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Fix nits, rework the second example of select (the one deliberately failing to compile)

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This commit is contained in:
Alexis Bourget 2020-07-24 19:33:52 +02:00
parent f8335fb1e4
commit 6af60d3e24
1 changed files with 15 additions and 33 deletions
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48
src/libstd/keyword_docs.rs
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@ -1680,19 +1680,15 @@ mod use_keyword {}
///
/// `where` can also be used for lifetimes.
///
/// This compiles because the lifetime of `longer` is superior to the lifetime
/// of `shorter`, thus the constraint is respected:
/// This compiles because `longer` outlives `shorter`, thus the constraint is
/// respected:
///
/// ```rust
/// fn select_where<'a, 'b>(s1: &'a str, s2: &'b str, second: bool) -> &'a str
/// fn select<'short, 'long>(s1: &'short str, s2: &'long str, second: bool) -> &'short str
/// where
/// 'b: 'a,
/// 'long: 'short,
/// {
/// if second {
/// s2
/// } else {
/// s1
/// }
/// if second { s2 } else { s1 }
/// }
///
/// let outer = String::from("Long living ref");
@ -1701,35 +1697,20 @@ mod use_keyword {}
/// let inner = String::from("Short living ref");
/// let shorter = &inner;
///
/// assert_eq!(select_where(shorter, longer, false), shorter);
/// assert_eq!(select_where(shorter, longer, true), longer);
/// assert_eq!(select(shorter, longer, false), shorter);
/// assert_eq!(select(shorter, longer, true), longer);
/// }
/// ```
///
/// On the other hand, this will not compile: `shorter` does not have a lifetime
/// that respects the constraint imposed by the `select_where` functions.
/// On the other hand, this will not compile because the `where 'b: 'a` clause
/// is missing: the `'b` lifetime is not known to live at least as long as `'a`
/// which means this function cannot ensure it always returns a valid reference:
///
/// ```rust,compile_fail,E0597
/// # fn select_where<'a, 'b>(s1: &'a str, s2: &'b str, second: bool) -> &'a str
/// # where
/// # 'b: 'a,
/// # {
/// # if second {
/// # s2
/// # } else {
/// # s1
/// # }
/// # }
/// let outer = String::from("Long living ref");
/// let longer = &outer;
/// let res;
/// ```rust,compile_fail,E0623
/// fn select<'a, 'b>(s1: &'a str, s2: &'b str, second: bool) -> &'a str
/// {
/// let inner = String::from("Short living ref");
/// let shorter = &inner;
///
/// res = select_where(longer, shorter, false);
/// if second { s2 } else { s1 }
/// }
/// assert_eq!(res, &outer);
/// ```
///
/// `where` can also be used to express more complicated constraints that cannot
@ -1749,7 +1730,8 @@ mod use_keyword {}
/// ```
///
/// `where` is available anywhere generic and lifetime parameters are available,
/// as can be seen in the [`Cow`](crate::borrow::Cow) from the standard library:
/// as can be seen with the [`Cow`](crate::borrow::Cow) type from the standard
/// library:
///
/// ```rust
/// # #![allow(dead_code)]