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Introduce `rustc_const_stable` and explain `rustc_const_unstable` (#542)

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Oliver Scherer 2019-12-24 01:16:23 +01:00 committed by Mazdak Farrokhzad
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@ -49,6 +49,28 @@ marks an item as stabilized. To do this, follow the instructions in
Note that stable functions may use unstable things in their body.
## rustc_const_unstable
The `#[rustc_const_unstable(feature = "foo", issue = "1234", reason = "lorem ipsum")]`
has the same interface as the `unstable` attribute. It is used to mark
`const fn` as having their constness be unstable. This allows you to make a
function stable without stabilizing its constness or even just marking an existing
stable function as `const fn` without instantly stabilizing the `const fn`ness.
Furthermore this attribute is needed to mark an intrinsic as `const fn`, because
there's no way to add `const` to functions in `extern` blocks for now.
## rustc_const_stable
The `#[stable(feature = "foo", "since = "1.420.69")]` attribute explicitly marks
a `const fn` as having its constness be `stable`. This attribute can make sense
even on an `unstable` function, if that function is called from another
`rustc_const_stable` function.
Furthermore this attribute is needed to mark an intrinsic as callable from
`rustc_const_stable` functions.
## allow_internal_unstable
Macros, compiler desugarings and `const fn`s expose their bodies to the call