Reserved words

These identifiers are reserved. None of them can be used as a variable, a parameter, a field, or a function name, including in an FFI signature.

Args        arch        as          blockexit   break
class       closure     const       constant    constructor
continue    danger_cast else        enum        export
external    false       fn          for         from
gcsafe      hide        if          impl        import
in          let         link        module      mutates
new         notrack     null        opaque      operator
platform    private     public      return      serial
state       static      style       super       switch
trait       true        variadic    while

The ones that catch people

Three of these read like ordinary nouns and are the usual cause of a confusing parse failure:

• fn, which is tempting as a shorthand parameter name for a function or a file name.
• in, which is natural for a parameter meaning input.
• arch, which is natural for a variable holding a processor architecture.

A reserved word used as an identifier does not produce one clear error. The parse fails at that point and the failure cascades, so the file reports many unrelated-looking errors elsewhere. If a small edit suddenly produces a large number of errors, look for a newly introduced reserved word before reading any of them.