> As for treating Latin-1 as a raw encoding, how can that be theoretically
> okay if the parser assumes UTF-8 and Latin-1 is not a superset of Latin-1?

The parser doesn't assume UTF-8, but "ascii+", i.e. it passes all
non-ASCII bytes on to the AST, which then needs to deal with them;
it then could (but apparently doesn't) take into account whether the
internal representation was UTF-8 or Latin-1: see ast.c:decode_unicode
for some remains of that.

The other case (besides string literals) where bytes > 127 matter is
tokenizer.c:verify_identifier; this indeed assumes UTF-8 only (but
could be easily extended to support Latin-1 as well).

The third case where non-ASCII bytes are allowed is comments; there
they are entirely ignored (i.e. it is not even verified that the
comment is well-formed UTF-8).

Removal of the special case should simplify the code; I would agree
that any speedup gained by not going through a codec is irrelevant.
I'm still puzzled why test_imp if the special case is removed.