Failure to parse "value as Type[index]"

[CensoredUsername]

Summary:

Parsing value as Type[index] leads to the error:

expected one of `!`, `(`, `::`, `;`, or `<`, found `[`

It is currently impossible for a cast expression to have a trailing index expression without surrounding the cast expression by parenthesis. This however seems to be a parser limitation, not a grammar limitation.

As currently the macro language guarantees that ty/path expressions can be followed by a "[", this seems like a bug in the parser to me, as it seems to be parsing it as value as (Type[index]) which makes no grammatical sense, compared to parsing it as (value as Type)[index] which should be valid if I read the lexical structure specification correctly.

rustc --version
>>> rustc 1.12.0-nightly (576f76659 2016-08-09)

[oli-obk]

I think it's parsed correctly, but should get a tailored error message to suggest surrounding the cast with parens.

[CensoredUsername]

Considering that a trailing "[" is never valid in the type syntax (otherwise macros break) and the fact that foo as Type * bar and variants are perfectly legal (even > which is another symbol that occurs in type syntax).

But if a tailored error message will be made instead, then this should also include other postfix operators like val as Type . attr which are currently rejected even though technically being legal by the lexical grammar specification. (another one is the proposed "?" operator)

[Mark-Simulacrum]

This is still lacking a tailored error message, and as far as I know no movement has been made here.

[daboross]

Submitted a pull request to fix this! #68985

[CensoredUsername]

Thanks!