Specify temporary lifetime extension through expressions #2051

dianne · 2025-10-12T20:28:49Z

Reference PR for rust-lang/rust#146098. This includes a reworked definition of extending expressions with the aim of expressing the new semantics more uniformly.

dianne · 2025-10-12T20:33:36Z

src/destructors.md

 r[destructors.scope.lifetime-extension.exprs.extending]
-For a let statement with an initializer, an *extending expression* is an
-expression which is one of the following:
+An *extending expression* is an expression which is one of the following:


A major change here: an extending expression is now any expression that preserves lifetime extension, defined non-inductively. I found this helps with generalizing the definition beyond let statement initializers, but I also often found myself having to refer to an expression being "extending when its parent is extending"; that's are now just an extending expression.

Yes, this greatly widens the definition of an extending expression, putting more load on the other rules.

dianne · 2025-10-12T20:36:23Z

src/destructors.md

-* The operand of an extending [borrow] expression.
-* The [super operands] of an extending [super macro call] expression.
-* The operand(s) of an extending [array][array expression], [cast][cast
+* The initializer expression of a `let` statement or the body expression of a [static][static item] or [constant item].


Something slightly weird here: the last expression of a const block morally should be here, but it'd be a bit messy to have to exclude it from the rule for blocks lower down. Given that this definition of extending expressions doesn't care about where scopes are extended to, it shouldn't be a semantic issue, but it might warrant reformatting and/or an admonition.

Let's see the admonition for this (with one or more examples).

dianne · 2025-10-12T20:55:36Z

src/destructors.md

+r[destructors.scope.lifetime-extension.exprs.parent]
+If a temporary scope is extended through the scope of an extending expression, it is extended through that scope's [parent][destructors.scope.nesting].


I struggled a bit with how to express this; in its current form it's a bit of a hack. Ideally, I feel like it wouldn't need to be a separate rule or to refer to the definition of scope nesting, but it's working around something subtle: by ensuring that expressions' temporary scopes are only extended by their scope-ancestors, we can work around const blocks having parent expressions that (to my understanding) shouldn't be considered ancestor scopes of the const block's body; temporaries extended by const blocks are extended to the end of the program¹. Maybe there's a simpler way to express this, and regardless it could probably use an admonition.

I do think that some sort of "extended by" or "extended through" or "extending based on" relation is necessary though, regardless of how exactly we choose to define/present it. I feel there's too much ambiguity if we can't precisely associate expressions we're extending the temporary scopes of with the scopes they're being extended to.

Footnotes

This PR doesn't make all the changes needed to iron that out, but see Further specify temporary scoping for statics and consts #2041. ↩

Let's see the admonition that you have in mind here (with one or more examples). Let's also add rules to define what it means exactly to "extend to", "extend through", etc.

In particular, what it means to "extend through" a parent scope is going to be worth careful explanation and one or more examples in an admonition.

dianne · 2025-10-12T20:59:09Z

src/destructors.md

+```rust,edition2024
+# fn temp() {}
+# fn use_temp(_: &()) {}
+// The final expression of a block is extending. Since the block below
+// is not itself extending, the temporary is extended to the block
+// expression's temporary scope, ending at the semicolon.
+use_temp({ &temp() });
+// As above, the final expressions of `if`/`else` blocks are
+// extending, which extends the temporaries to the `if` expression's
+// temporary scope.
+use_temp(if true { &temp() } else { &temp() });
+```


Some additional examples would probably be good. Maybe it would help to have one where temporaries are extended through a block but not to the end of a statement? That could also be used as a compile_fail example.

Maybe it could also use some additional text (or even an admonition?) to make clear the interaction with if block scopes and Rust 2024's tail expression scopes. I'm not sure exactly how much explaining is needed for that, though.

This is all rather subtle. Erring on the side of more examples and explanation will be better here.

dianne · 2025-10-12T21:06:26Z

src/destructors.md

+r[destructors.scope.lifetime-extension.exprs.let]
+A temporary scope extended through a `let` statement scope is [extended] to the scope of the block containing the `let` statement ([destructors.scope.lifetime-extension.let]).
+
+r[destructors.scope.lifetime-extension.exprs.static]
+A temporary scope extended through a [static][static item] or [constant item] scope or a [const block][const block expression] scope is [extended] to the end of the program ([destructors.scope.lifetime-extension.static]).


I wonder if there's a way to cut down on the duplication here. I felt these rules were necessary to be precise about where temporaries' scopes are extended to, but having them in the introduction to the lifetime extension feels necessary too.

Agreed they felt necessary here.

dianne · 2025-10-12T21:08:01Z

src/destructors.md

+r[destructors.scope.lifetime-extension.exprs.other]
+A temporary scope extended through the scope of a non-extending expression is [extended] to that expression's [temporary scope].


Alternative to de-duplicating the above two rules, maybe there should be a more detailed section alongside destructors.scope.lifetime-extension.let and destructors.scope.lifetime-extension.static for this?

Not sure what you mean here exactly.

traviscross · 2025-10-12T21:38:22Z

Thanks @dianne; will have a look.

traviscross · 2025-10-28T22:46:42Z

@ehuss and I looked carefully through this today. Some thoughts.

After staring at this awhile, I see why this makes it easier to express the core idea of rust-lang/rust#146098 -- that the relative drop order within an expression should be consistent regardless of where that expression is. Expressing the rules inductively for defining an extending expression ends up pushing against this.

So it does, I think, make a deep sort of sense to drop the inductive approach, as this PR does.

At the same time, the inductive approach was carrying a lot of load. The reader could visualize walking down an expression, outside to inside, at each step checking whether it was still an extending expression. Then there was essentially one key rule that leaned on that:

The operand of an extending [borrow] expression has its [temporary scope] [extended].

What this PR does, in dropping the inductive framing, is to widen the definition of an extending expression significantly, and thereby put a lot more load on the rules that follow to narrow this back down. That's where it ends up getting a bit subtle. With this approach, both @ehuss and I found building intuition and checking the correctness of the rules to be more difficult. So, in dropping the inductive approach, I think we have some work to do in order to recover the same level of clarity.

Adding some !EXAMPLE admonitions that walk through applying these rules would help and be a good start here. It'll also be worth more carefully defining what it means to "extend to" or "extend through" something. The bit where we define some things as "extending through" but then "clamp" them so as to only "extend to" with the final rule ("A temporary scope extended through the scope of a non-extending expression is [extended] to that expression's [temporary scope].") is another bit of subtlety that it might be worth finding another way to express.

I'll leave some other thoughts inline.

traviscross · 2025-10-28T23:01:05Z

src/destructors.md

+A temporary scope extended through a [static][static item] or [constant item] scope or a [const block][const block expression] scope is [extended] to the end of the program ([destructors.scope.lifetime-extension.static]).
+
+r[destructors.scope.lifetime-extension.exprs.other]
+A temporary scope extended through the scope of a non-extending expression is [extended] to that expression's [temporary scope].


This rule is particular is one where I wonder how we might do better. The idea, currently, is that the rules above define when a temporary scope would be "extended through" the scope of an expression, and then this rule says, well, if it's a non-extending expression, then we only "extend to" it.

traviscross · 2025-10-28T23:05:45Z

src/destructors.md

-The operand of an extending [borrow] expression has its [temporary scope] [extended].
+The [temporary scope] of the operand of a [borrow] expression is *extended through* the scope of the borrow expression.


It's a bit subtle that, after having defined above what is an extending expression, that this rule then doesn't use that definition at all. It makes sense -- all operands of any borrow expression are extending. But it'll be worth adding an admonition under each of these rules that elaborates the rationale and gives one or more examples.

dianne · 2025-10-29T05:50:10Z

I'll try and address individual points soon, but first one higher-level note that I'll try to use as guidance when restructuring:

At the same time, the inductive approach was carrying a lot of load. The reader could visualize walking down an expression, outside to inside, at each step checking whether it was still an extending expression.

This PR's approach still has a visual intuition to it, so it should be presented in a way that makes that clear: instead of walking down an extending expression to check whether an &'s operand is extended, you walk up from an & and check whether you're in an extending expression at each step to determine the temporary scope of the &'s operand. Possibly a separate inductive definition would be useful for making that clear? Along with examples, of course. I was worried defining too many things all at once would make it difficult to follow, but examples should help.

traviscross · 2025-10-29T06:23:44Z

This PR's approach still has a visual intuition to it, so it should be presented in a way that makes that clear: instead of walking down an extending expression to check whether an &'s operand is extended, you walk up from an & and check whether you're in an extending expression at each step to determine the temporary scope of the &'s operand. Possibly a separate inductive definition would be useful for making that clear? Along with examples, of course.

Yes, exactly. This upward walk is what I'd been expecting but it has to be teased out a bit from the presentation here. Hopefully making that more clear will make the rules here more clear.

Specify lifetime extension through expressions

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dianne commented Oct 12, 2025

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dianne marked this pull request as ready for review October 12, 2025 21:13

rustbot added the S-waiting-on-review Status: The marked PR is awaiting review from a maintainer label Oct 12, 2025

dianne mentioned this pull request Oct 12, 2025

Temporary lifetime extension for blocks rust-lang/rust#146098

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dianne changed the title ~~Specify lifetime extension through expressions~~ Specify temporary lifetime extension through expressions Oct 12, 2025

traviscross reviewed Oct 28, 2025

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		r[destructors.scope.lifetime-extension.exprs.parent]
		If a temporary scope is extended through the scope of an extending expression, it is extended through that scope's [parent][destructors.scope.nesting].

		r[destructors.scope.lifetime-extension.exprs.other]
		A temporary scope extended through the scope of a non-extending expression is [extended] to that expression's [temporary scope].

		The operand of an extending [borrow] expression has its [temporary scope] [extended].
		The [temporary scope] of the operand of a [borrow] expression is extended through the scope of the borrow expression.

Specify temporary lifetime extension through expressions #2051

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