`unstable feature` error needs to specify feature names for better usability.

[nejucomo]

Synopsis: The unstable feature compiler error does not specify the explicit feature, so when many are enabled in a single line, the user cannot tell which subset are unstable.

What I did: Tried to build rust-peg with rustc 1.0.0-beta (9854143cb 2015-04-02) (built 2015-04-02)

What I expected: A specific error message that let's me know which unstable features I must explicitly enable in order to build rust-peg.

What happened: I got the following error (rust-peg ticket #72):

src/peg_syntax_ext.rs:1:1: 1:107 error: unstable feature
src/peg_syntax_ext.rs:1 #![feature(plugin_registrar, quote, box_syntax, core, collections, rustc_private, box_patterns, str_char)]
                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
note: this feature may not be used in the beta release channel
error: aborting due to previous error
Could not compile `peg`.

Which feature(s) are unstable? The error message needs to be more specific, otherwise I need to try each combination in the superset. :-<

Meta:

$ rustc --version --verbose
rustc 1.0.0-beta (9854143cb 2015-04-02) (built 2015-04-02)
binary: rustc
commit-hash: 9854143cba679834bc4ef932858cd5303f015a0e
commit-date: 2015-04-02
build-date: 2015-04-02
host: x86_64-unknown-linux-gnu
release: 1.0.0-beta

[steveklabnik]

Ah ha! So, until right before beta, we used to say "use feature flag x to enable this feature", but since that doesn't work in beta, we removed it, meaning we lost this bit of info.

[huonw]

All features are unstable in beta, I believe the compiler is trying to say that the feature attribute itself is illegal.

[steveklabnik]

While that's true, the wording can certainly be better.

[hgrecco]

I was going to report the same thing (even with the same library, peg). In my ignorance I tried to find out which "feature" was not available in beta.

[Jfromchina]

I encountered the same problem, but my version is Release1.0.0, is it the latest version? which one is stable Release?
$ rustc main.rs
main.rs:1:1: 1:25 error: unstable feature
main.rs:1 #![feature(collections)]
^~~~~~~~~~~~~~~~~~~~~~~~
note: this feature may not be used in the stable release channel
error: aborting due to previous error

$ rustc --version --verbose
rustc 1.0.0 (a59de37 2015-05-13) (built 2015-05-14)
binary: rustc
commit-hash: a59de37
commit-date: 2015-05-13
build-date: 2015-05-14
host: x86_64-unknown-linux-gnu
release: 1.0.0

[steveklabnik]

@Jfromchina that is stable, yes. If it were beta, it would say 1.0.0-beta, or nightly, 1.0.0-nightly.

[steveklabnik]

Nominating. While it's sort of a papercut, it's also pretty poor UX.

[nikomatsakis]

cc @brson

[nikomatsakis]

triage: P-medium

[brson]

This is an easy matter of modifying the emit_feature_err function.