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Two lifetime clarification #350

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May 7, 2022
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Two lifetime clarification #350

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15 changes: 8 additions & 7 deletions src/lifetimes.md
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Original file line number Diff line number Diff line change
Expand Up @@ -80,8 +80,8 @@ z = y;
'b: {
let z: &'b i32;
'c: {
// Must use 'b here because this reference is
// being passed to that scope.
// Must use 'b here because the reference to x is
// being passed to the scope 'b.
let y: &'b i32 = &'b x;
z = y;
}
Expand Down Expand Up @@ -208,11 +208,12 @@ violate the *second* rule of references.

However this is *not at all* how Rust reasons that this program is bad. Rust
doesn't understand that `x` is a reference to a subpath of `data`. It doesn't
understand `Vec` at all. What it *does* see is that `x` has to live for `'b` to
be printed. The signature of `Index::index` subsequently demands that the
reference we take to `data` has to survive for `'b`. When we try to call `push`,
it then sees us try to make an `&'c mut data`. Rust knows that `'c` is contained
within `'b`, and rejects our program because the `&'b data` must still be alive!
understand `Vec` at all. What it *does* see is that `x` has to live for `'b` in
order to be printed. The signature of `Index::index` subsequently demands that
the reference we take to `data` has to survive for `'b`. When we try to call
`push`, it then sees us try to make an `&'c mut data`. Rust knows that `'c` is
contained within `'b`, and rejects our program because the `&'b data` must still
be alive!

Here we see that the lifetime system is much more coarse than the reference
semantics we're actually interested in preserving. For the most part, *that's
Expand Down