diff --git a/src/subtyping.md b/src/subtyping.md index 79b6408..80e329e 100644 --- a/src/subtyping.md +++ b/src/subtyping.md @@ -188,6 +188,10 @@ some sense "fundamental". All the others can be understood by analogy to the oth * `*const T` follows the logic of `&T` * `*mut T` follows the logic of `&mut T` (or `UnsafeCell`) +For more types, see the ["Variance" section][variance-table] on the reference. + +[variance-table]: ../reference/subtyping.html#variance + > NOTE: the *only* source of contravariance in the language is the arguments to > a function, which is why it really doesn't come up much in practice. Invoking > contravariance involves higher-order programming with function pointers that @@ -265,7 +269,7 @@ enough into the place expecting something long-lived. Here it is: -```rust,ignore +```rust,compile_fail fn evil_feeder(input: &mut T, val: T) { *input = val; } @@ -285,15 +289,16 @@ And what do we get when we run this? ```text error[E0597]: `spike` does not live long enough - --> src/main.rs:9:32 + --> src/main.rs:9:31 | -9 | let spike_str: &str = &spike; - | ^^^^^ borrowed value does not live long enough -10 | evil_feeder(&mut mr_snuggles, spike_str); +6 | let mut mr_snuggles: &'static str = "meow! :3"; // mr. snuggles forever!! + | ------------ type annotation requires that `spike` is borrowed for `'static` +... +9 | let spike_str: &str = &spike; // Only lives for the block + | ^^^^^^ borrowed value does not live long enough +10 | evil_feeder(&mut mr_snuggles, spike_str); // EVIL! 11 | } - | - borrowed value only lives until here - | - = note: borrowed value must be valid for the static lifetime... + | - `spike` dropped here while still borrowed ``` Good, it doesn't compile! Let's break down what's happening here in detail.