated Size         Count     Item Size
----------------------------------------------------------------
TypeBinding                    2_280            57            40
Mod                            3_560            89            40
PathListItem                   6_516           181            36
Variant                        7_872            82            96
LifetimeDef                   21_280           380            56
StructField                   22_880           260            88
Lifetime                      23_800         1_190            20
Local                         30_192           629            48
ForeignItem                   31_504           179           176
Arm                           42_880           670            64
Mac                           46_960           587            80
FnDecl                        57_792         1_204            48
TraitItem                     69_504           362           192
TyParamBound                  98_280           945           104
Block                        108_384         2_258            48
Stmt                         144_720         3_618            40
ImplItem                     230_272         1_028           224
Item                         467_456         1_826           256
Pat                          517_776         4_623           112
Attribute                    745_680        15_535            48
Ty                         1_114_848         9_954           112
PathSegment                1_218_528        16_924            72
Expr                       3_082_408        20_279           152
----------------------------------------------------------------
Total                      8_095_372

POST EXPANSION AST STATS

Name                Accumulated Size         Count     Item Size
----------------------------------------------------------------
MacroDef                       1_056            12            88
Mod                            3_400            85            40
TypeBinding                    4_280           107            40
PathListItem                   6_516           181            36
Variant                        7_872            82            96
StructField                   24_904           283            88
ForeignItem                   31_504           179           176
TraitItem                     69_504           362           192
Local                         85_008         1_771            48
Arm                          100_288         1_567            64
Lifetime                     123_980         6_199            20
LifetimeDef                  126_728         2_263            56
TyParamBound                 297_128         2_857           104
FnDecl                       305_856         6_372            48
Block                        481_104        10_023            48
Stmt                         535_120        13_378            40
Item                       1_469_952         5_742           256
Attribute                  1_629_840        33_955            48
ImplItem                   1_732_864         7_736           224
Pat                        2_360_176        21_073           112
PathSegment                5_888_448        81_784            72
Ty                         6_237_168        55_689           112
Expr                      12_013_320        79_035           152
----------------------------------------------------------------
Total                     33_536_016

HIR STATS

Name                Accumulated Size         Count     Item Size
----------------------------------------------------------------
MacroDef                         864            12            72
Mod                            2_720            85            32
TypeBinding                    3_424           107            32
PathListItem                   5_068           181            28
Variant                        6_560            82            80
StructField                   20_376           283            72
ForeignItem                   27_208           179           152
WherePredicate                43_776           684            64
TraitItem                     52_128           362           144
Decl                          68_992         2_156            32
Local                         89_184         1_858            48
Arm                           94_368         1_966            48
LifetimeDef                  108_624         2_263            48
Lifetime                     123_980         6_199            20
Stmt                         168_000         4_200            40
TyParamBound                 251_416         2_857            88
FnDecl                       254_880         6_372            40
Block                        583_968        12_166            48
Item                       1_240_272         5_742           216
ImplItem                   1_361_536         7_736           176
Attribute                  1_620_480        33_760            48
Pat                        2_073_120        21_595            96
Path                       2_385_856        74_558            32
Ty                         4_455_040        55_688            80
PathSegment                5_587_904        87_311            64
Expr                       7_588_992        79_052            96
----------------------------------------------------------------
Total                     28_218_736
```

reference full path DefaultHasher

Since `DefaultHasher` does not in scope, it's not obvious where to find it.

Shrink `hir::Expr` slightly

r? @EddyB

Fix issues with the Add/AddAssign impls for Cow<str>

* Correct the stability attributes.
* Make Add and AddAssign actually behave the same.
* Use String::with_capacity when allocating a new string.
* Fix the tests.

macros: improve expansion performance

This PR fixes that regression, further improves performance on recursive, `tt`-heavy workloads, and makes a variety of other improvements to parsing and expansion performance.

Expansion performance improvements:

| Test case      | Run-time | Memory usage |
| -------------- | -------- | ------------ |
| libsyntax      | 8%       | 10%          |
| librustc       | 15%      | 6%           |
| librustc_trans | 30%      | 6%           |
| #37074         | 20%      | 15%          |
| #34630         | 40%      | 8%           |

r? @EddyB

Set RUSTC_BOOTSTRAP to some value.

Environment variables on windows can't be empty.

Peekable::peek(): Use Option::as_ref()

Replace the match expression in .peek() with Option::as_ref() since it's the same functionality.

Fix ICE when querying DefId on Def::Err.

Also moves computations into check that `kind_id` is `Ok(_)`, which is in theory an optimization, though I expect it's minor.

Fixes #37534.

r? @EddyB.

Use DefId's in const eval for cross-crate const fn's

Fixes #36954.

r? @EddyB

cc @raphaelcohn

set frame pointer elimination attribute for main

The rustc-generated function `main` should respect the same config for
frame pointer elimination as the rest of code.

Add missing urls for ErrorKind's variants

r? @steveklabnik

rustc: Add knowledge of Windows subsystems.

This commit is an implementation of [RFC 1665] which adds support for the
`#![windows_subsystem]` attribute. This attribute allows specifying either the
"windows" or "console" subsystems on Windows to the linker.

[RFC 1665]: https://github.com/rust-lang/rfcs/blob/master/text/1665-windows-subsystem.md

Previously all Rust executables were compiled as the "console" subsystem which
meant that if you wanted a graphical application it would erroneously pop up a
console whenever opened. When compiling an application, however, this is
undesired behavior and the "windows" subsystem is used instead to have control
over user interactions.

This attribute is validated, but ignored on all non-Windows platforms.

cc #37499

Don't reuse RandomState seeds

cc #36481

Reduce the number of bytes hashed by IchHasher.

IchHasher uses blake2b hashing, which is expensive, so the fewer bytes hashed
the better. There are two big ways to reduce the number of bytes hashed.
- Filenames in spans account for ~66% of all bytes (for builds with debuginfo).
  The vast majority of spans have the same filename for the start of the span
  and the end of the span, so hashing the filename just once in those cases is
  a big win.
- u32 and u64 and usize values account for ~25%--33% of all bytes (for builds
  with debuginfo). The vast majority of these are small, i.e. fit in a u8, so
  shrinking them down before hashing is also a big win.

This PR implements these two optimizations. I'm certain the first one is safe.
I'm about 90% sure that the second one is safe.

Here are measurements of the number of bytes hashed when doing
debuginfo-enabled builds of stdlib and
rustc-benchmarks/syntex-0.42.2-incr-clean.

```
                    stdlib   syntex-incr
                    ------   -----------
original       156,781,386   255,095,596
half-SawSpan   106,744,403   176,345,419
short-ints      45,890,534   118,014,227
no-SawSpan[*]    6,831,874    45,875,714

[*] don't hash the SawSpan at all. Not part of this PR, just implemented for
    comparison's sake.
```

For debug builds of syntex-0.42.2-incr-clean, the two changes give a 1--2%
speed-up.

Add .wrapping_offset() methods

.wrapping_offset() exposes the arith_offset intrinsic in the core
module (as methods on raw pointers, next to offset). This is the
first step in making it possible to stabilize the interface later.

`arith_offset` is a useful tool for developing iterators for two
reasons:
1. `arith_offset` is used by the slice's iterator, the most important
   iterator in libcore, and it is natural that Rust users need the same
   power available to implement similar iterators.
2. It is a good way to implement raw pointer iterations with step
   greater than one.

The name seems to fit the style of methods like "wrapping_add".

[5/n] rustc: record the target type of every adjustment.

_This is part of a series ([prev](https://github.com/rust-lang/rust/pull/37404) | [next](https://github.com/rust-lang/rust/pull/37412)) of patches designed to rework rustc into an out-of-order on-demand pipeline model for both better feature support (e.g. [MIR-based](https://github.com/solson/miri) early constant evaluation) and incremental execution of compiler passes (e.g. type-checking), with beneficial consequences to IDE support as well.
If any motivation is unclear, please ask for additional PR description clarifications or code comments._

<hr>

The first commit rearranges `tcx.tables` so that all users go through `tcx.tables()`. This in preparation for per-body `Tables` where they will be requested for a specific `DefId`. Included to minimize churn.

The rest of the changes focus on adjustments, there are some renamings, but the main addition is the target type, always available in all cases (as opposed to just for unsizing where it was previously needed).

Possibly the most significant effect of this change is that figuring out the final type of an expression is now _always_ just one successful `HashMap` lookup (either the adjustment or, if that doesn't exist, the node type).

Add release notes for 1.12.1

It completely slipped my mind that this is something I'm supposed to do as part of the release process...

Fix some mistakes in HRTB docs

The example code for higher-ranked trait bounds on closures had an unnecessary `mut` which was confusing, and the text referred to an mutable reference which does not exist in the code (and isn't needed).  Removed the `mut`s and fixed the text to better describe the actual error for the failing example.

Thanks to csd_ on IRC for pointing out these problems!

r? @steveklabnik

It seems to prevent it from being inlined.

Add Iterator trait TrustedLen to enable better FromIterator / Extend

This trait attempts to improve FromIterator / Extend code by enabling it to trust the iterator to produce an exact number of elements, which means that reallocation needs to happen only once and is moved out of the loop.

`TrustedLen` differs from `ExactSizeIterator` in that it attempts to include _more_ iterators by allowing for the case that the iterator's len does not fit in `usize`. Consumers must check for this case (for example they could panic, since they can't allocate a collection of that size).

For example, chain can be TrustedLen and all numerical ranges can be TrustedLen. All they need to do is to report an exact size if it fits in `usize`, and `None` as the upper bound otherwise.

The trait describes its contract like this:

```
An iterator that reports an accurate length using size_hint.

The iterator reports a size hint where it is either exact
(lower bound is equal to upper bound), or the upper bound is `None`.
The upper bound must only be `None` if the actual iterator length is
larger than `usize::MAX`.

The iterator must produce exactly the number of elements it reported.

This trait must only be implemented when the contract is upheld.
Consumers of this trait must inspect `.size_hint()`’s upper bound.
```

Fixes #37232

detect extra region requirements in impls

The current "compare method" check fails to check for the "region obligations" that accrue in the fulfillment context. This branch switches that code to create a `FnCtxt` so that it can invoke the regionck code. Previous crater runs (I haven't done one with the latest tip) have found some small number of affected crates, so I went ahead and introduced a warning cycle. I will kick off a crater run with this branch shortly.

This is a [breaking-change] because previously unsound code was accepted. The crater runs also revealed some cases where legitimate code was no longer type-checking, so the branch contains one additional (but orthogonal) change. It improves the elaborator so that we elaborate region requirements more thoroughly. In particular, if we know that `&'a T: 'b`, we now deduce that `T: 'b` and `'a: 'b`.

I invested a certain amount of effort in getting a good error message. The error message looks like this:

```
error[E0276]: impl has stricter requirements than trait
  --> traits-elaborate-projection-region.rs:33:5
   |
21 |     fn foo() where T: 'a;
   |     --------------------- definition of `foo` from trait
...
33 |     fn foo() where U: 'a { }
   |     ^^^^^^^^^^^^^^^^^^^^^^^^ impl has extra requirement `U: 'a`
   |
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #18937 <https://github.com/rust-lang/rust/issues/18937>
note: lint level defined here
  --> traits-elaborate-projection-region.rs:12:9
   |
12 | #![deny(extra_requirement_in_impl)]
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^
```

Obviously the warning only prints if this is a _new_ error (that resulted from the bugfix). But all existing errors that fit this description are updated to follow the general template. In order to get the lint to preserve the span-labels and the error code, I separate out the core `Diagnostic` type (which encapsulates the error code, message, span, and children) from the `DiagnosticBuilder` (which layers on a `Handler` that can be used to report errors). I also extended `add_lint` with an alternative `add_lint_diagnostic` that takes in a full diagnostic (cc @jonathandturner for those changes). This doesn't feel ideal but feels like it's moving in the right direction =).

r? @pnkfelix
cc @arielb1

Fixes #18937

Add conversions from `io:ErrorKind` to `io::Error`

Filing to help with discussion around the possibility of doing this.

Current changes are clearly backwards incompatible, but I think adding a new function (with a bikeshed on naming) like `Error::new_str` should be possible (or some other way of specializing the string error message case) to fix #36658.

A way to remove otherwise unused locals from MIR

There is a certain amount of desire for a pass which cleans up the provably unused variables (no assignments or reads). There has been an implementation of such pass by @scottcarr, and another (two!) implementations by me in my own dataflow efforts.

PR like https://github.com/rust-lang/rust/pull/35916 proves that this pass is useful even on its own, which is why I cherry-picked it out from my dataflow effort.

@nikomatsakis previously expressed concerns over this pass not seeming to be very cheap to run and therefore unsuitable for regular cleanup duties. Turns out, regular cleanup of local declarations is not at all necessary, at least now, because majority of passes simply do not (or should not) care about them. That’s why it is viable to only run this pass once (perhaps a few more times in the future?) per function, right before translation.

r? @EddyB or @nikomatsakis

On 64-bit platforms this reduces the size of `Expr_` from  64 bytes to
56 bytes, and reduces the size of `Expr` from 88 bytes to 80 bytes.

On 64-bit platforms this reduces the size of `Expr` from 96 bytes to 88
bytes.

* Correct the stability attributes.
* Make Add and AddAssign actually behave the same.
* Use String::with_capacity when allocating a new string.
* Fix the tests.