Module name 

What may spell a variable name.

A name is an identifier under UAX #31 — XID_Start then XID_Continue, plus _ — widened with emoji, and closed against characters that do not show themselves.

The rule behind all three parts is that a reader must be able to see what the name is. café and 名前 are visible. 😁 is visible. A non-breaking space is not: a\u{a0}b renders as a b and is one name that looks like two words. A zero-width space is worse — a\u{200b}b renders as ab and is a different variable from ab. A right-to-left override reorders the text around it, so the source shows an order the parser does not see. Each of those is rejected, and the error names the character.

A name that mixes scripts is a different problem: every character shows itself, and the name still reads as something it is not. PАTH — with CYRILLIC CAPITAL LETTER A where Latin A belongs — binds a second variable and leaves $PATH alone. That one is a warning, not a refusal (mixed_script), because refusing it would refuse 変数x and every other name a writing system spells in two scripts.

Structs

MixedScript

A name spelled in more than one script, and the character that shows it.

NameError

Why a name was refused, carrying the character that caused it.

Enums

NameErrorKind

The class of character that made a name unreadable.

Functions

is_name_continue

May this character continue a name? Joiners are accepted here and checked for context by validate — a character class alone cannot see what came before it.

is_name_start

May this character begin a name?

mixed_script

Is this name spelled in more than one script?

validate

Check a whole name, returning the first character that makes it unreadable.