add option of outputting doc attributes to crates + a doc fetching tool

[thestinger]

Similar to Python's pydoc tool, it should just take a path and resolve it to the correct docstring. The REPL could also take advantage of this with a :help meta-command.

[pnkfelix]

The word "path" in the description could refer to a path on the filesystem denoting a whole crate (i.e. one provides the library file for the crate to the tool), or "path" could refer to a path in a Rust program, denoting some item within a crate. (It could also refer to some combination of the two notions, but I will leave that possibility aside.)

Both interpretations sound potentially useful. I suspect only the latter interpretation is consistent with your use-case of putting this into a REPL :help meta-command.

In the former interpretation, I imagine the docstring extractor would just be another command-line utility program.

In the latter interpretation, the docstring extractor sounds like it would have to be some sort of operation one expresses within Rust programs. (It is not quite a reflection facility, since it is only extracting statically available meta-data, not runtime values.)

Regarding the latter interpretation, I am a little concerned about these sentences from the Rust manual:

Every item has a canonical path within its crate, but the path naming an item is only meaningful within a given
crate. There is no global namespace across crates; an item's canonical path merely identifies it within the crate.

This implies to me that one would want to resolve a path with respect to a particular crate (provided as an additional argument) when doing doc-string extraction; but that contradicts the interface from the first sentence of the description. (That, or implicitly use the current crate as the basis for the resolution; I cannot tell if always using the current crate would suffice for the cases that thestinger is anticipating.)

@thestinger Would you mind elaborating a little bit more about which of the above cases you were asking for? It will help me determine where this falls in our milestone scheme.

[emberian]

docstrings can be fetched from the JSON output of rustdoc_ng, but if the repl is already linking to the crates, it can just look for the attribute directly (though it will need to do a bit of a dance to get the item from the AST). untagging rustdoc.

[thestinger]

(accidentally marked this bug for closure, ignore the mentions here - has been amended)

[steveklabnik]

I wonder what a Cargo subcommand for this would be like.

[steveklabnik]

I'm pulling a massive triage effort to get us ready for 1.0. As part of this, I'm moving stuff that's wishlist-like to the RFCs repo, as that's where major new things should get discussed/prioritized.

This issue has been moved to the RFCs repo: rust-lang/rfcs#650