nvim-treesitter/queries/rust/injections.scm

(macro_invocation
  (token_tree) @injection.content
  (#set! injection.language "rust")
  (#set! injection.include-children))

(macro_definition
  (macro_rule
    left: (token_tree_pattern) @injection.content
    (#set! injection.language "rust")))

(macro_definition
  (macro_rule
    right: (token_tree) @injection.content
    (#set! injection.language "rust")))

([
  (line_comment)
  (block_comment)
] @injection.content
  (#set! injection.language "comment"))

((macro_invocation
  macro: (identifier) @injection.language
  (token_tree) @injection.content)
  (#any-of? @injection.language "html" "json"))

(call_expression
  function:
    (scoped_identifier
      path: (identifier) @_regex
      (#eq? @_regex "Regex")
      name: (identifier) @_new
      (#eq? @_new "new"))
  arguments:
    (arguments
      (raw_string_literal) @injection.content)
  (#set! injection.language "regex"))

(call_expression
  function:
    (scoped_identifier
      path:
        (scoped_identifier
          (identifier) @_regex
          (#eq? @_regex "Regex") .)
      name: (identifier) @_new
      (#eq? @_new "new"))
  arguments:
    (arguments
      (raw_string_literal) @injection.content)
  (#set! injection.language "regex"))

((block_comment) @injection.content
  (#match? @injection.content "/\\*!([a-zA-Z]+:)?re2c")
  (#set! injection.language "re2c"))