unused-features only warns unused non-language features

[fhahn]

For the following snippet, rustc only warns about rustc_privat being unused:

#![feature(rustc_private)]
#![feature(box_syntax)]
#![feature(trace_macros)]
#![feature(slicing_syntax)]
#![feature(log_syntax)]

fn main() {}

According to the documentation, check_unused_or_stable_features only checks unused non-langauge features, but it is the only place that emits UNSUSED_FEATURES.

[fhahn]

I would like to work on a patch for this issue. Would it make sense to collect language feature usage in synatx::feature_gate::PostExpansionVisitor and check for unused language features in ``synatx::feature_gate::check_crate`?

[steveklabnik]

Triage: today, this says that slicing_syntax is stable, but only complains about rustc_private. Removing that and leaving box_syntax, trace_macros, and log_syntax actually compiles with no warnings whatsoever, even though they don't appear to be used?

[leonardo-m]

Some more undetected ones:

#![feature(slice_patterns)]
#![feature(inclusive_range_syntax)]
#![feature(type_ascription)]
#![feature(asm)]
#![feature(stmt_expr_attributes)]
#![feature(conservative_impl_trait)]
#![feature(const_fn)]

[bjorn3]

When enabling incremental compilation we removed the global state that contained all used unstable features and thus had to remove the unused features warning.