This allows us to write

fn char_to_string() -> String {
    'a'.into()
}

which was not possible before.

Fixed grammar and sentence structure on appropriate instances.

I know very little about rust, so I saw this example and tried to generalize it by writing,
```
    let layout = Layout::new::<T>();
    let new_obj = unsafe {
        let ptr = alloc(layout) as *mut T;
        *ptr = obj;
        Box::from_raw(ptr)
    };
```
for some more complicated `T`, which ended up crashing with SIGSEGV,
because it tried to `drop_in_place` the previous object in `ptr` which is
of course garbage. I also added a comment that explains why `.write`
is used, but I think adding that comment is optional and may be too verbose
here. I do however think that changing this example is a good idea to
suggest the correct generalization. `.write` is also used in most of the rest
of the documentation here, even if the example is `i32`, so it would additionally
be more consistent.

Co-authored-by: Niko Matsakis <niko@alum.mit.edu>

Linked the logo/svg to https://www.rust-lang.org/
(change if required)

Added responsiveness to image logo based on device width (set at 90% of device width, which can be changed as per requirement)

update master

They are gated by internal feature gate const_likely

This commit applies the existing 'extra angle bracket recovery' logic
when parsing fields in struct definitions. This allows us to continue
parsing the struct's fields, avoiding spurious 'missing field' errors in
code that tries to use the struct.

Co-authored-by: Oliver Scherer <github35764891676564198441@oli-obk.de>

…xamples

Does not yet make its constness stable, though. Use of
`Location::caller` in const contexts is still gated by
`#![feature(const_caller_location)]`.

Stabilize `#[track_caller]`.

# Stabilization Report

RFC: [2091]
Tracking issue: rust-lang#47809

## Summary

From the [rustc-dev-guide chapter][dev-guide]:

> Take this example program:

```rust
fn main() {
    let foo: Option<()> = None;
    foo.unwrap(); // this should produce a useful panic message!
}
```

> Prior to Rust 1.42, panics like this `unwrap()` printed a location in libcore:

```
$ rustc +1.41.0 example.rs; example.exe
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value',...core\macros\mod.rs:15:40
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
```

> As of 1.42, we get a much more helpful message:

```
$ rustc +1.42.0 example.rs; example.exe
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', example.rs:3:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

> These error messages are achieved through a combination of changes to `panic!` internals to make use of `core::panic::Location::caller` and a number of `#[track_caller]` annotations in the standard library which propagate caller information.

The attribute adds an implicit caller location argument to the ABI of annotated functions, but does not affect the type or MIR of the function. We implement the feature entirely in codegen and in the const evaluator.

## Bottom Line

This PR stabilizes the use of `#[track_caller]` everywhere, including traits and extern blocks. It also stabilizes `core::panic::Location::caller`, although the use of that function in a const context remains gated by `#![feature(const_caller_location)]`.

The implementation for the feature already changed the output of panic messages for a number of std functions, as described in the [1.42 release announcement]. The attribute's use in `Index` and `IndexMut` traits is visible to users since 1.44.

## Tests

All of the tests for this feature live under [src/test/ui/rfc-2091-track-caller][tests] in the repo.

Noteworthy cases:

* [use of attr in std]
  * validates user-facing benefit of the feature
* [trait attribute inheritance]
  * covers subtle behavior designed during implementation and not RFC'd
* [const/codegen equivalence]
  * this was the result of a suspected edge case and investigation
* [diverging function support]
  * covers an unresolved question from the RFC
* [fn pointers and shims]
  * covers important potential sources of unsoundness

## Documentation

The rustc-dev-guide now has a chapter on [Implicit Caller Location][dev-guide].

I have an [open PR to the reference][attr-reference-pr] documenting the attribute.

The intrinsic's [wrapper] includes some examples as well.

## Implementation History

* 2019-10-02: [`#[track_caller]` feature gate (RFC 2091 1/N) rust-lang#65037](rust-lang#65037)
  * Picked up the patch that @ayosec had started on the feature gate.
* 2019-10-13: [Add `Instance::resolve_for_fn_ptr` (RFC 2091 rust-lang#2/N) rust-lang#65182](rust-lang#65182)
* 2019-10-20: ~~[WIP Add MIR argument for #[track_caller] (RFC 2091 3/N) rust-lang#65258](rust-lang#65258
  * Abandoned approach to send location as a MIR argument.
* 2019-10-28: [`std::panic::Location` is a lang_item, add `core::intrinsics::caller_location` (RFC 2091 3/N) rust-lang#65664](rust-lang#65664)
* 2019-12-07: [Implement #[track_caller] attribute. (RFC 2091 4/N) rust-lang#65881](rust-lang#65881)
* 2020-01-04: [libstd uses `core::panic::Location` where possible. rust-lang#67137](rust-lang#67137)
* 2020-01-08: [`Option::{expect,unwrap}` and `Result::{expect, expect_err, unwrap, unwrap_err}` have `#[track_caller]` rust-lang#67887](rust-lang#67887)
* 2020-01-20: [Fix #[track_caller] and function pointers rust-lang#68302](rust-lang#68302) (fixed rust-lang#68178)
* 2020-03-23: [#[track_caller] in traits rust-lang#69251](rust-lang#69251)
* 2020-03-24: [#[track_caller] on core::ops::{Index, IndexMut}. rust-lang#70234](rust-lang#70234)
* 2020-04-08 [Support `#[track_caller]` on functions in `extern "Rust" { ... }` rust-lang#70916](rust-lang#70916)

## Unresolveds

### From the RFC

> Currently the RFC simply prohibit applying #[track_caller] to trait methods as a future-proofing
> measure.

**Resolved.** See the dev-guide documentation and the tests section above.

> Diverging functions should be supported.

**Resolved.** See the tests section above.

> The closure foo::{{closure}} should inherit most attributes applied to the function foo, ...

**Resolved.** This unknown was related to specifics of the implementation which were made irrelevant by the final implementation.

### Binary Size

I [instrumented track_caller to use custom sections][measure-size] in a local build and discovered relatively minor binary size usage for the feature overall. I'm leaving the issue open to discuss whether we want to upstream custom section support.

There's an [open issue to discuss mitigation strategies][mitigate-size]. Some decisions remain about the "right" strategies to reduce size without overly constraining the compiler implementation. I'd be excited to see someone carry that work forward but my opinion is that we shouldn't block stabilization on implementing compiler flags for redaction.

### Specialization

There's an [open issue][specialization] on the semantics of the attribute in specialization chains. I'm inclined to move forward with stabilization without an exact resolution here given that specialization is itself unstable, but I also think it should be an easy question to resolve.

### Location only points to the start of a call span

rust-lang#69977 was resolved by rust-lang#73182, and the next step should probably be to [extend `Location` with a notion of the end of a call](rust-lang#73554).

### Regression of std's panic messages

rust-lang#70963 should be resolved by serializing span hygeine to crate metadata: rust-lang#68686.

[2091]: https://github.com/rust-lang/rfcs/blob/master/text/2091-inline-semantic.md
[dev-guide]: https://rustc-dev-guide.rust-lang.org/codegen/implicit-caller-location.html
[specialization]: rust-lang#70293
[measure-size]: rust-lang#70579
[mitigate-size]: rust-lang#70580
[attr-reference-pr]: rust-lang/reference#742
[wrapper]: https://doc.rust-lang.org/nightly/core/panic/struct.Location.html#method.caller
[tests]: https://github.com/rust-lang/rust/tree/master/src/test/ui/rfc-2091-track-caller
[const/codegen equivalence]: https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2091-track-caller/caller-location-fnptr-rt-ctfe-equiv.rs
[diverging function support]: https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2091-track-caller/diverging-caller-location.rs
[use of attr in std]: https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2091-track-caller/std-panic-locations.rs
[fn pointers and shims]: https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2091-track-caller/tracked-fn-ptr-with-arg.rs
[trait attribute inheritance]: https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2091-track-caller/tracked-trait-impls.rs
[1.42 release announcement]: https://blog.rust-lang.org/2020/03/12/Rust-1.42.html#useful-line-numbers-in-option-and-result-panic-messages

…olnay

impl From<char> for String

This allows us to write

````rust
fn char_to_string() -> String {
    'a'.into()
}
````

which was not possible before.

…static-morse

remove rustdoc warnings

Fix sentence structure

Fixed grammar and sentence structure on appropriate instances.

Update Box::from_raw example to generalize better

I know very little about rust, so I saw the example here
```
use std::alloc::{alloc, Layout};

unsafe {
    let ptr = alloc(Layout::new::<i32>()) as *mut i32;
    *ptr = 5;
    let x = Box::from_raw(ptr);
}
```
and tried to generalize it by writing,
```
    let layout = Layout::new::<T>();
    let new_obj = unsafe {
        let ptr = alloc(layout) as *mut T;
        *ptr = obj;
        Box::from_raw(ptr)
    };
```
for some more complicated `T`, which ended up crashing with SIGSEGV,
because it tried to `drop_in_place` the previous object in `ptr` which is
of course garbage. I think that changing this example to use `.write` instead
would be a good idea to suggest the correct generalization. It is also more
consistent with other documentation items in this file, which use `.write`.
I also added a comment to explain it, but I'm not too attached to that,
and can see it being too verbose in this place.

stop taking references in Relate

Adds a `Copy` bound to `Relate` and changes the type signatures to `T` from `&T`. While the `Copy` bound is not strictly necessary (i.e. the `Clone` bound of `TypeRelation` would be good enough), we don't need non `Copy` types and it simplifies the implementation.

Removes the afaict unused impls for `Vec<ty::PolyExistentialProjection<'tcx>>`, `Rc<T>` and `Box<T>`. If they end up being relevant again the bound of `Relate` can be reduced to `T: Clone`.

This also changes signature of `Binder::skip_binder` to `fn skip_binder(self) -> T`.

`TypeError::ProjectionBoundsLength` was never used and is also removed in this PR.

r? @nikomatsakis maybe 🤔 feel free to reassign

…sKalbertodt

Document the static keyword

Partial fix of rust-lang#34601.

This documents the `static` keyword. It's basically a simplified version of the reference with more examples.

@rustbot modify labels: T-doc,C-enhancement

…LukasKalbertodt

Remap Windows ERROR_INVALID_PARAMETER to ErrorKind::InvalidInput from Other

I don't know if this is acceptable or how likely it is to break existing code, but it seem to me ERROR_INVALID_PARAMETER "The parameter is incorrect" should map to ErrorKind::InvalidInput "A parameter was incorrect". Previously this value fell through to ErrorKind::Other.

I can't speak for anyone but myself, but I instinctively thought it would be InvalidInput.

…ywiser

Move terminator to new module

Moves terminator types from `librustc_middle/mir/mod.rs` into submodule
Fixes rust-lang#73723

Make `likely` and `unlikely` const, gated by feature `const_unlikely`

This PR also contains a fix to allow `#[allow_internal_unstable]` to work properly with `#[rustc_const_unstable]`.

cc @RalfJung @nagisa

r? @oli-obk

…ery, r=matthewjasper

Recover extra trailing angle brackets in struct definition

This commit applies the existing 'extra angle bracket recovery' logic
when parsing fields in struct definitions. This allows us to continue
parsing the struct's fields, avoiding spurious 'missing field' errors in
code that tries to use the struct.

Document the type keyword

Partial fix of rust-lang#34601.

Two small examples, one clarifying that `type` only defines an alias, not a completely new type, the other explaining the use in traits.

@rustbot modify labels: T-doc,C-enhancement

ast_pretty: Pass some token streams and trees by reference

Salvaged from an intermediate version of rust-lang#73345.

…bank

Fix wording for anonymous parameter name help

```
 --> exercises/functions/functions2.rs:8:15
  |
8 | fn call_me(num) {
  |               ^ expected one of `:`, `@`, or `|`
  |
  = note: anonymous parameters are removed in the 2018 edition (see RFC 1685)
help: if this is a `self` type, give it a parameter name
  |
8 | fn call_me(self: num) {
  |            ^^^^^^^^^
help: if this was a parameter name, give it a type
  |
8 | fn call_me(num: TypeName) {
  |            ^^^^^^^^^^^^^
help: if this is a type, explicitly ignore the parameter name
  |
8 | fn call_me(_: num) {
  |
```
This commit changes "if this was a parameter name" to "if this is a parameter name" to match the wording of similar errors.

…-morse

Some refactoring around intrinsic type checking

So... This PR went a bit overboard. I wanted to make the `rustc_peek` intrinsic safe (cc @ecstatic-morse ), and remembered a long-standing itch of mine. So I made that huge `&str` match for the intrinsic name a match on `Symbol`s (so basically `u32`s). This is unlikely to have a positive perf effect, even if it likely has better codegen (intrinsics are used rarely, mostly once in their wrapper), so it's mostly a consistency thing since other places actually match on the symbol name of the intrinsics.