In a declarative macro matcher, expr matches one Rust expression and binds it to a macro variable such as $x. The dollar sign distinguishes the metavariable from a literal token. section 2.1.4
Use expression matchers to accept computed values while keeping the macro pattern more specific than tt:
- Match $x:expr, push the captured value into a vector, and return the vector. section 2.1.4
- Prefer expr when the macro expects an expression; reserve the broader tt matcher for cases that genuinely require almost any token sequence. section 2.1.4
- Wrap multiple statements and the final value in braces when the macro must expand to one expression usable in a let binding. section 2.2.2
Capturing one expression
A matcher such as {$x:expr} can expand into a block that creates a vector, pushes $x, and returns v. Calling it with 1 produces a vector containing 1; the block remains a single expression. section 2.1.4 section 2.2.2
{$x:expr} => {
{
let mut v = Vec::new();
v.push($x);
v
}
};
Inspect the matcher and transcriber that capture an expression, perform statements inside a block, and return the resulting vector. section 2.1.4
