Properly deduce object lifetime defaults in projections & trait refs

[fmease]

Object Lifetime Defaults (Primer, Refresher & Definitions)

You can read this section in The Reference but has several issues¹. 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 object lifetime bound from context (without relying on rustc_infer's region inference as we might not be in a body²). The "context" means the closest — what I call — eligible generic container C<X0, …, Xn> that wraps this trait object type. (Eligible generic) container is almost synonymous with type constructor but it also includes type aliases, traits & enum variants.

So if we have C<…, dyn Trait, …> (e.g., &'r dyn Trait or Struct<'r, dyn Trait>) or C<…, N<…, dyn Trait, …>, …> (e.g., &'r (dyn Trait,) or Struct<'r, (dyn Trait,)>) where N denotes a generic type that is not an eligible generic container, we use the explicit³ outlives-bounds on the corresp. type param of C to determine the object lifetime bound (the details⁴ aren't relevant here) (e.g., given struct Struct<'a, T: 'a + ?Sized>(…);, we elaborate Struct<'r, dyn Trait> to Struct<'r, dyn Trait + 'r>).

Lastly, I call object lifetime bounds used as the default for constituent trait object types of an eligible generic container C the ambient object lifetime defaults for / induced by C (these ambient defaults may be shadowed by inner containers).

---

Changes Made by This PR

1. Make resolved associated type paths (projections) eligible generic containers.
   • <Y0 as TraitRef<X0, …, Xn>>::AssocTy<Y1, …, Ym> now induces ambient object lifetime defaults for constituents Y0 to Ym (TraitRef is considered a separate container, see also list item (2)).
   • 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).
2. In type-relative paths Y0::AssocTy<Y1, …, Ym> consider the ambient object lifetime default indeterminate
   • Meaning if we're in an item context ("item signature", "non-body") & the trait object type isn't bounded by any outlives-bounds (which would take precedence over the default) we will reject the type
   • Reason: Limitations of the current implementation which can't be easily overcome
     • RBV (which resolves 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
     • See also this GH thread
     • See also #t-types/meetings > 2025-09-16 weekly @ 💬
   • IINM this should still be maximally forward compatible
3. Fixes 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 gnarly 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(!) (playground)

These changes are theoretically breaking because in certain cases they lead to different object lifetime bounds getting deduced compared to master which is obviously user observable. However, the latest crater run found 0 non-spurious regressions.

Motivation: Both object lifetime default RFCs never explicitly specify what constitutes an — what I call — eligible generic container but it only makes sense to include any type constructor or (generic) type alias that can bear outlives-bounds … like associated types. So it's only consistent to make this change.

Fixes #115379.

TODO(#129543 (comment)): Add a clear example regression to the PR description.
TODO: Add lcnr's tests.
TODO: #129543 (comment) needs to be decided.

Footnotes

1. https://github.com/rust-lang/reference/issues/1407 ↩

2. If we are in a body, we do however use to normal region inference as a fallback. ↩

3. Indeed, we don't consider implied bounds (inferred outlives-bounds). ↩

4. Like how we deal with 'ambiguities' or how bounds 'on' inner TOTs take precedence. ↩

[fmease]

@bors try

[bors]

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

[bors]

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

[fmease]

@craterbot check

[craterbot]

👌 Experiment pr-129543 created and queued.
🤖 Automatically detected try build 24cd45d
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[compiler-errors]

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

[craterbot]

📝 Configuration of the pr-129543 experiment changed.

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[traviscross]

@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]

📝 Configuration of the pr-129543 experiment changed.

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[craterbot]

🚧 Experiment pr-129543 is now running

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[craterbot]

🎉 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!
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[rustbot]

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann

[rustbot]

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.

[fmease]

please add a test for

trait Trait<T> {
    type Assoc<'a>
    where
        T: 'a;
}

I would assume that we don't have a default for (): Trait<dyn Inner>. What about (): Trait<dyn Inner, Assoc<'r> = ()> or <() as Trait<dyn Inner>>::Assoc<'r>

In all three cases, the following holds:

1. trait Inner presumably isn't bounded by a lifetime
2. We're in an item ctxt / item signature / non-body
   • Cases 1 & 2: in a where-clause to be precise
   • Case 3: the projection could be inside of a body, we don't know, but then it'd resolve to a fresh region var, so that case isn't interesting
3. Trait's T doesn't impose any lifetime requirements (we don't traverse into the associated items) aka it's Empty

We therefore default to 'static which is the fallback / used for Empty in non-bodies.

I don't know, do you expect that we default to 'r in the third case? I think I could make that work, just let me know.

[fmease]

I've now dropped the final commit again, the one that attempted to resolve type-relative paths inside RBV. See #t-types/meetings > 2025-09-16 weekly @ 💬 for a rationale.

I'm now fully set on considering ambient object lifetime defaults induced by type-relative paths indeterminate, so this PR has a realistic chance of getting merged within a reasonable time frame.

I've updated the PR description. Still three minor-ish TODOs left (see the PR description).
I might make a patch available somewhere which impls object lifetime defaulting in type-relative paths (which is pretty hacky be warned). The more principled approach would be to somehow intertwine RBV & HIR ty lowering (requires rearchitecture).

[rustbot]

⚠️ Warning ⚠️

• This PR is based on an upstream commit that is older than 28 days.

  It's recommended to update your branch according to the rustc-dev-guide.

[fmease]

I've pushed a tmp commit to address #115379 (comment) which I'll need to crater. It's good to rerun crater anyway cuz the last one was almost a year ago.

@bors try

[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[TIMING:end] tool::ToolBuild { build_compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu, tool: "tidy", path: "src/tools/tidy", mode: ToolBootstrap, source_type: InTree, extra_features: [], allow_features: "", cargo_args: [], artifact_kind: Binary } -- 11.850
[TIMING:end] tool::Tidy { compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu } -- 0.000
fmt check
Diff in /checkout/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs:1826:
         // in the trait ref `YY<...>` in `Item: YY<...>`.
         for constraint in generic_args.constraints {
             let scope = Scope::ObjectLifetimeDefault {
-                lifetime: if has_lifetime_parameters || constraint.gen_args.args.iter().any(|arg| matches!(arg, GenericArg::Lifetime(_))) {
+                lifetime: if has_lifetime_parameters
+                    || constraint
+                        .gen_args
+                        .args
+                        .iter()
+                        .any(|arg| matches!(arg, GenericArg::Lifetime(_)))
+                {
                     None
                 } else {
                     Some(ResolvedArg::StaticLifetime)
fmt: checked 6650 files

[rust-bors]

☀️ Try build successful (CI)
Build commit: 5ff9315 (5ff93158e08fa2205cd4b86cddf9eae95952cc96, parent: 86a49fd71fecd25b0fd20247db0ba95eeceaba28)

[fmease]

@craterbot check

[craterbot]

👌 Experiment pr-129543-2 created and queued.
🤖 Automatically detected try build 5ff9315
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