Temporary lifetime extension for blocks

[dianne]

This implements a revised version of the temporary lifetime extension semantics I suggested in #145838 (comment), with the goal of making temporary lifetimes and drop order more consistent between extending and non-extending blocks. As a consequence, this undoes the breaking change introduced by #145838 (but in exchange has a much larger surface area).

The change this PR hopes to enforce is a general rule: any expression's temporaries should have the same relative drop order regardless of whether the expression is in an extending context or not: let _ = $expr; and drop($expr); should have the same drop order. To achieve that, this PR applies lifetime extension rules to blocks:

// This `temp()` is now extended past the block tail in all contexts.
{ &temp() }

now extends the lifetime of temp() to outlive the block tail in Rust 2024 regardless of whether the block is an extending expression in a let statement initializer (in which context it was already extended to outlive the block before this PR). The scoping rules for tails of extending blocks remain the same: extending subexpressions' temporary scopes are extended based on the source of the lifetime extension (e.g. to match the scope of a parent let statement's bindings). For blocks not extended by any other source, extending borrows in the tail expression now share a temporary scope with the result of the block. This can in turn extend nested blocks within blocks' tail expressions:

// This `temp()` is extended past the outer block tail.
// It is now dropped after the reference to it at the `;`.
f({{ &temp() }});

// This context-sensitivity is consistent with `let`:
// This `temp()` was already extended.
// It is still dropped after `x` at the end of its scope.
let x = {{ &temp() }};

Since this uses the same rules as let, it only applies to extending sub-expressions.

// This `temp()` is still never extended in any context.
// In Rust 2024, it is dropped at the end of the block tail.
{ identity(&temp()) }

This also applies to if expressions' blocks since lifetime extension applies to if blocks' tail expressions, meaning it affects all editions. This is where breakage from #145838 was observed:

if cond { &temp() } else { &temp() }

now extends temp() to have the same temporary scope as the result of the if expression.

As a further consequence, this makes super let in if expressions' blocks more consistent with block expressions:

if cond() {
    super let x = temp();
    &temp
} else {
    super let x = temp();
    &temp
}

previously only worked in extending contexts (since the super lets would be extended), and now it works everywhere.

I don't think this is ready to merge yet. It needs to be optimized, it should have a Reference PR, it will need a lang FCP, and it may need other things as well.

@rustbot label +T-lang

[dianne]

@rustbot label -stable-nominated

I'm not intending to stable-nominate this, at least. Someone else can, but I don't expect it's needed or that it would be accepted.

[traviscross]

Does this only affect code in Rust 2024, or would you expect any visible difference in earlier editions?

[dianne]

It should only be visible in Rust 2024. The only extending expressions that introduce temporary drop scopes are Rust 2024 block tail expressions. Edit: this is also visible on earlier editions through if expressions' blocks.

Suppose we have a macro extending!, for which $expr is extending if extending!($expr) is extending. Under this PR, in a non-extending context, { extending!(&temp()) } would give temp() the same temporary scope as the result of the block. Prior to Rust 2021, they're already in the same scope, due to extending! being unable to introduce temporary scopes.

Or to generalize this, the aim of this PR is that in a non-extending context, extending!(&temp()) should give temp() the same temporary scope as the expansion, similar to how let x = extending!(&temp()); gives temp() the same scope as x. This already holds in Rust 2021 and prior.

If new expressions are added to Rust that are both extending and temporary scopes, I'd want this behavior to apply to them as well.

[traviscross]

Since this would effectively reduce the scope of the Rust 2024 tail expression temporary scope change, we'd also want to be sure to reflect that in the behavior of the tail-expr-drop-order lint.

[dianne]

I haven't done extensive testing, but see this test diff for that lint: lint-tail-expr-drop-order-borrowck.rs. I'm applying the lifetime extension rules on all editions, and lifetime extension prevents the temporary scope from being registered as potentially forwards-incompatible (even though the extended scopes are technically the same as the old scopes in old editions). Though I think I've convinced myself at this point that lifetime extension doesn't need to be applied to block tails of non-extending old-edition blocks¹, so potentially the lint change could be implemented in some other way instead.

Footnotes

1. I was worried about mixed-edition code, but I don't think it's an issue anymore. ↩

[bors]

☔ The latest upstream changes (presumably #146666) made this pull request unmergeable. Please resolve the merge conflicts.

[dianne]

I've made some revisions. This should now properly handle if expressions' blocks, meaning it affects all editions (since if blocks are both terminating in all editions and extending when the if expression is extending). Of note, I didn't notice at the time, but I think #145838 affected all editions as well (including the real-world breakage), due to if blocks working like that.

I think the implementation will likely need optimization and cleanup, but it might take a bit of refactoring to get it to a good place, so I'd like to get a vibe check on the design first, if there's room for it in a lang team meeting.

@rustbot label +I-lang-nominated

[rustbot]

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[nnethercote]

Hmm, these kinds of subtle language behaviour PRs are ones that I'm really not comfortable reviewing. Not in my wheelhouse :/

View changes since this review

[nnethercote]

r? compiler

[traviscross]

@dianne, with respect to understanding this rule in terms of the Reference's language, what would we be changing in destructors?

If I understand correctly, we'd be extending this temporary

let _x = f({ &temp() });

to live until the end of the statement but not until the end of the surrounding block. So we can't just say, e.g., that trailing expressions of non-extending blocks are extending expressions without saying something else here. Maybe what we're trying to say is that we're walking a chain of extending expressions (which now include trailing expressions of non-extending blocks and ifs), and if the start of the chain is an initializer expression, we extend to the end of the block, and if it's a block or if tail expression, we extend to the end of the statement. Does that sound right, or how would you think to frame this?

[dianne]

The way I've been thinking about it is we'd add a new base case for lifetime extension alongside destructors.scope.lifetime-extension.let and destructors.scope.lifetime-extension.static: informally, lifetime extension would apply to tail expressions of non-extending¹ blocks and to the consequent, else if, and else blocks of non-extending¹ if expressions, where it extends temporaries to the temporary scope of the block or if expression as defined by destructors.scope.temporary.enclosing². We'd also need to change destructors.scope.lifetime-extension.exprs so that the base case isn't specific to let statement initializers³.

Footnotes

1. The "non-extending" requirement could be removed, but then it would need something else to disambiguate the scope used for extended temporaries in extending blocks' and if expressions' tails. e.g. an additional rule could be added to specify the start of a chain of extending expressions, or it could specify that the largest applicable scope is used, or such. ↩ ↩²

2. Importantly, this isn't always the end of the statement if the block or if expression is contained within a smaller temporary scope (e.g. the operand of a logical operator, a for loop desugaring, a non-pattern-matching if condition, etc.). ↩

3. Regardless of this PR, I think this also needs updating account for const and static items' bodies, which are treated like let statement initializers by scope resolution in the compiler and by destructors.scope.lifetime-extension.static. ↩