Make trait refs & assoc ty paths properly induce trait object lifetime defaults by fmease · Pull Request #129543 · rust-lang/rust

fmease · 2024-08-25T03:08:55Z

Trait Object Lifetime Defaults

Primer & Definitions

You could read this section in the Reference but it has several issues (see rust-lang/reference#1407). Here's a small explainer by me that only mentions the parts relevant to this PR:

Basically, given dyn Trait (≠ dyn Trait + '_) we want to deduce its trait object lifetime bound from context without relying on normal region inference as we might not be in a body¹. The "context" means the closest – what I call – (eligible) container C<X0, …, Xn> that wraps this trait object type. A container is to be understood as a use site of a "parametrized definition" (more general than type constructors). Currently eligible are ADTs, type aliases, traits and enum variants.

So if we have C<dyn Trait> (e.g., &'r dyn Trait or Struct<'r, dyn Trait>), D<C<dyn Trait>> or C<N<dyn Trait>> (e.g., Struct<'r, (dyn Trait,)>), we use the explicit² outlives-bounds on the corresponding type parameter of C to determine the trait object lifetime bound. Here, C & D denote (eligible) containers and N denotes a generic type that is not an eligible container. E.g., given struct Struct<'a, T: 'a + ?Sized>(…);, we elaborate Struct<'r, dyn Trait> to Struct<'r, dyn Trait + 'r>.

Finally, we call lifetime bounds used as the default for constituent trait object types of an eligible container C the trait object lifetime defaults (induced by C). These defaults may of course end up getting shadowed in parts of the type by the defaults induced by any inner eligible containers.

Changes Made

These changes are theoretically breaking.

Make resolved associated type paths / projections eligible containers.

<Y0 as TraitRef<X0, …, Xn>>::AssocTy<Y1, …, Ym> now induces trait object lifetime defaults for constituents Y0 to Ym (TraitRef is considered a separate container, see also list item (3)).
Notably, for the self type Y0 of (resolved) projections we now look at the bounds on the Self type param of the relevant trait (e.g., given trait Outer<'a>: 'a { type Proj; } or trait Outer<'a> where Self: 'a { type Proj; } we elaborate <dyn Inner as Outer<'r>>::Proj to <dyn Inner + 'r as Outer<'r>>::Proj).

Example breakages:

trait Outer<'a> { type Ty<T: ?Sized + 'a>; }
impl<'a> Outer<'a> for () { type Ty<T: ?Sized + 'a> = &'a T; }
trait Inner {}

fn f<'r>(x: <() as Outer<'r>>::Ty<dyn Inner>) {
//                                ~~~~~~~~~
//                                this branch:  dyn Inner + 'r       (due to bound `'a` on `T`)
//                                stable/main:  dyn Inner + 'static  (due to item signature fallback)
    let _: <() as Outer<'r>>::Ty<dyn Inner + 'static> = x;
//         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
//         this branch:  error: lifetime may not live long enough // `'r` must outlive `'static`
//         stable/main:  OK
}

trait Outer { type Ty; }
trait Inner {}

impl<'a> Outer for dyn Inner + 'a { type Ty = &'a (); }

fn f<'r>(x: *mut &'r <dyn Inner as Outer>::Ty) {
//                    ~~~~~~~~~
//                    this branch:  dyn Inner + 'static  (due to lack of bounds on `Outer`)
//                    stable/main:  dyn Inner + 'r       (since the assoc type path now shadows the default induced by the ref typ ctor `&`)
    let _: *mut &'r <dyn Inner + 'r as Outer>::Ty = x;
//         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
//         this branch:  error: lifetime may not live long enough // `'r` must outlive `'static`
//         stable/main:  OK
}

In type-relative paths Y0::Name<Y1, …, Ym> consider the trait object lifetime default indeterminate
- Meaning if we're in an "item context" / "item signature" / "non-body" (& the principal trait isn't bounded by any outlives-bounds which would take precedence over the default) we will reject any implicit trait object lifetime bounds that would take on that default
- Reason: Limitations of the current implementation which can't be easily overcome
  - RBV (which resolves trait object lifetime defaults by recursing into the local crate "in one sitting") would require the resolution of type-relative paths in order to look up the generics but these paths are only resolved in HIR ty lowering (that can selectively lower local items) which depends on the results of RBV (cyclic dependency!)
  - While one might be able to resolve type-relative paths in RBV in an ad-hoc fashion, it would require a lot of duplication with HIR ty lowering and its impl would be very brittle (RTN does something like that in RBV but we require a more sophisticated resolver)
  - I did attempt that but it got too gnarly and brittle and would've likely been incomplete anyway
  - See also this GH thread
  - See also #t-types/meetings > 2025-09-16 weekly @ 💬
- This should still be maximally forward compatible and allow us to implement the desired behavior in the future.
- Example breakage:
```
trait Outer { type Ty<'a, T: 'a + ?Sized>; }
trait Inner {}

fn f<'r, T: Outer>(x: T::Ty<'r, dyn Inner>) {}
//                              ~~~~~~~~~
//                              this branch:  error: indeterminate  (reservation)
//                              stable/main:  dyn Inner + 'static   (due to item signature fallback)
```

Fixes trait object lifetime defaults inside trait refs TraitRef<X0, …, Xn> (this fell out from the previous changes). They used to be completely broken due to a nasty off-by-one error for not accounting for the implicit Self type param of traits which lead to cases like

Outer<'r, dyn Inner> (with trait Outer<'a, T: 'a + ?Sized> {}) getting rejected as indeterminate (it tries to access a lifetime at index 1 instead 0) (playground)
Outer<'r, 's, dyn Inner> (with trait Outer<'a, 'b, T: 'a + ?Sized> {}) elaborating dyn Inner to dyn Inner + 's instead of dyn Inner + 'r(!) which subsequently get rejected of course since 's isn't known to outlive 'r (playground)
The same applies to trait alias refs (feature trait_alias)

Example breakage:

trait Outer<'a, 'b, T: 'a + ?Sized> {}
trait Inner {}

struct F<'r, T>
where
    T: Outer<'r, 'static, dyn Inner>
//                        ~~~~~~~~~
//                        this branch:  dyn Inner + 'r       (correctly mapping `'a` => `'r`)
//                        stable/main:  dyn Inner + 'static  (incorrectly mapping `'a` => `'static` due to off-by-one)
{
    g: G<'r, T>,
//     ~~~~~~~~
//     this branch:  error: mismatched types
//                          expected: `Outer<'r, 'static, (dyn Inner + 'static)>`
//                             found: `Outer<'r, 'static, (dyn Inner + 'r)>`
//     stable/main:  OK
}

struct G<'r, T>(&'r (), T)
where
    T: Outer<'r, 'static, dyn Inner + 'static>;

In associated type binding TraitRef<AssocTy<X0, …, Xn> = Y> consider the trait object lifetime default indeterminate (in X0, …, Xn and Y) if X0, …, Xn contains any lifetime arguments.
- Meaning if we're in an item context (& the principal trait isn't bounded) we will reject any implicit trait object lifetime bounds that would take on that default
- This reserves us the right to (1) take into account the item bounds of AssocTy in the future when computing the default for Y (2) take into account the parameter bounds of AssocTy in the future when computing the defaults for X0, …, Xn.
- This extends a preexisting hack that – given TraitRef<X0, …, Xn, AssocTy<Y0, …, Ym> = Z> – treats the default indeterminate in Y0, …, Ym and Z if X0, …, Xn contains any lifetime arguments.
- Rephrased, this hack / reservation previously didn't account for GAT args, only trait ref args, which is insufficient
- See also this GH comment of mine
- Example breakages:
```
trait Outer { type Ty<'a>: ?Sized; }
trait Inner {}

fn f<'r>(_: impl Outer<Ty<'r> = dyn Inner>) {}
//                              ~~~~~~~~~
//                              this branch:  error: indeterminate  (reservation)
//                              stable/main:  dyn Inner + 'static   (forced)
```
```
trait Outer { type Ty<'a, T: ?Sized + 'a>; }
trait Inner {}

fn f<'r>(_: impl Outer<Ty<'r, dyn Inner> = ()>) {}
//                            ~~~~~~~~~
//                            this branch:  error: indeterminate  (reservation)
//                            stable/main:  dyn Inner + 'static   (forced)
```

Motivation

Both trait object lifetime default RFCs (599 and 1156) never explicitly specify what constitutes a — what I call — (eligible) container but it only makes sense to include anything that can be parametrized by generics and can be mentioned in places where we don't perform full region inference … like associated types. So it's only consistent to make this change.

Breakages

These changes are theoretically breaking because they can lead to different trait object lifetime bounds getting deduced compared to main which is obviously user observable. Moreover, we're now explicitly rejecting implicit trait object lifetime bounds inside type-relative paths (excl. the self type) and on the RHS of assoc type bindings if the assoc type has lifetime params.
However, the latest crater run found 0 non-spurious regressions (see here and here).

Fixes #115379.
Fixes #140710.
Fixes #141997.

If we are in a body we do use to normal region inference as a fallback. ↩
Indeed, we don't consider implied bounds (inferred outlives-bounds). ↩

fmease · 2024-08-25T03:09:05Z

@bors try

[crater] Properly deduce the object lifetime default in GAT paths Fixes rust-lang#115379. r? ghost

bors · 2024-08-25T03:10:17Z

⌛ Trying commit 8bfcd86 with merge 24cd45d...

bors · 2024-08-25T05:05:36Z

☀️ Try build successful - checks-actions
Build commit: 24cd45d (24cd45d7714ba43afd4b8e62fb677b069b21c4a5)

fmease · 2024-08-25T05:06:34Z

@craterbot check

craterbot · 2024-08-25T05:06:42Z

👌 Experiment pr-129543 created and queued.
🤖 Automatically detected try build 24cd45d
🔍 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

[CRATER] Crater Rollup This is a " crater rollup" of: * rust-lang#126452 * rust-lang#128784 * rust-lang#129392 * rust-lang#129422 * rust-lang#129543 **What is a crater rollup?** It's simply a crater job that is run on all of the containing PRs *together*, and then we can set the crates list for each of these jobs to just the failures after it's done. It should cut out on the bulk of "normal" crates that do nothing and simply just take time to build. r? `@ghost`

[CRATER] Crater Rollup This is a " crater rollup" of: * rust-lang#126452 * rust-lang#128784 * rust-lang#129392 * rust-lang#129422 * rust-lang#129543 * rust-lang#129604 **What is a crater rollup?** It's simply a crater job that is run on all of the containing PRs *together*, and then we can set the crates list for each of these jobs to just the failures after it's done. It should cut out on the bulk of "normal" crates that do nothing and simply just take time to build. r? `@ghost`

compiler-errors · 2024-08-31T12:23:28Z

@craterbot crates=https://gist.githubusercontent.com/compiler-errors/4a09d64cd15dc3dca50edeea26cc9938/raw/b4181c225709e120a11d91cce69d0d4da3e652d0/regressed.txt

craterbot · 2024-08-31T12:23:37Z

📝 Configuration of the pr-129543 experiment changed.

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

traviscross · 2024-08-31T19:27:22Z

@craterbot crates=https://gist.githubusercontent.com/compiler-errors/4a09d64cd15dc3dca50edeea26cc9938/raw/b4181c225709e120a11d91cce69d0d4da3e652d0/regressed.txt p=1

(Bump it up the queue as this will go quickly.)

craterbot · 2024-08-31T19:27:26Z

📝 Configuration of the pr-129543 experiment changed.

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

craterbot · 2024-09-01T07:57:21Z

🚧 Experiment pr-129543 is now running

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

craterbot · 2024-09-01T12:35:33Z

🎉 Experiment pr-129543 is completed!
📊 6 regressed and 0 fixed (98236 total)
📰 Open the full report.

⚠️ If you notice any spurious failure please add them to the blacklist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

fmease · 2026-04-08T20:23:29Z

In the list of examples, can you match them to each of the changes from this PR

Done.

I've addressed all of your review comments. To 3 of these comments I have replied with a counterquestion, namely: (1) #129543 (comment), (2) #129543 (comment), (3) #129543 (comment).

The final commit (Introduce fn eligible_container & rename depth to rev seg index) is new, the other commits are basically unchanged.

@rustbot review

rustbot · 2026-04-11T14:17:02Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

…ir own args (excl. self ty) This automatically fixes trait object lifetime defaulting for trait refs and trait alias refs, too. These used to be broken because the index calculation for going from middle generic args back to HIR ones didn't take into account the implicit self ty param present on traits. Moreover, in item signatures reject hidden trait object lifetime bounds inside type-relative paths (excl. the self ty) on grounds of the default being indeterminate.

…or their self ty

* print `Empty` as it's called in code, otherwise it's unnecessarily confusing * go through the middle::ty Generics instead of the HIR ones, so we can dump the default for the implicit `Self` type parameter of traits, too

…ntiated assoc ty bindings Namely, on the RHS and in the args of the bindings.

lcnr · 2026-04-13T15:09:12Z

-                        self.resolve_object_lifetime_default(&*lifetime)
+                        // If the user doesn't write *anything*, we apply the
+                        // trait object lifetime defaulting rules.
+                        // E.g., `Box<dyn Debug>` becomes `Box<dyn Debug + 'static>`.


View changes since the review

Suggested change

// E.g., `Box<dyn Debug>` becomes `Box<dyn Debug + 'static>`.

// E.g. `Box<dyn Debug>` becomes `Box<dyn Debug + 'static>`.

A comma after "e.g." is a stylistic choice, very prevalent in American English (Chicago Style, AP Style, etc.). See also https://linguaholic.com/linguablog/comma-after-e-g.

lcnr · 2026-04-13T15:21:18Z

+
+fn is_static<T>(_: T) where T: 'static {}
+
+// FIXME: Ideally, we'd elaborate `dyn Bar` to `dyn Bar + 'static` instead of rejecting it.


...induce a lifetime default that's determined by the item bounds

here that would be ~~'static~~indeterminate since OLD(Foo::Item) would be Empty which turns into ~~'static~~indeterminate for associated type bindings in item signatures

is what I'd expect

lcnr · 2026-04-13T15:44:17Z

gonna start a new FCP with this. see the PR description.

@rfcbot fcp merge types

lcnr · 2026-04-14T09:54:29Z

@rfcbot fcp cancel

lcnr · 2026-04-14T09:54:37Z

@rfcbot fcp merge types

rust-rfcbot · 2026-04-14T09:54:42Z

Team member @lcnr has proposed to merge this. The next step is review by the rest of the tagged team members:

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Aug 25, 2024

bors added a commit to rust-lang-ci/rust that referenced this pull request Aug 25, 2024

Auto merge of rust-lang#129543 - fmease:obj-lt-def-gat, r=<try>

24cd45d

[crater] Properly deduce the object lifetime default in GAT paths Fixes rust-lang#115379. r? ghost

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fmease changed the title ~~[crater] Properly deduce the object lifetime default in GAT paths~~ [crater] Properly deduce object lifetime defaults in GAT paths Aug 25, 2024

compiler-errors mentioned this pull request Aug 27, 2024

[CRATER] Crater Rollup #129660

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fmease mentioned this pull request Apr 9, 2026

[IGNORE] Abolish trait object lifetime defaults in bodies #155020

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apiraino mentioned this pull request Apr 9, 2026

Provided team `` is invalid rust-lang/rfcbot-rs#343

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fmease added 8 commits April 11, 2026 19:00

Split out fn compute_object_lifetime_defaults

e891f94

Don't needlessly search for already-found HIR generic param

c939fab

Make resolved assoc ty paths induce a trait object lifetime default f…

74b15be

…or their self ty

Tweak #[rustc_dump_object_lifetime_defaults]

665e1e2

* print `Empty` as it's called in code, otherwise it's unnecessarily confusing * go through the middle::ty Generics instead of the HIR ones, so we can dump the default for the implicit `Self` type parameter of traits, too

Rename hir::GenericArgs::{num,has}_generic_params to *_generic_args

4eddfb0

Render trait object lifetime defaults indeterminate in lifetime-insta…

a07e8e6

…ntiated assoc ty bindings Namely, on the RHS and in the args of the bindings.

Introduce fn eligible_container & rename depth to rev seg index

9b4931c

lcnr reviewed Apr 13, 2026

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	// E.g., `Box<dyn Debug>` becomes `Box<dyn Debug + 'static>`.
	// E.g. `Box<dyn Debug>` becomes `Box<dyn Debug + 'static>`.


		fn is_static<T>(_: T) where T: 'static {}

		// FIXME: Ideally, we'd elaborate `dyn Bar` to `dyn Bar + 'static` instead of rejecting it.

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Trait Object Lifetime Defaults

Primer & Definitions

Changes Made

Motivation

Breakages

Footnotes

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