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.

Reference PR: rust-lang/reference#2051

@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]

r? @nnethercote

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They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[BoxyUwU]

r? BoxyUwU

feel free to review it too if you like but I've been getting on with this over the past few weeks so would like to finish that out :)

[jackh726]

Sounds good!

[dianne]

I ended up doing some cleanup in addition to the necessary refactors. If it'd help with review, I can pull that into a separate PR or two or three.

The notable changes are:

• I added a test for some existing behavior where super let doesn't quite match the identities commonly used for reasoning about it, to demonstrate that this PR doesn't affect that behavior. In particular, this shows pin! won't always assign the same scope to its pinned temporary as &mut does to its operand. I think this only affects code unlikely to be written though, e.g. &mut *pin!(temp()) vs &mut *&mut temp() (assuming temp(): Unpin. or replace the outer &mut with &). I believe the difference predates pin! being implemented in terms of super let as well; the old version of pin! wouldn't have permitted let _ = &mut *pin!(temp()); either.
• I merged RvalueScopes into ScopeTree and got rid of RvalueCandidate. This refactor turned out not strictly to be necessary, but I think removing the indirection and consolidating the implementation of temporary scoping is a win; this gets rid of 2 of the ~4 files it was spread out between.
• I made ScopeTree::temporary_scope (previously RvalueScopes::temporary_scope) return a struct rather than a tuple to make it clearer which scope is the current old-edition scope and which is the shortened Rust 2024 scope. This also cleaned up THIR building, since it no longer needs to untuple and put the results into a struct.
• I rewrote the implementation of temporary lifetime extension in ScopeResolutionVisitor to compute the scopes of extended temporaries as it walks the HIR rather than by doing a separate pass when it hits a let statement. This both simplifies both the implementation of this PR's actual change to semantics and the implementation of super let.
• I reimplemented said actual change on top of that, made it able to register scoping incompatibility hints for the tail_expr_drop_order lint, and added another test for that.

cc @dingxiangfei2009 for the RvalueScopes and BID-related refactors

I'll fire off a fresh try build and perf run once CI is green. It may also need a new crater run since match arm scopes weren't handled correctly before (and/or to catch any bugs missed by the test suite; it is a substantial rewrite).

@rustbot ready

[dianne]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 9f93af2 (9f93af291970322f4f1c6315ccde4d7078201159, parent: 1ef7943ee607160a564655b6596f83670ef95df5)

[traviscross]

@craterbot run mode=build-and-test

[craterbot]

👌 Experiment pr-146098-6 created and queued.
🤖 Automatically detected try build 9f93af2
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[rust-timer]

Finished benchmarking commit (9f93af2): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.3%	[0.2%, 0.4%]	2
Regressions ❌ (secondary)	0.6%	[0.6%, 0.6%]	3
Improvements ✅ (primary)	-0.4%	[-0.9%, -0.2%]	5
Improvements ✅ (secondary)	-0.1%	[-0.3%, -0.1%]	9
All ❌✅ (primary)	-0.2%	[-0.9%, 0.4%]	7

Max RSS (memory usage)

Results (primary -1.8%, secondary 1.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	1.8%	[1.8%, 1.8%]	1
Regressions ❌ (secondary)	3.2%	[1.7%, 5.5%]	10
Improvements ✅ (primary)	-3.0%	[-3.6%, -2.1%]	3
Improvements ✅ (secondary)	-2.6%	[-3.6%, -1.9%]	5
All ❌✅ (primary)	-1.8%	[-3.6%, 1.8%]	4

Cycles

Results (primary 3.4%, secondary 2.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	3.4%	[3.4%, 3.4%]	1
Regressions ❌ (secondary)	5.2%	[1.8%, 10.7%]	9
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.8%	[-5.0%, -2.6%]	5
All ❌✅ (primary)	3.4%	[3.4%, 3.4%]	1

Binary size

Results (primary 0.0%, secondary 0.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.0%, 0.3%]	3
Regressions ❌ (secondary)	0.0%	[0.0%, 0.0%]	2
Improvements ✅ (primary)	-0.1%	[-0.1%, -0.0%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[-0.1%, 0.3%]	5

Bootstrap: 474.243s -> 475.297s (0.22%)
Artifact size: 390.98 MiB -> 390.72 MiB (-0.07%)