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Document sub-namespaces. #1043

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Jun 19, 2021
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Document sub-namespaces. #1043

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16 changes: 16 additions & 0 deletions src/names/namespaces.md
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Expand Up @@ -104,24 +104,40 @@ imported.

<!-- TODO: describe how `use` works on the use-declarations page, and link to it here. -->

## Sub-namespaces

The macro namespace is split into two sub-namespaces: one for [bang-style macros] and one for [attributes].
When an attribute is resolved, any bang-style macros in scope will be ignored.
And conversely resolving a bang-style macro will ignore attribute macros in scope.
This prevents one style from shadowing another.

For example, the [`cfg` attribute] and the [`cfg` macro] are two different entities with the same name in the macro namespace, but they can still be used in their respective context.

It is still an error for a [`use` import] to shadow another macro, regardless of their sub-namespaces.

[^rustc-lifetime-shadow]: `rustc` currently warns about shadowing when using
the same name for a label and lifetime in the same scope, but it still
treats them independently. This is intended as a future-compatibility
warning about a possible extension to the language. See [PR
#24162](https://github.com/rust-lang/rust/pull/24162).

[`cfg` attribute]: ../conditional-compilation.md#the-cfg-attribute
[`cfg` macro]: ../conditional-compilation.md#the-cfg-macro
[`for`]: ../expressions/loop-expr.md#iterator-loops
[`if let`]: ../expressions/if-expr.md#if-let-expressions
[`let`]: ../statements.md#let-statements
[`macro_rules` declarations]: ../macros-by-example.md
[`match`]: ../expressions/match-expr.md
[`Self` constructors]: ../paths.md#self-1
[`Self` type]: ../paths.md#self-1
[`use` import]: ../items/use-declarations.md
[`while let`]: ../expressions/loop-expr.md#predicate-pattern-loops
[Associated const declarations]: ../items/associated-items.md#associated-constants
[Associated function declarations]: ../items/associated-items.md#associated-functions-and-methods
[Associated type declarations]: ../items/associated-items.md#associated-types
[Attribute macros]: ../procedural-macros.md#attribute-macros
[attributes]: ../attributes.md
[bang-style macros]: ../macros.md
[boolean]: ../types/boolean.md
[Built-in attributes]: ../attributes.md#built-in-attributes-index
[closure parameters]: ../expressions/closure-expr.md
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