|
| 1 | +# Lifetime elision |
| 2 | + |
| 3 | +Rust has rules that allow lifetimes to be elided in various places where the |
| 4 | +compiler can infer a sensible default choice. |
| 5 | + |
| 6 | +## Lifetime elision in functions |
| 7 | + |
| 8 | +In order to make common patterns more ergonomic, Rust allows lifetimes to be |
| 9 | +*elided* in [function item], [function pointer] and [closure trait] signatures. |
| 10 | +The following rules are used to infer lifetime parameters for elided lifetimes. |
| 11 | +It is an error to elide lifetime parameters that cannot be inferred. |
| 12 | + |
| 13 | +* Each elided lifetime in the parameters becomes a distinct lifetime parameter. |
| 14 | +* If there is exactly one lifetime used in the parameters (elided or not), that |
| 15 | + lifetime is assigned to *all* elided output lifetimes. |
| 16 | + |
| 17 | +In method signatures there is another rule |
| 18 | + |
| 19 | +* If the receiver has type `&Self` or `&mut Self`, then the lifetime of that |
| 20 | + reference to `Self` is assigned to all elided output lifetime parameters. |
| 21 | + |
| 22 | +Examples: |
| 23 | + |
| 24 | +```rust,ignore |
| 25 | +fn print(s: &str); // elided |
| 26 | +fn print<'a>(s: &'a str); // expanded |
| 27 | +
|
| 28 | +fn debug(lvl: usize, s: &str); // elided |
| 29 | +fn debug<'a>(lvl: usize, s: &'a str); // expanded |
| 30 | +
|
| 31 | +fn substr(s: &str, until: usize) -> &str; // elided |
| 32 | +fn substr<'a>(s: &'a str, until: usize) -> &'a str; // expanded |
| 33 | +
|
| 34 | +fn get_str() -> &str; // ILLEGAL |
| 35 | +
|
| 36 | +fn frob(s: &str, t: &str) -> &str; // ILLEGAL |
| 37 | +
|
| 38 | +fn get_mut(&mut self) -> &mut T; // elided |
| 39 | +fn get_mut<'a>(&'a mut self) -> &'a mut T; // expanded |
| 40 | +
|
| 41 | +fn args<T: ToCStr>(&mut self, args: &[T]) -> &mut Command; // elided |
| 42 | +fn args<'a, 'b, T: ToCStr>(&'a mut self, args: &'b [T]) -> &'a mut Command; // expanded |
| 43 | +
|
| 44 | +fn new(buf: &mut [u8]) -> BufWriter; // elided |
| 45 | +fn new<'a>(buf: &'a mut [u8]) -> BufWriter<'a>; // expanded |
| 46 | +
|
| 47 | +type FunPtr = fn(&str) -> &str; // elided |
| 48 | +type FunPtr = for<'a> fn(&'a str) -> &'a str; // expanded |
| 49 | +
|
| 50 | +type FunTrait = Fn(&str) -> &str; // elided |
| 51 | +type FunTrait = for<'a> Fn(&'a str) -> &'a str; // expanded |
| 52 | +``` |
| 53 | + |
| 54 | +## Default trait object lifetimes |
| 55 | + |
| 56 | +The assumed lifetime of references held by a [trait object] is called its |
| 57 | +_default object lifetime bound_. These were defined in [RFC 599] and amended in |
| 58 | +[RFC 1156]. Default object lifetime bounds are used instead of the lifetime |
| 59 | +parameter elision rules defined above. |
| 60 | + |
| 61 | +If the trait object is used as a type argument of a generic type then the |
| 62 | +containing type is first used to try to infer a bound. |
| 63 | + |
| 64 | +* If there is a unique bound from the containing type then that is the default |
| 65 | +* If there is more than one bound from the containing type then an explicit |
| 66 | + bound must be specified |
| 67 | + |
| 68 | +If neither of those rules apply, then the bounds on the trait are used: |
| 69 | + |
| 70 | +* If the trait is defined with a single lifetime _bound_ then that bound is |
| 71 | + used. |
| 72 | +* If `'static` is used for any lifetime bound then `'static` is used. |
| 73 | +* If the trait has no lifetime bounds, then the lifetime is inferred in |
| 74 | + expressions and is `'static` outside of expressions. |
| 75 | + |
| 76 | +```rust,ignore |
| 77 | +// For the following trait... |
| 78 | +trait Foo { } |
| 79 | +
|
| 80 | +// These two are the same as Box<T> has no lifetime bound on T |
| 81 | +Box<Foo> |
| 82 | +Box<Foo + 'static> |
| 83 | +
|
| 84 | +// ...and so are these: |
| 85 | +impl Foo {} |
| 86 | +impl Foo + 'static {} |
| 87 | +
|
| 88 | +// ...so are these, because &'a T requires T: 'a |
| 89 | +&'a Foo |
| 90 | +&'a (Foo + 'a) |
| 91 | +
|
| 92 | +// std::cell::Ref<'a, T> also requires T: 'a, so these are the same |
| 93 | +std::cell::Ref<'a, Foo> |
| 94 | +std::cell::Ref<'a, Foo + 'a> |
| 95 | +
|
| 96 | +// This is an error: |
| 97 | +struct TwoBounds<'a, 'b, T: ?Sized + 'a + 'b> |
| 98 | +TwoBounds<'a, 'b, Foo> // Error: the lifetime bound for this object type cannot |
| 99 | + // be deduced from context |
| 100 | +``` |
| 101 | + |
| 102 | +Note that the innermost object sets the bound, so `&'a Box<Foo>` is still `&'a |
| 103 | +Box<Foo + 'static>`. |
| 104 | + |
| 105 | +```rust,ignore |
| 106 | +// For the following trait... |
| 107 | +trait Bar<'a>: 'a { } |
| 108 | +
|
| 109 | +// ...these two are the same: |
| 110 | +Box<Bar<'a>> |
| 111 | +Box<Bar<'a> + 'a> |
| 112 | +
|
| 113 | +// ...and so are these: |
| 114 | +impl<'a> Foo<'a> {} |
| 115 | +impl<'a> Foo<'a> + 'a {} |
| 116 | +
|
| 117 | +// This is still an error: |
| 118 | +struct TwoBounds<'a, 'b, T: ?Sized + 'a + 'b> |
| 119 | +TwoBounds<'a, 'b, Foo<'c>> |
| 120 | +``` |
| 121 | + |
| 122 | +## `'static` lifetime elision |
| 123 | + |
| 124 | +Both [constant] and [static] declarations of reference types have *implicit* |
| 125 | +`'static` lifetimes unless an explicit lifetime is specified. As such, the |
| 126 | +constant declarations involving `'static` above may be written without the |
| 127 | +lifetimes. |
| 128 | + |
| 129 | +```rust |
| 130 | +// STRING: &'static str |
| 131 | +const STRING: &str = "bitstring"; |
| 132 | + |
| 133 | +struct BitsNStrings<'a> { |
| 134 | + mybits: [u32; 2], |
| 135 | + mystring: &'a str, |
| 136 | +} |
| 137 | + |
| 138 | +// BITS_N_STRINGS: BitsNStrings<'static> |
| 139 | +const BITS_N_STRINGS: BitsNStrings = BitsNStrings { |
| 140 | + mybits: [1, 2], |
| 141 | + mystring: STRING, |
| 142 | +}; |
| 143 | +``` |
| 144 | + |
| 145 | +Note that if the `static` or `const` items include function or closure |
| 146 | +references, which themselves include references, the compiler will first try |
| 147 | +the standard elision rules. If it is unable to resolve the lifetimes by its |
| 148 | +usual rules, then it will error. By way of example: |
| 149 | + |
| 150 | +```rust,ignore |
| 151 | +// Resolved as `fn<'a>(&'a str) -> &'a str`. |
| 152 | +const RESOLVED_SINGLE: fn(&str) -> &str = .. |
| 153 | +
|
| 154 | +// Resolved as `Fn<'a, 'b, 'c>(&'a Foo, &'b Bar, &'c Baz) -> usize`. |
| 155 | +const RESOLVED_MULTIPLE: &Fn(&Foo, &Bar, &Baz) -> usize = .. |
| 156 | +
|
| 157 | +// There is insufficient information to bound the return reference lifetime |
| 158 | +// relative to the argument lifetimes, so this is an error. |
| 159 | +const RESOLVED_STATIC: &Fn(&Foo, &Bar) -> &Baz = .. |
| 160 | +``` |
| 161 | + |
| 162 | +[closure trait]: types.html#closure-types |
| 163 | +[constant]: items.html#constant-items |
| 164 | +[function item]: types.html#function-item-types |
| 165 | +[function pointer]: types.html#function-pointers |
| 166 | +[implementation]: items/implementations.html |
| 167 | +[RFC 599]: https://github.com/rust-lang/rfcs/blob/master/text/0599-default-object-bound.md |
| 168 | +[RFC 1156]: https://github.com/rust-lang/rfcs/blob/master/text/1156-adjust-default-object-bounds.md |
| 169 | +[static]: items.html#static-items |
| 170 | +[trait object]: types.html#trait-objects |
| 171 | +[type aliases]: items/type-aliases.html |
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