Lex nested block comments

[brson]

Currently we don't support nested block comments. This allows lexing to be regular, but it widely seen as an inconvenience. Modify the lexer to keep track of comment nesting.

Nominating backcompat.

[erickt]

I thought we were going to grow a macro to handle block quotes. Did that ever get implemented?

[brson]

@erickt I don't know what you mean by block quotes.

[catamorphism]

Accepted for feature-complete

[ben0x539]

Don't line comments address pretty much all uses of nested block comments?

[erickt]

@brson: by block quote, I mean in the style of CPP's #ifdef 0 ... #endif to allow for nesting comments. Something like this:

comment!(
fn foo(x: int) -> int {
    comment!(
    printfln!("%?", x);
    )
    x
}
)

[madjar]

From what I've gathered from the code, there are two implementations of the comment parsing, one of which supports nested comments, so I guess #9701 should be fixed before this bug.

[brson]

That's pretty funny that the pretty printer already supports nested comments. I don't think they necessarily have to be merged to fix this bug, though our lexing is a bit of a mess.

[brson]

@madjar Are you going to tackle this?

[madjar]

@brson If there's no massive refactoring to do, I can try! Expect a PR soon.

[jibal]

The reference manual says "Comments in Rust code follow the general C++ style of line and block-comment forms, with no nesting of block-comment delimiters" ... right after production rules for nested comments.

[huonw]

@jibal looks like it was forgotten to be updated, thanks for noticing! (I opened #14767.)