Fragment specifier
A fragment specifier is a declarative-macro matcher annotation that specifies the syntactic category of tokens a metavariable can match.
Write Powerful Rust Macros
A fragment specifier is the annotation in a declarative macro matcher that selects the syntactic category accepted by a metavariable.
- How it is used
- Selects the syntactic categories accepted by metavariables in declarative macro matchers.
- How deep it goes
- Explains common categories and the precision trade-off between concrete specifiers and TokenTree matching.
- Examples
- Uses expression capture to build vectors and contrasts concrete fragment matching with broad TokenTree matching.
- What you can do after
- Apply via declarative macros by choosing fragment specifiers that constrain inputs and testing broader TokenTree matchers carefully.
Read how Write Powerful Rust Macros explains fragment specifier →
Learn Rust in a Month of Lunches
A fragment specifier is a macro pattern annotation that declares what syntactic category of token input a metavariable can match.
- How it is used
- Guides macro input matching by restricting metavariables to chosen syntactic categories.
- How deep it goes
- Explains matching with expression and statement examples, then lists the available specifiers without requiring memorization.
- Examples
- Uses `$input:expr` with values and vectors, contrasts it with `$input:stmt`, and demonstrates `ident` and `tt` patterns.
- What you can do after
- Build macro_rules! patterns that accept appropriately categorized input and interpret compiler matching errors.
Read how Learn Rust in a Month of Lunches explains fragment specifier →
Related concepts
- metavariableequivalent to · A metavariable is a declarative-macro binding whose fragment specifier determines the syntactic category of captured input.
- type fragment specifierincludes · A type fragment specifier is a declarative-macro matcher specifier that matches Rust type syntax.
- macroconfigures · A macro is a named construct that expands or transforms source code, commands, or templates before or during execution.
- token treeused by · A token tree is a recursive Rust macro syntax structure consisting of tokens and nested delimited sequences of tokens.
- expression fragment specifiercontrasts with · An expression fragment specifier is a Rust declarative-macro matcher annotation that matches syntax representing an expression.
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