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