This partially implements the feature staging described in the
[release channel RFC][rc]. It does not yet fully conform to the RFC as
written, but does accomplish its goals sufficiently for the 1.0 alpha
release.
It has three primary user-visible effects:
* On the nightly channel, use of unstable APIs generates a warning.
* On the beta channel, use of unstable APIs generates a warning.
* On the beta channel, use of feature gates generates a warning.
Code that does not trigger these warnings is considered 'stable',
modulo pre-1.0 bugs.
Disabling the warnings for unstable APIs continues to be done in the
existing (i.e. old) style, via `#[allow(...)]`, not that specified in
the RFC. I deem this marginally acceptable since any code that must do
this is not using the stable dialect of Rust.
Use of feature gates is itself gated with the new 'unstable_features'
lint, on nightly set to 'allow', and on beta 'warn'.
The attribute scheme used here corresponds to an older version of the
RFC, with the `#[staged_api]` crate attribute toggling the staging
behavior of the stability attributes, but the user impact is only
in-tree so I'm not concerned about having to make design changes later
(and I may ultimately prefer the scheme here after all, with the
`#[staged_api]` crate attribute).
Since the Rust codebase itself makes use of unstable features the
compiler and build system to a midly elaborate dance to allow it to
bootstrap while disobeying these lints (which would otherwise be
errors because Rust builds with `-D warnings`).
This patch includes one significant hack that causes a
regression. Because the `format_args!` macro emits calls to unstable
APIs it would trigger the lint. I added a hack to the lint to make it
not trigger, but this in turn causes arguments to `println!` not to be
checked for feature gates. I don't presently understand macro
expansion well enough to fix. This is bug #20661.
Closes#16678
[rc]: https://github.com/rust-lang/rfcs/blob/master/text/0507-release-channels.md
There's been some debate over the precise form that these APIs should take, and
they've undergone some changes recently, so these APIs are going to be left
unstable for now to be fleshed out during the next release cycle.
fmt::Show is for debugging, and can and should be implemented for
all public types. This trait is used with `{:?}` syntax. There still
exists #[derive(Show)].
fmt::String is for types that faithfully be represented as a String.
Because of this, there is no way to derive fmt::String, all
implementations must be purposeful. It is used by the default format
syntax, `{}`.
This will break most instances of `{}`, since that now requires the type
to impl fmt::String. In most cases, replacing `{}` with `{:?}` is the
correct fix. Types that were being printed specifically for users should
receive a fmt::String implementation to fix this.
Part of #20013
[breaking-change]
parameters on impls must now also appear in the trait ref, self type,
or some associated type declared on the impl. This ensures that they
are constrianed in some way and that the semantics of the trait system
are well-defined (always a good thing).
There are three major ways to fix this error:
1. Convert the trait to use associated types; most often the type
parameters are not constrained because they are in fact outputs of
the impl.
2. Move the type parameters to methods.
3. Add an additional type parameter to the self type or trait so that
the unused parameter can appear there.
In some cases, it is not possible to fix the impl because the trait
definition needs to be changed first (and that may be out of your
control). In that case, for the time being, you can opt out of these
rules by using `#[old_impl_check]` on the impl and adding a
`#![feature(old_impl_check)]` to your crate declaration.
See RFC 550 (https://github.com/rust-lang/rfcs/pull/550) for the motivation
and details.
If this breaks your code, add one of the listed tokens after the relevant
non-terminal in your matcher.
[breaking-change]
These aren't in their final form, but are all aiming to be part of 1.0, so at the very least encouraging usage now to find the bugs is nice.
Also, the widespread roll-out of associated types in the standard library indicates they're getting good, and it's lame to have to activate a feature in essentially every crate ever.
Fixes#17904. All the cases that I believe we should support are detailed in the test case, let me know if there is there is any more desired behavior. cc @japaric.
r? @nikomatsakis or whoever is appropriate.
macro_rules! is like an item that defines a macro. Other items don't have a
trailing semicolon, or use a paren-delimited body.
If there's an argument for matching the invocation syntax, e.g. parentheses for
an expr macro, then I think that applies more strongly to the *inner*
delimiters on the LHS, wrapping the individual argument patterns.
