64 lines
2.3 KiB
Markdown
64 lines
2.3 KiB
Markdown
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% Lifetime Elision
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In order to make common patterns more ergonomic, Rust allows lifetimes to be
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*elided* in function signatures.
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A *lifetime position* is anywhere you can write a lifetime in a type:
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```rust
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&'a T
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&'a mut T
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T<'a>
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```
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Lifetime positions can appear as either "input" or "output":
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* For `fn` definitions, input refers to the types of the formal arguments
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in the `fn` definition, while output refers to
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result types. So `fn foo(s: &str) -> (&str, &str)` has elided one lifetime in
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input position and two lifetimes in output position.
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Note that the input positions of a `fn` method definition do not
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include the lifetimes that occur in the method's `impl` header
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(nor lifetimes that occur in the trait header, for a default method).
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* In the future, it should be possible to elide `impl` headers in the same manner.
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Elision rules are as follows:
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* Each elided lifetime in input position becomes a distinct lifetime
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parameter.
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* If there is exactly one input lifetime position (elided or not), that lifetime
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is assigned to *all* elided output lifetimes.
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* If there are multiple input lifetime positions, but one of them is `&self` or
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`&mut self`, the lifetime of `self` is assigned to *all* elided output lifetimes.
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* Otherwise, it is an error to elide an output lifetime.
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Examples:
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```rust
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fn print(s: &str); // elided
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fn print<'a>(s: &'a str); // expanded
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fn debug(lvl: uint, s: &str); // elided
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fn debug<'a>(lvl: uint, s: &'a str); // expanded
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fn substr(s: &str, until: uint) -> &str; // elided
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fn substr<'a>(s: &'a str, until: uint) -> &'a str; // expanded
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fn get_str() -> &str; // ILLEGAL
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fn frob(s: &str, t: &str) -> &str; // ILLEGAL
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fn get_mut(&mut self) -> &mut T; // elided
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fn get_mut<'a>(&'a mut self) -> &'a mut T; // expanded
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fn args<T:ToCStr>(&mut self, args: &[T]) -> &mut Command // elided
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fn args<'a, 'b, T:ToCStr>(&'a mut self, args: &'b [T]) -> &'a mut Command // expanded
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fn new(buf: &mut [u8]) -> BufWriter; // elided
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fn new<'a>(buf: &'a mut [u8]) -> BufWriter<'a> // expanded
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```
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