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Grammar: macros #458
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Original file line number | Diff line number | Diff line change |
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@@ -9,6 +9,95 @@ There are two ways to define new macros: | |
* [Macros by Example] define new syntax in a higher-level, declarative way. | ||
* [Procedural Macros] can be used to implement custom derive. | ||
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## Macro Invocation | ||
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> **<sup>Syntax</sup>**\ | ||
> _MacroInvocation_ :\ | ||
> [_SimplePath_] `!` _DelimTokenTree_ | ||
> | ||
> _DelimTokenTree_ :\ | ||
> `(` _TokenTree_<sup>\*</sup> `)`\ | ||
> | `[` _TokenTree_<sup>\*</sup> `]`\ | ||
> | `{` _TokenTree_<sup>\*</sup> `}` | ||
> | ||
> _TokenTree_ :\ | ||
> [_Token_]<sub>_except delimiters_</sub> | _DelimTokenTree_ | ||
> | ||
> _MacroInvocationSemi_ :\ | ||
> [_SimplePath_] `!` `(` _TokenTree_<sup>\*</sup> `)` `;`\ | ||
> | [_SimplePath_] `!` `[` _TokenTree_<sup>\*</sup> `]` `;`\ | ||
> | [_SimplePath_] `!` `{` _TokenTree_<sup>\*</sup> `}` | ||
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A macro invocation executes a macro at compile time and replaces the | ||
invocation with the result of the macro. Macros may be invoked in the | ||
following situations: | ||
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* [Expressions] and [statements] | ||
* [Patterns] | ||
* [Types] | ||
* [Items] including [associated items] | ||
* [`macro_rules`] transcribers | ||
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When used as an item or a statement, the _MacroInvocationSemi_ form is used | ||
where a semicolon is required at the end when not using curly braces. | ||
[Visibility qualifiers] are never allowed before a macro invocation or | ||
[`macro_rules`] definition. | ||
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```rust | ||
// Used as an expression. | ||
let x = vec![1,2,3]; | ||
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// Used as a statement. | ||
println!("Hello!"); | ||
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// Used in a pattern. | ||
macro_rules! pat { | ||
($i:ident) => (Some($i)) | ||
} | ||
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if let pat!(x) = Some(1) { | ||
assert_eq!(x, 1); | ||
} | ||
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// Used in a type. | ||
macro_rules! Tuple { | ||
{ $A:ty, $B:ty } => { ($A, $B) }; | ||
} | ||
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type N2 = Tuple!(i32, i32); | ||
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// Used as an item. | ||
# use std::cell::RefCell; | ||
thread_local!(static FOO: RefCell<u32> = RefCell::new(1)); | ||
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// Used as an associated item. | ||
macro_rules! const_maker { | ||
($t:ty, $v:tt) => { const CONST: $t = $v; }; | ||
} | ||
trait T { | ||
const_maker!{i32, 7} | ||
} | ||
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// Macro calls within macros. | ||
macro_rules! example { | ||
() => { println!("Macro call in a macro!") }; | ||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. There should be some mention that the outer macro expands to the inner macro, which is then expanded. |
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} | ||
// Outer macro `example` is expanded, then inner macro `println` is expanded. | ||
example!(); | ||
``` | ||
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[Macros by Example]: macros-by-example.html | ||
[Procedural Macros]: procedural-macros.html | ||
[_SimplePath_]: paths.html#simple-paths | ||
[_Token_]: tokens.html | ||
[associated items]: items/associated-items.html | ||
[compiler plugins]: ../unstable-book/language-features/plugin.html | ||
[delimiters]: tokens.html#delimiters | ||
[expressions]: expressions.html | ||
[items]: items.html | ||
[`macro_rules`]: macros-by-example.html | ||
[patterns]: patterns.html | ||
[statements]: statements.html | ||
[tokens]: tokens.html | ||
[types]: types.html | ||
[visibility qualifiers]: visibility-and-privacy.html |
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Could you name the relevant production in the grammar for all of these that have one?