rust fragment specifier

Specifier	Description	Examples
ident	Identifier	x, foo
path	Qualified name	std::collection::HashSet, Vec::new
ty	Type	i32, &T, Vec<(char, String)>
expr	Expression	2+2, f(42), if true { 1 } else
pat	Pattern	_, c @ 'a' ... 'z', (true, &x), Badger
stmt	Statement	let x = 3, return 42
block	Brace-delimited block	{ foo(); bar(); },
item	Item	fn foo() {}, struct Bar;, use std::io;
meta	Inside of attribute	cfg!(windows), doc="comment"
tt	Token tree	+, foo, 5, [?!(???)]

原文：https://riptutorial.com/rust/example/5646/fragment-specifiers---kind-of-patterns