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pull/340/head
Eric Huss 2 years ago
parent 15174604f9
commit 54ca7d1a34

@ -12,7 +12,7 @@ Let's start with an example.
```rust ```rust
// Note: debug expects two parameters with the *same* lifetime // Note: debug expects two parameters with the *same* lifetime
fn debug<'a>(a: &'a str, b: &'a str) { fn debug<'a>(a: &'a str, b: &'a str) {
println!("a = {:?} b = {:?}", a, b); println!("a = {a:?} b = {b:?}");
} }
fn main() { fn main() {
@ -25,7 +25,7 @@ fn main() {
} }
``` ```
In a conservative implementation of lifetimes, since `hello` and `world` have differing lifetimes, In a conservative implementation of lifetimes, since `hello` and `world` have different lifetimes,
we might see the following error: we might see the following error:
```text ```text
@ -72,12 +72,12 @@ so we need to understand how this stuff really works, and how we can mess it up.
Going back to our example above, we can say that `'static <: 'world`. Going back to our example above, we can say that `'static <: 'world`.
For now, let's also accept the idea that subtypes of lifetimes can be passed through references For now, let's also accept the idea that subtypes of lifetimes can be passed through references
(more on this in [Variance](#variance)), (more on this in [Variance](#variance)),
_e.g._ `&'static str` is a subtype of `&'world str`, then we can let a `&'static str` "downgrade" into a `&'world str`. _e.g._ `&'static str` is a subtype of `&'world str`, then we can "downgrade" `&'static str` into a `&'world str`.
With that, the example above will compile: With that, the example above will compile:
```rust ```rust
fn debug<T: std::fmt::Debug>(a: T, b: T) { fn debug<'a>(a: &'a str, b: &'a str) {
println!("a = {:?} b = {:?}", a, b); println!("a = {a:?} b = {b:?}");
} }
fn main() { fn main() {
@ -95,7 +95,7 @@ fn main() {
Above, we glossed over the fact that `'static <: 'b` implied that `&'static T <: &'b T`. This uses a property known as _variance_. Above, we glossed over the fact that `'static <: 'b` implied that `&'static T <: &'b T`. This uses a property known as _variance_.
It's not always as simple as this example, though. To understand that, let's try to extend this example a bit: It's not always as simple as this example, though. To understand that, let's try to extend this example a bit:
```rust,compile_fail ```rust,compile_fail,E0597
fn assign<T>(input: &mut T, val: T) { fn assign<T>(input: &mut T, val: T) {
*input = val; *input = val;
} }
@ -106,7 +106,7 @@ fn main() {
let world = String::from("world"); let world = String::from("world");
assign(&mut hello, &world); assign(&mut hello, &world);
} }
println!("{}", hello); // use after free 😿 println!("{hello}"); // use after free 😿
} }
``` ```
@ -177,7 +177,7 @@ For more types, see the ["Variance" section][variance-table] on the reference.
Now that we have some more formal understanding of variance, Now that we have some more formal understanding of variance,
let's go through some more examples in more detail. let's go through some more examples in more detail.
```rust,compile_fail ```rust,compile_fail,E0597
fn assign<T>(input: &mut T, val: T) { fn assign<T>(input: &mut T, val: T) {
*input = val; *input = val;
} }
@ -188,7 +188,7 @@ fn main() {
let world = String::from("world"); let world = String::from("world");
assign(&mut hello, &world); assign(&mut hello, &world);
} }
println!("{}", hello); println!("{hello}");
} }
``` ```
@ -230,7 +230,7 @@ This is counter to the `&T` case:
```rust ```rust
fn debug<T: std::fmt::Debug>(a: T, b: T) { fn debug<T: std::fmt::Debug>(a: T, b: T) {
println!("a = {:?} b = {:?}", a, b); println!("a = {a:?} b = {b:?}");
} }
``` ```

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