Use `trait_object_dummy_self` more & heavily fix+update related docs by fmease · Pull Request #153497 · rust-lang/rust

fmease · 2026-03-06T14:38:58Z

Use trait_object_dummy_self (Infer(FreshTy(0))) in more places
- Two places used to construct this dummy self type manually (Ty::new_fresh(tcx, 0))
- One used to use usize as a dummy self type!
Really flesh out the docs of trait_object_dummy_self
Rewrite severely outdated & wrong (doc) comments from scratch (some date back to 2018)
Uncomment a debug assertion that was accidentally commented out in a refactoring PR back in 2018!

fmease · 2026-03-06T14:39:54Z

compiler/rustc_hir_analysis/src/outlives/implicit_infer.rs


-/// Infer predicates for the items in the crate.
-///
-/// `global_inferred_outlives`: this is initially the empty map that


(this parameter no longer exists, it's a local variable now)

fmease · 2026-03-06T14:46:46Z

compiler/rustc_type_ir/src/predicate.rs


-    /// Object types don't have a self type specified. Therefore, when
-    /// we convert the principal trait-ref into a normal trait-ref,
-    /// you must give *some* self type. A common choice is `mk_err()`


A common choice is mk_err()

No longer up to date.

First of all, mk_err() no longer exists, it's Ty::new_error from middle which type IR shouldn't know about.

Second of all, we no longer use {type error} for that since that one carries an ErrorGuaranteed now, so it can no longer be used as a flexible dummy type. It's superseded by, you guessed it, trait_object_dummy_self (cc #70866, #71938).

fmease · 2026-03-06T14:47:38Z

compiler/rustc_type_ir/src/predicate.rs

+    ///
+    /// Existential trait refs don't contain a self type, it's erased.
+    /// Therefore, you must specify *some* self type to perform the conversion.
+    /// A common choice is the trait object type itself or some kind of dummy type.


We're not mentioning trait_object_dummy_self from middle by name because type IR shouldn't know about it.

fmease · 2026-03-06T14:48:11Z

compiler/rustc_type_ir/src/predicate.rs

+/// For example, the trait object type `Trait<'a, T, N>` can be understood as:
 /// ```ignore (illustrative)
-/// exists T. T: Trait<'a, 'b, X, Y>
+/// exists<X> X: Trait<'a, T, N>


Using a more Rust-like syntax instead of a Haskell-based one.

fmease · 2026-03-06T14:50:11Z

compiler/rustc_middle/src/ty/context.rs

    pub never: Ty<'tcx>,
    pub self_param: Ty<'tcx>,

-    /// Dummy type used for the `Self` of a `TraitRef` created for converting


Context: "converting" is old terminology for (HIR ty) lowering before the ASTconv → HIR ty lowering renaming.

fmease · 2026-03-06T14:51:30Z

compiler/rustc_type_ir/src/predicate.rs

+    /// A common choice is the trait object type itself or some kind of dummy type.
    pub fn with_self_ty(self, interner: I, self_ty: I::Ty) -> TraitRef<I> {
        // otherwise the escaping vars would be captured by the binder
-        // debug_assert!(!self_ty.has_escaping_bound_vars());


This debug assert was accidentally commented out in a refactoring PR from 2018! See PR #53816, path src/librustc/ty/sty.rs.

fmease · 2026-03-06T14:53:15Z

compiler/rustc_hir_analysis/src/outlives/implicit_infer.rs

-    for (outlives_predicate, &span) in explicit_predicates.as_ref().skip_binder() {
-        debug!("outlives_predicate = {outlives_predicate:?}");
-
-        // Careful: If we are inferring the effects of a `dyn Trait<..>`


I've "moved" this lengthy explainer to the ty::Dynamic branch (or rather replaced it with a more concise & up to date one) because it is about trait object types specifically and therefore shouldn't belong in this general function!

fmease · 2026-03-06T15:16:39Z

compiler/rustc_hir_analysis/src/outlives/implicit_infer.rs

-        // Note that we do this check for self **before** applying `args`. In the
-        // case that `args` come from a `dyn Trait` type, our caller will have
-        // included `Self = usize` as the value for `Self`. If we were
-        // to apply the args, and not filter this predicate, we might then falsely
-        // conclude that e.g., `X: 'x` was a reasonable inferred requirement.


The provided example ("we might then falsely conclude that e.g., X: 'x was a reasonable inferred requirement") doesn't make any sense whatsoever! That's because this comment wasn't properly updated over time:

Originally, it didn't say Self = usize but Self = dyn Trait<'x, X> which makes a whole lot more sense since back in ancient times, we did use the trait object type itself for the erased self type which was problematic and thus changed to usize (sic!).

So the problem this paragraph is trying to convey is:

Given:

struct Struct<'x, X>(Box<dyn Trait<'x, X>>);

trait Trait<'a, T> where Self: 'a {}

We'd convert existential trait ref Trait<'x, X> to normal trait ref <dyn Trait<'x, X> as Trait<'x, X>> (setting the self type to the trait object type itself)

Extract its args, so [dyn Trait<'x, X>, 'x, X]

Instantiate Self: 'a from the trait with these args to obtain dyn Trait<'x, X>: 'x

We then split the trait object type in outlives-components, namely X: 'x (and 'x: 'x) which would be bad.

Therefore we filter out outlives-bounds mentioning Self before substitution/instantiation.

However, using the trait object type itself is still problematic since it can lead to escaping bound vars (issue #53419), therefore it was replaced with the dummy usize (PR #53441) but the comment wasn't updated (PR #56003 later updated it to Self = usize but didn't update the rest of the paragraph).

fmease · 2026-03-06T15:21:20Z

compiler/rustc_hir_analysis/src/outlives/implicit_infer.rs

-                    // predicates in `check_explicit_predicates` we
-                    // need to ignore checking the explicit_map for
-                    // Self type.
-                    let args = ex_trait_ref.with_self_ty(tcx, tcx.types.usize).skip_binder().args;


Instead of a dummy usize we now use the "canonical" trait_object_dummy_self which is way less janky.

fmease · 2026-03-06T15:27:27Z

compiler/rustc_hir_analysis/src/outlives/implicit_infer.rs

+        if let IgnorePredicatesReferencingSelf::Yes = ignore_preds_refing_self
+            && arg.walk().any(|arg| arg == tcx.types.self_param.into())


Note that this whole "filter out things before substitution/instantiation" topic is completely irrelevant now due to us using trait_object_dummy_self instead of usize. The former is guaranteed to never occur in the outlives-predicates (that's a guarantee by HIR ty lowering / HIR analysis's type collection). The latter, usize, can obviously occur in the predicates.

So theoretically I could just look for fn param ignored_self_ty (set to the dummy Self for branch ty::Dynamic) after instantiation if I so wanted to.

However, I feel like looking for self_param before instantiation plus just having a Boolean-like parameter is a lot cleaner.

rustbot · 2026-03-06T15:28:04Z

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rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Mar 6, 2026

fmease commented Mar 6, 2026

View reviewed changes

fmease marked this pull request as ready for review March 6, 2026 15:28

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Mar 6, 2026

rustbot assigned dingxiangfei2009 Mar 6, 2026

rustbot removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Mar 6, 2026

fmease force-pushed the trait-obj-dummy-self-improvs branch 2 times, most recently from aeaf232 to 03b5e8e Compare March 6, 2026 15:40

Use trait_object_dummy_self more & heavily fix+update related docs

4e7b116

fmease force-pushed the trait-obj-dummy-self-improvs branch from 03b5e8e to 4e7b116 Compare March 6, 2026 16:10

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Use `trait_object_dummy_self` more & heavily fix+update related docs#153497

Use `trait_object_dummy_self` more & heavily fix+update related docs#153497
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		if let IgnorePredicatesReferencingSelf::Yes = ignore_preds_refing_self
		&& arg.walk().any(\|arg\| arg == tcx.types.self_param.into())

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