rhai/doc/src/language/keywords.md

Keywords

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The following are reserved keywords in Rhai:

Keywords	Usage	Not available under feature
true, false	Boolean constants
let, const	Variable declarations
if, else	Control flow
while, loop, for, in, continue, break	Looping
fn, private	Functions	[no_function]
return	Return values
throw	throw exceptions
import, export, as	Modules	[no_module]
Fn, call	Function pointers
type_of, print, debug, eval	Special functions

Keywords cannot be the name of a [function] or [variable], unless the relevant feature is enabled. For example, fn is a valid variable name under [no_function].