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Auto merge of #75960 - camelid:patch-6, r=jyn514

Improve docs for the `const` keyword

@rustbot modify labels: T-doc
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bors 2020-09-02 17:35:46 +00:00
commit a167485e27

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@ -107,7 +107,7 @@ mod break_keyword {}
/// Sometimes a certain value is used many times throughout a program, and it can become /// 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 /// inconvenient to copy it over and over. What's more, it's not always possible or desirable to
/// make it a variable that gets carried around to each function that needs it. In these cases, the /// make it a variable that gets carried around to each function that needs it. In these cases, the
/// `const` keyword provides a convenient alternative to code duplication. /// `const` keyword provides a convenient alternative to code duplication:
/// ///
/// ```rust /// ```rust
/// const THING: u32 = 0xABAD1DEA; /// const THING: u32 = 0xABAD1DEA;
@ -115,10 +115,12 @@ mod break_keyword {}
/// let foo = 123 + THING; /// let foo = 123 + THING;
/// ``` /// ```
/// ///
/// Constants must be explicitly typed, unlike with `let` you can't ignore its type and let the /// Constants must be explicitly typed; unlike with `let`, you can't ignore their type and let the
/// compiler figure it out. Any constant value can be defined in a const, which in practice happens /// compiler figure it out. Any constant value can be defined in a `const`, which in practice happens
/// to be most things that would be reasonable to have a constant (barring `const fn`s). For /// to be most things that would be reasonable to have in a constant (barring `const fn`s). For
/// example, you can't have a File as a `const`. /// example, you can't have a [`File`] as a `const`.
///
/// [`File`]: crate::fs::File
/// ///
/// The only lifetime allowed in a constant is `'static`, which is the lifetime that encompasses /// The only lifetime allowed in a constant is `'static`, which is the lifetime that encompasses
/// all others in a Rust program. For example, if you wanted to define a constant string, it would /// all others in a Rust program. For example, if you wanted to define a constant string, it would
@ -128,7 +130,7 @@ mod break_keyword {}
/// const WORDS: &'static str = "hello rust!"; /// const WORDS: &'static str = "hello rust!";
/// ``` /// ```
/// ///
/// Thanks to static lifetime elision, you usually don't have to explicitly use 'static: /// Thanks to static lifetime elision, you usually don't have to explicitly use `'static`:
/// ///
/// ```rust /// ```rust
/// const WORDS: &str = "hello convenience!"; /// const WORDS: &str = "hello convenience!";
@ -136,19 +138,19 @@ mod break_keyword {}
/// ///
/// `const` items looks remarkably similar to `static` items, which introduces some confusion as /// `const` items looks remarkably similar to `static` items, which introduces some confusion as
/// to which one should be used at which times. To put it simply, constants are inlined wherever /// to which one should be used at which times. To put it simply, constants are inlined wherever
/// they're used, making using them identical to simply replacing the name of the const with its /// they're used, making using them identical to simply replacing the name of the `const` with its
/// value. Static variables on the other hand point to a single location in memory, which all /// value. Static variables, on the other hand, point to a single location in memory, which all
/// accesses share. This means that, unlike with constants, they can't have destructors, and act as /// accesses share. This means that, unlike with constants, they can't have destructors, and act as
/// a single value across the entire codebase. /// a single value across the entire codebase.
/// ///
/// Constants, as with statics, should always be in SCREAMING_SNAKE_CASE. /// Constants, like statics, should always be in `SCREAMING_SNAKE_CASE`.
/// ///
/// The `const` keyword is also used in raw pointers in combination with `mut`, as seen in `*const /// The `const` keyword is also used in raw pointers in combination with `mut`, as seen in `*const
/// T` and `*mut T`. More about that can be read at the [pointer] primitive part of the Rust docs. /// 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] /// For more detail on `const`, see the [Rust Book] or the [Reference].
/// ///
/// [pointer]: primitive.pointer.html /// [pointer primitive]: primitive.pointer.html
/// [Rust Book]: /// [Rust Book]:
/// ../book/ch03-01-variables-and-mutability.html#differences-between-variables-and-constants /// ../book/ch03-01-variables-and-mutability.html#differences-between-variables-and-constants
/// [Reference]: ../reference/items/constant-items.html /// [Reference]: ../reference/items/constant-items.html