Proposal: strict ordering of PtrTypeStart and ArrayTypeStart qualifiers

[andersfr]

The pointer and array types can be qualified with align, const, volatile and allowzero.

In the current grammar they are specified as any amount and no specific ordering. The reality is that they can only occur once.

Having relaxed ordering gives no particular benefit to the language and can potentially confuse users into believing the order carries meaning.

Parsing can also be performed more efficiently with strict ordering.

I propose that we fix the order changing them into the following:

PrefixTypeOp
     / ArrayTypeStart KEYWORD_allowzero? ByteAlign? KEYWORD_const? KEYWORD_volatile?
     / PtrTypeStart KEYWORD_allowzero? (KEYWORD_align LPAREN Expr (COLON INTEGER COLON INTEGER)? RPAREN)? KEYWORD_const? KEYWORD_volatile?

EDIT: Moved KEYWORD_allowzero? first to be consistent with zig fmt

[hryx]

See a brief previous discussion of this here: #2405 (comment)

[andersfr]

If you run your code through zig fmt you will automatically get a strict ordering of these qualifiers. This is afaik the only place you will observe a reordering.

All existing code can thus be made into compliance with this chance by simply running the formatter once.

Notable examples of strict ordering are FnProto and VarDecl with both pre and post qualifiers (even more so in a TopDecl context).