Bug: ambiguous + detection for Trait and lifetime bounds is broken

[ruabmbua]

Example: playground
Related: issue #279

There are multiple cases, where the ambiguity of + is not detected. Note, that the analysis is inside libsyntax (e.g. rustfmt throws the ambiguous + error in all cases). This leads to code being able to pass the syntax check, and yielding incorrect results.

List of cases, where additional bounds via + are invalid (only the ones I could find / did know):

• shared & mutable reference types
• const and mutable raw pointer types
• Closure return type position (these are handled wrong in libsyntax according to language spec)
• bare function return type position (I think libsyntax got that one right)
• and of course the type cast expression, which is mentioned in issue Bug: operator precedence around **as** keyword is wrong #279 (the fix should probably be replaced by a more general solution)

This should be solved by properly implementing TypeNoBounds as a sub parser for all type expressions.

TypeNoBounds can then be invoked in all of these 5 cases. Note, that the allow_bound mechanism in the path_() parser should probably be replaced by something else. E.g. by using TypeParamBounds in all the correct places and implementing it correctly.

[ruabmbua]

I will try to create a PR, but this might be a little bit over my head.

[matklad]

@ruabmbua I think the best way to fix this might be not in the parser itself, but in the validation layer. That is, after the parsig is done, you traverse the tree and check that all disambiguating parenthesis are present.

[ruabmbua]

A second pass seems like a good idea.

Maybe I can find some time to test this strategy.