- Feature Name: N/A
- Start Date: 2022-01-19
- RFC PR: [rust-lang/rfcs#3373](https://github.com/rust-lang/rfcs/pull/3373)
- Tracking Issue: [rust-lang/rust#120363](https://github.com/rust-lang/rust/issues/120363)

## Summary
[summary]: #summary

Add a warn-by-default lint for items inside functions or expressions that
implement methods or traits that are visible outside the function or
expression. Consider ramping that lint to deny-by-default for Rust 2024, and
evaluating a hard error for 2027.

## Motivation
[motivation]: #motivation

Currently, tools cross-referencing uses and definitions (such as IDEs) must
either search inside all function bodies and other expression-containing items
to find potential definitions corresponding to uses within another function, or
not cross-reference those definitions at all.

Humans cross-referencing such uses and definitions may find themselves
similarly baffled.

This change helps humans limit the scope of their search and avoid looking for
definitions inside other functions or items, without missing any relevant
definitions. If in the future we manage to forbid it entirely within a
subsequent Rust edtion, tools will be able to rely on this as well.

## Explanation
[explanation]: #explanation

The following types of items, "expression-containing items", can contain
expressions, including the definitions of other items:
- Functions
- Closures
- The values assigned to `static` items or non-anonymous `const` items.
- The discriminant values assigned to `enum` variants

Rust will emit a warn-by-default lint for all of the following cases:
- An item nested inside an expression-containing item (through any level of
  nesting) may not define an `impl Type` block unless the `Type` is also nested
  inside the same expression-containing item.
- An item nested inside an expression-containing item (through any level of
  nesting) may not define an `impl Trait for Type` unless either the `Trait` or
  the `Type` is also nested inside the same expression-containing item.
- An item nested inside an expression-containing item (through any level of
  nesting) may not define an exported macro visible outside the
  expression-containing item (e.g. using `#[macro_export]`).

In a future edition, we may consider making this lint deny-by-default, or
eventually making it a hard error. We'll evaluate the impact on the ecosystem
and existing use cases before doing so.

The lint is considered to attach to the `impl` token of an `impl` block, or the
`macro_rules!` token of a macro definition.

## Drawbacks
[drawbacks]: #drawbacks

Some existing code makes use of this pattern, and would need to migrate to a
different pattern. In particular, this pattern may occur in macro-generated
code, or in code generated by tools like rustdoc. Making this change would
require such code and tools to restructure to meet this requirement.

## Prior art
[prior-art]: #prior-art

Other aspects of Rust's design attempt to enable local reasoning and avoid
global reasoning, including non-inference of function signatures, and not
having the function body affect non-opaque properties of `impl Trait` uses in
the signature without reflecting those properties in the signature.

## Unresolved questions
[unresolved-questions]: #unresolved-questions

We'll need a crater run to look at how widespread this pattern is in existing
code.

Should we flag these definitions in anonymous `const` items as well, or would
that produce unwanted warnings?

## Future possibilities
[future-possibilities]: #future-possibilities

If in the future Rust provides a "standalone `derive`" mechanism (e.g. `derive
Trait for Type` as a standalone definition separate from `Type`), the `impl`
produced by that mechanism would be subject to the same requirements.
