How deeply does Learn Rust in a Month of Lunches cover fragment specifier?

Explains matching with expression and statement examples, then lists the available specifiers without requiring memorization.

Fragment specifier

As explained in 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.

A fragment specifier tells a Rust macro what syntactic category a metavariable may match. In `$input:expr`, `expr` accepts an expression; in `$input:stmt`, `stmt` accepts a statement. section 22.2

Expression versus statement

A macro expecting `$input:expr` rejects `let x = 9` with a “no rules expected” error. Changing the specifier to `$input:stmt` makes that input match. The compiler feedback identifies both the unexpected token and the metavariable pattern being matched. section 22.2

block | expr | ident | item | lifetime | literal  | meta | pat | path 
| stmt | tt | ty | vis.

Browse the available fragment specifiers, including `expr`, `ident`, `stmt`, `tt`, `ty`, and others. section 22.2

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Write Powerful Rust MacrosA fragment specifier is the annotation in a declarative macro matcher that selects the syntactic category accepted by a metavariable.

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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.

This explainer was generated by Manning's knowledge graph from the text of Learn Rust in a Month of Lunches. The section links point to the parts of the book it draws on.