…ros, r=petrochenkov

Explicitly export core and std macros

Currently all core and std macros are automatically added to the prelude via #[macro_use]. However a situation arose where we want to add a new macro `assert_matches` but don't want to pull it into the standard prelude for compatibility reasons. By explicitly exporting the macros found in the core and std crates we get to decide on a per macro basis and can later add them via the rust_20xx preludes.

Closes #53977
Unlocks #137487

Reference PR:

- rust-lang/reference#2077

# Stabilization report lib

Everything N/A or already covered by lang report except, breaking changes: The unstable and never intended for public use `format_args_nl` macro is no longer publicly accessible as requested by @petrochenkov. Affects <10 crates including dependencies.

# Stabilization report lang

## Summary

Explicitly export core and std macros.

This change if merged would change the code injected into user crates to no longer include #[macro_use] on extern crate core and extern crate std. This change is motivated by a near term goal and a longer term goal. The near term goal is to allow a macro to be defined at the std or core crate root but not have it be part of the implicit prelude. Such macros can then be separately promoted to the prelude in a new edition. Specifically this is blocking the stabilization of assert_matches #137487. The longer term goal is to gradually deprecate #[macro_use]. By no longer requiring it for standard library usage, this serves as a step towards that goal. For more information see #53977.

PR link: #139493

Tracking:

- #147319

Reference PRs:

- #139493

cc @rust-lang/lang @rust-lang/lang-advisors

### What is stabilized

Stabilization:
    
* `#[macro_use]` is no longer automatically included in the crate root module. This allows the explicit import of macros in the `core` and `std` prelude e.g. `pub use crate::dbg;`.

* `ambiguous_panic_imports` lint. Code that previously passed without warnings, but included the following or equivalent - only pertaining to core vs std panic - will now receive a warning:

  ```rust
  #![no_std]
  extern crate std;
  use std::prelude::v1::*;
  fn xx() {
      panic!(); // resolves to core::panic
      //~^ WARNING `panic` is ambiguous
      //~| WARNING this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  }
  ```

  This lint is tied to a new exception to the name resolution logic in [compiler/rustc_resolve/src/ident.rs](https://github.com/rust-lang/rust/pull/139493/files#diff-c046507afdba3b0705638f53fffa156cbad72ed17aa01d96d7bd1cc10b8d9bce) similar to an exception added for #145575. Specifically this only happens if the import of two builtin macros is ambiguous and they are named `sym::panic`. I.e. this can only happen for `core::panic` and `std::panic`. While there are some tiny differences in what syntax is allowed in `std::panic` vs `core::panic` in editions 2015 and 2018, [see](#139493 (comment)). The behavior at runtime will always be the same if it compiles, implying minimal risk in what specific macro is resolved. At worst some closed source project not captured by crater will stop compiling because a different panic is resolved than previously and they were using obscure syntax like `panic!(&String::new())`.

## Design

N/A

### Reference

> What updates are needed to the Reference? Link to each PR. If the Reference is missing content needed for describing this feature, discuss that.

- rust-lang/reference#2077

### RFC history

> What RFCs have been accepted for this feature?

N/A

### Answers to unresolved questions

N/A

### Post-RFC changes

> What other user-visible changes have occurred since the RFC was accepted? Describe both changes that the lang team accepted (and link to those decisions) as well as changes that are being presented to the team for the first time in this stabilization report.

N/A

### Key points

> What decisions have been most difficult and what behaviors to be stabilized have proved most contentious? Summarize the major arguments on all sides and link to earlier documents and discussions.

- Nothing was really contentious.

### Nightly extensions

> Are there extensions to this feature that remain unstable? How do we know that we are not accidentally committing to those?

N/A

### Doors closed

> What doors does this stabilization close for later changes to the language? E.g., does this stabilization make any other RFCs, lang experiments, or known in-flight proposals more difficult or impossible to do later?

No known doors are closed.

## Feedback

### Call for testing

> Has a "call for testing" been done? If so, what feedback was received?

No.

### Nightly use

> Do any known nightly users use this feature? Counting instances of `#![feature(FEATURE_NAME)]` on GitHub with grep might be informative.

N/A

## Implementation

### Major parts

> Summarize the major parts of the implementation and provide links into the code and to relevant PRs.
>
> See, e.g., this breakdown of the major parts of async closures:
>
> - <https://rustc-dev-guide.rust-lang.org/coroutine-closures.html>

The key change is [compiler/rustc_builtin_macros/src/standard_library_imports.rs](https://github.com/rust-lang/rust/pull/139493/files#diff-be08752823b8f862bb0c7044ef049b0f4724dbde39306b98dea2adb82ec452b0) removing the macro_use inject and the `v1.rs` preludes now explicitly `pub use`ing the macros https://github.com/rust-lang/rust/pull/139493/files#diff-a6f9f476d41575b19b399c6d236197355556958218fd035549db6d584dbdea1d + https://github.com/rust-lang/rust/pull/139493/files#diff-49849ff961ebc978f98448c8990cf7aae8e94cb03db44f016011aa8400170587.

### Coverage

> Summarize the test coverage of this feature.
>
> Consider what the "edges" of this feature are. We're particularly interested in seeing tests that assure us about exactly what nearby things we're not stabilizing. Tests should of course comprehensively demonstrate that the feature works. Think too about demonstrating the diagnostics seen when common mistakes are made and the feature is used incorrectly.
>
> Within each test, include a comment at the top describing the purpose of the test and what set of invariants it intends to demonstrate. This is a great help to our review.
>
> Describe any known or intentional gaps in test coverage.
>
> Contextualize and link to test folders and individual tests.

A variety of UI tests including edge cases have been added.

### Outstanding bugs

> What outstanding bugs involve this feature? List them. Should any block the stabilization? Discuss why or why not.

An old bug is made more noticeable by this change #145577 but it was recommended to not block on it #139493 (comment).

### Outstanding FIXMEs

> What FIXMEs are still in the code for that feature and why is it OK to leave them there?

```
// Turn ambiguity errors for core vs std panic into warnings.
// FIXME: Remove with lang team approval.
```

https://github.com/rust-lang/rust/pull/139493/files#diff-c046507afdba3b0705638f53fffa156cbad72ed17aa01d96d7bd1cc10b8d9bce

### Tool changes

> What changes must be made to our other tools to support this feature. Has this work been done? Link to any relevant PRs and issues.

- ~~rustfmt~~
- ~~rust-analyzer~~
- ~~rustdoc (both JSON and HTML)~~
- ~~cargo~~
- ~~clippy~~
- ~~rustup~~
- ~~docs.rs~~

No known changes needed or expected.

### Breaking changes

> If this stabilization represents a known breaking change, link to the crater report, the analysis of the crater report, and to all PRs we've made to ecosystem projects affected by this breakage. Discuss any limitations of what we're able to know about or to fix.

Breaking changes:

* It's possible for user code to invoke an ambiguity by defining their own macros with standard library names and glob importing them, e.g. `use nom::*` importing `nom::dbg`. In practice this happens rarely based on crater data. The 3 public crates where this was an issue, have been fixed. The ambiguous panic import is more common and affects a non-trivial amount of the public - and likely private - crate ecosystem. To avoid a breaking change, a new future incompatible lint was added ambiguous_panic_imports see #147319. This allows current code to continue compiling, albeit with a new warning. Future editions of Rust make this an error and future versions of Rust can choose to make this error. Technically this is a breaking change, but crater gives us the confidence that the impact will be at worst a new warning for 99+% of public and private crates.

  ```rust
  #![no_std]
  extern crate std;
  use std::prelude::v1::*;
  fn xx() {
      panic!(); // resolves to core::panic
      //~^ WARNING `panic` is ambiguous
      //~| WARNING this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  }
  ```

* Code using `#![no_implicit_prelude]` *and* Rust edition 2015 will no longer automatically have access to the prelude macros. The following works on nightly but would stop working with this change:

  ```rust
  #![no_implicit_prelude]
  // Uncomment to fix error.
  // use std::vec;
  fn main() {
      let _ = vec![3, 6];
  }
  ```

  Inversely with this change the `panic` and `unreachable` macro will always be in the prelude even if `#![no_implicit_prelude]` is specified.

  Error matrix when using `#![no_implicit_prelude]`, ✅ means compiler passes 🚫 means compiler error:
  
  Configuration | Rust 2015 | Rust 2018+
  --------------|-----------|-----------
  Nightly (panic\|unreachable) macro | ✅ | 🚫
  PR (panic\|unreachable) macro | ✅ | ✅
  Nightly (column\|concat\|file\|line\|module_path\|stringify) macro | ✅ | ✅
  PR (column\|concat\|file\|line\|module_path\|stringify) macro | ✅ | ✅
  Nightly remaining macros | ✅ | 🚫
  PR remaining macros | 🚫 | 🚫

  Addressing this issue is deemed expensive.

Crater found no instance of this pattern in use. Affected code can fix the issue by directly importing the macros. The new behavior matches the behavior of `#![no_implicit_prelude]` in Rust editions 2018 and beyond and it's intuitive meaning.

Crater report:

- https://crater-reports.s3.amazonaws.com/pr-139493-2/index.html (latest run, but partial run)
- https://crater-reports.s3.amazonaws.com/pr-139493-1/index.html (previous full run, one fix missing)

Crater analysis:

- Discussed in breaking changes.

PRs to affected crates:

- Michael-F-Bryan/gcode-rs#57
- stbuehler/rust-ipcrypt#1
- jcreekmore/dmidecode#55

## Type system, opsem

### Compile-time checks

> What compilation-time checks are done that are needed to prevent undefined behavior?
>
> Link to tests demonstrating that these checks are being done.

N/A

### Type system rules

> What type system rules are enforced for this feature and what is the purpose of each?

N/A

### Sound by default?

> Does the feature's implementation need specific checks to prevent UB, or is it sound by default and need specific opt-in to perform the dangerous/unsafe operations? If it is not sound by default, what is the rationale?

N/A

### Breaks the AM?

> Can users use this feature to introduce undefined behavior, or use this feature to break the abstraction of Rust and expose the underlying assembly-level implementation? Describe this if so.

N/A

## Common interactions

### Temporaries

> Does this feature introduce new expressions that can produce temporaries? What are the scopes of those temporaries?

N/A

### Drop order

> Does this feature raise questions about the order in which we should drop values? Talk about the decisions made here and how they're consistent with our earlier decisions.

N/A

### Pre-expansion / post-expansion

> Does this feature raise questions about what should be accepted pre-expansion (e.g. in code covered by `#[cfg(false)]`) versus what should be accepted post-expansion? What decisions were made about this?

N/A

### Edition hygiene

> If this feature is gated on an edition, how do we decide, in the context of the edition hygiene of tokens, whether to accept or reject code. E.g., what token do we use to decide?

N/A

### SemVer implications

> Does this feature create any new ways in which library authors must take care to prevent breaking downstreams when making minor-version releases? Describe these. Are these new hazards "major" or "minor" according to [RFC 1105](https://rust-lang.github.io/rfcs/1105-api-evolution.html)?

No.

### Exposing other features

> Are there any other unstable features whose behavior may be exposed by this feature in any way? What features present the highest risk of that?

No.

## History

> List issues and PRs that are important for understanding how we got here.

- This change was asked for here #137487 (comment)

## Acknowledgments

> Summarize contributors to the feature by name for recognition and so that those people are notified about the stabilization. Does anyone who worked on this *not* think it should be stabilized right now? We'd like to hear about that if so.

More or less solo developed by @Voultapher with some help from @petrochenkov.

## Open items

> List any known items that have not yet been completed and that should be before this is stabilized.

None.