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Rollup merge of #77838 - RalfJung:const-fn, r=kennytm 

const keyword: brief paragraph on 'const fn'

`const fn` were mentioned in the title, but called "deterministic functions" which is not their main property (though at least currently it is a consequence of being const-evaluable). This adds a brief paragraph discussing them, also in the hopes of clarifying that they do *not* have any effect on run-time uses.
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Yuki Okushi 2020-10-20 12:11:02 +09:00 committed by GitHub
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library/std/src/keyword_docs.rs
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@ -102,7 +102,9 @@ mod break_keyword {}
#[doc(keyword = "const")]
//
/// Compile-time constants and deterministic functions.
/// Compile-time constants and compile-time evaluable functions.
///
/// ## Compile-time constants
///
/// Sometimes a certain value is used many times throughout a program, and it can become
/// inconvenient to copy it over and over. What's more, it's not always possible or desirable to
@ -145,15 +147,28 @@ mod break_keyword {}
///
/// Constants, like statics, should always be in `SCREAMING_SNAKE_CASE`.
///
/// For more detail on `const`, see the [Rust Book] or the [Reference].
///
/// ## Compile-time evaluable functions
///
/// The other main use of the `const` keyword is in `const fn`. This marks a function as being
/// callable in the body of a `const` or `static` item and in array initializers (commonly called
/// "const contexts"). `const fn` are restricted in the set of operations they can perform, to
/// ensure that they can be evaluated at compile-time. See the [Reference][const-eval] for more
/// detail.
///
/// Turning a `fn` into a `const fn` has no effect on run-time uses of that function.
///
/// ## Other uses of `const`
///
/// The `const` keyword is also used in raw pointers in combination with `mut`, as seen in `*const
/// T` and `*mut T`. More about `const` as used in raw pointers can be read at the Rust docs for the [pointer primitive].
///
/// For more detail on `const`, see the [Rust Book] or the [Reference].
///
/// [pointer primitive]: primitive.pointer.html
/// [Rust Book]:
/// ../book/ch03-01-variables-and-mutability.html#differences-between-variables-and-constants
/// [Reference]: ../reference/items/constant-items.html
/// [const-eval]: ../reference/const_eval.html
mod const_keyword {}
#[doc(keyword = "continue")]