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Add #[allow_internal_unstable] to track stability for macros better. #22899

Merged
merged 4 commits into from
Mar 6, 2015
Merged

Add #[allow_internal_unstable] to track stability for macros better. #22899

merged 4 commits into from
Mar 6, 2015

Commits on Mar 5, 2015

  1. Add #[allow_internal_unstable] to track stability for macros better. 

    Unstable items used in a macro expansion will now always trigger
stability warnings, *unless* the unstable items are directly inside a
macro marked with `#[allow_internal_unstable]`. IOW, the compiler warns
unless the span of the unstable item is a subspan of the definition of a
macro marked with that attribute.

E.g.

    #[allow_internal_unstable]
    macro_rules! foo {
        ($e: expr) => {{
            $e;
            unstable(); // no warning
            only_called_by_foo!();
        }}
    }

    macro_rules! only_called_by_foo {
        () => { unstable() } // warning
    }

    foo!(unstable()) // warning

The unstable inside `foo` is fine, due to the attribute. But the
`unstable` inside `only_called_by_foo` is not, since that macro doesn't
have the attribute, and the `unstable` passed into `foo` is also not
fine since it isn't contained in the macro itself (that is, even though
it is only used directly in the macro).

In the process this makes the stability tracking much more precise,
e.g. previously `println!("{}", unstable())` got no warning, but now it
does. As such, this is a bug fix that may cause [breaking-change]s.

The attribute is definitely feature gated, since it explicitly allows
side-stepping the feature gating system.
    @huonw
    huonw committed Mar 5, 2015
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  2. Use #[allow_internal_unstable] for thread_local! 

    This destabilises all the implementation details of `thread_local!`,
since they do not *need* to be stable with the new attribute.
    @huonw
    huonw committed Mar 5, 2015
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  3. Add more debugging to syntax::feature_gate.

    @huonw
    huonw committed Mar 5, 2015
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  4. Run feature-gating on the final AST passed to the compiler. 

    This ensures we catch everything; previously, an unknown attribute
inserted by #[cfg_attr(...)] in a macro expansion would not be detected.
    @huonw
    huonw committed Mar 5, 2015
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