Filter doc comments from macro input

[Binero]

This seems to be a very inconsistant issue, but I think it's related to macros.

Here's an example of a list of comments that don't get parsed properly:

bitflags! {
    flags Tests: Test {
        /// Test
        static TestA = 0x80,
        // TODO: Document this.
        static TestB          = 0x40
    }
}

This code will error saying "no rules expected the token /// Test".
http://is.gd/Zkpgvd

[lifthrasiir]

That's mainly because /// foo is a separate token equivalent to #[doc="foo"] unlike comments (no token sent to the macro). Not sure if bitflags! can account for this, but it still highlights the limitation of the current macro parser.

[pczarn]

Possibly at odds with or duplicate of #14197
To be fixed in #17830

@lifthrasiir Perhaps the macro parser shouldn't be burdened with such details. #17830 starts expanding the DOC_COMMENT token into the equivalent sequence in tokentree reader.

[steveklabnik]

#17830 was merged, so I think this has been fixed.