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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. 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 ## allow_internal_unstable
Macros, compiler desugarings and `const fn`s expose their bodies to the call Macros, compiler desugarings and `const fn`s expose their bodies to the call