Emit ForbiddenBound for bounds of type params when parsing higher ranked binders by mu001999 · Pull Request #149728 · rust-lang/rust

mu001999 · 2025-12-07T03:44:49Z

Bounds in binders are denied, hir items won't contain and index them. But nested items in the bounds will still be lowered to hir. And their parents, i.e., the block in bounds is not in hir. So that ICE happens when error handling requires visiting hir parents.

~~This PR collects hirless def ids and skips them when iterating parents.~~

This PR checks such bounds used in higher ranked binders and emit error ForbiddenBound when parsing. And make sure no such bounds appear in hir.

rustbot · 2025-12-07T03:44:54Z

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mu001999 · 2025-12-18T03:50:15Z

@rustbot reroll

compiler/rustc_ast_passes/src/feature_gate.rs

jdonszelmann · 2026-01-08T23:42:59Z

@rustbot author

rustbot · 2026-01-08T23:43:03Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

mu001999 · 2026-01-09T01:41:50Z

@rustbot review

jdonszelmann · 2026-01-09T09:38:23Z

@rusbot author

jdonszelmann · 2026-01-09T09:38:57Z

@rustbot blocked on #149728 (comment)

mu001999 · 2026-01-09T18:09:10Z

Hi @jdonszelmann, in the lastest rough change, I tried to filter nested items in the bound, and not lower them to hir.

dbg! shows it works:

[compiler/rustc_ast_lowering/src/lib.rs:527:17] "skipping lowering of {:?}" = "skipping lowering of {:?}"
[compiler/rustc_ast_lowering/src/lib.rs:527:17] def_id = DefId(0:6 ~ bad_bounds[bee6]::produce::{constant#0}::A)
[compiler/rustc_ast_lowering/src/lib.rs:527:17] "skipping lowering of {:?}" = "skipping lowering of {:?}"
[compiler/rustc_ast_lowering/src/lib.rs:527:17] def_id = DefId(0:7 ~ bad_bounds[bee6]::produce::{constant#0}::A::{constructor#0})
[compiler/rustc_ast_lowering/src/lib.rs:527:17] "skipping lowering of {:?}" = "skipping lowering of {:?}"
[compiler/rustc_ast_lowering/src/lib.rs:527:17] def_id = DefId(0:8 ~ bad_bounds[bee6]::produce::{constant#0}::A::A)
[compiler/rustc_ast_lowering/src/lib.rs:527:17] "skipping lowering of {:?}" = "skipping lowering of {:?}"
[compiler/rustc_ast_lowering/src/lib.rs:527:17] def_id = DefId(0:10 ~ bad_bounds[bee6]::produce::{constant#0}::A#1)
[compiler/rustc_ast_lowering/src/lib.rs:527:17] "skipping lowering of {:?}" = "skipping lowering of {:?}"
[compiler/rustc_ast_lowering/src/lib.rs:527:17] def_id = DefId(0:11 ~ bad_bounds[bee6]::produce::{constant#0}::{impl#0})

But there still be errors, because they still have LocalDefId, so that they can be result of many other queries. For example, after the change, a new ICE appears because type_of(bad_bounds[bee6]::produce::{constant#0}::{impl#0}).

Concrete content of the new ICE is:

error: internal compiler error: compiler/rustc_middle/src/hir/mod.rs:385:32: No HirId for DefId(0:11 ~ bad_bounds[bee6]::produce::{constant#0}::{impl#0})


thread 'rustc' (6902108) panicked at compiler/rustc_middle/src/hir/mod.rs:385:32:
Box<dyn Any>
stack backtrace:
   0: std::panicking::begin_panic::<rustc_errors::ExplicitBug>
             at /rustc/1b6e21e163baa0b20f119e17e3871910978a60b6/library/std/src/panicking.rs:761:5
   1: std::panic::panic_any::<rustc_errors::ExplicitBug>
             at /rustc/1b6e21e163baa0b20f119e17e3871910978a60b6/library/std/src/panic.rs:260:5
   2: <rustc_errors::diagnostic::BugAbort as rustc_errors::diagnostic::EmissionGuarantee>::emit_producing_guarantee
             at ./compiler/rustc_errors/src/diagnostic.rs:63:9
   3: <rustc_errors::diagnostic::Diag<rustc_errors::diagnostic::BugAbort>>::emit
             at ./compiler/rustc_errors/src/diagnostic.rs:1405:9
   4: <rustc_errors::DiagCtxtHandle>::bug::<alloc::string::String>
             at ./compiler/rustc_errors/src/lib.rs:1231:30
   5: rustc_middle::util::bug::opt_span_bug_fmt::<rustc_span::span_encoding::Span>::{closure#0}
             at ./compiler/rustc_middle/src/util/bug.rs:39:48
   6: rustc_middle::ty::context::tls::with_opt::<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, !>::{closure#0}
             at ./compiler/rustc_middle/src/ty/context/tls.rs:136:23
   7: rustc_middle::ty::context::tls::with_context_opt::<rustc_middle::ty::context::tls::with_opt<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
             at ./compiler/rustc_middle/src/ty/context/tls.rs:79:18
   8: rustc_middle::ty::context::tls::with_opt::<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, !>
             at ./compiler/rustc_middle/src/ty/context/tls.rs:134:5
   9: rustc_middle::util::bug::opt_span_bug_fmt::<rustc_span::span_encoding::Span>
             at ./compiler/rustc_middle/src/util/bug.rs:33:5
  10: rustc_middle::util::bug::bug_fmt
             at ./compiler/rustc_middle/src/util/bug.rs:16:5
  11: rustc_middle::hir::provide::{closure#0}
             at ./compiler/rustc_middle/src/macros.rs:18:9
  12: <rustc_middle::hir::provide::{closure#0} as core::ops::function::FnOnce<(rustc_middle::ty::context::TyCtxt, rustc_span::def_id::LocalDefId)>>::call_once
             at /rustc/1b6e21e163baa0b20f119e17e3871910978a60b6/library/core/src/ops/function.rs:250:5
  13: rustc_query_impl::query_impl::local_def_id_to_hir_id::dynamic_query::{closure#2}::{closure#0}
             at ./compiler/rustc_query_impl/src/plumbing.rs:298:9
      [... omitted 22 frames ...]
  14: rustc_middle::query::inner::query_get_at::<rustc_data_structures::vec_cache::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::Erased<[u8; 8]>, rustc_query_system::dep_graph::graph::DepNodeIndex>>
             at ./compiler/rustc_middle/src/query/inner.rs:33:17
  15: <rustc_middle::query::plumbing::TyCtxtAt>::local_def_id_to_hir_id::<rustc_span::def_id::LocalDefId>
             at ./compiler/rustc_middle/src/query/plumbing.rs:415:31
  16: <rustc_middle::ty::context::TyCtxt>::local_def_id_to_hir_id::<rustc_span::def_id::LocalDefId>
             at ./compiler/rustc_middle/src/query/plumbing.rs:406:35
  17: rustc_hir_analysis::collect::type_of::type_of::{closure#0}
             at ./compiler/rustc_hir_analysis/src/collect/type_of.rs:56:22
  18: rustc_hir_analysis::collect::type_of::type_of
             at ./compiler/rustc_hir_analysis/src/collect/type_of.rs:21:1
  19: rustc_query_impl::query_impl::type_of::dynamic_query::{closure#2}::{closure#0}
             at ./compiler/rustc_query_impl/src/plumbing.rs:302:13
      [... omitted 22 frames ...]
  20: rustc_middle::query::inner::query_get_at::<rustc_query_system::query::caches::DefIdCache<rustc_middle::query::erase::Erased<[u8; 8]>>>
             at ./compiler/rustc_middle/src/query/inner.rs:33:17
  21: <rustc_middle::query::plumbing::TyCtxtAt>::type_of::<rustc_span::def_id::DefId>
             at ./compiler/rustc_middle/src/query/plumbing.rs:415:31
  22: <rustc_middle::ty::context::TyCtxt>::type_of::<rustc_span::def_id::DefId>
             at ./compiler/rustc_middle/src/query/plumbing.rs:406:35
  23: rustc_middle::ty::trait_def::trait_impls_of_provider
             at ./compiler/rustc_middle/src/ty/trait_def.rs:227:32
  24: rustc_query_impl::query_impl::trait_impls_of::dynamic_query::{closure#2}::{closure#0}
             at ./compiler/rustc_query_impl/src/plumbing.rs:298:9
      [... omitted 22 frames ...]
  25: rustc_middle::query::inner::query_get_at::<rustc_query_system::query::caches::DefIdCache<rustc_middle::query::erase::Erased<[u8; 8]>>>
             at ./compiler/rustc_middle/src/query/inner.rs:33:17
  26: <rustc_middle::query::plumbing::TyCtxtAt>::trait_impls_of::<rustc_span::def_id::DefId>
             at ./compiler/rustc_middle/src/query/plumbing.rs:415:31
  27: <rustc_middle::ty::context::TyCtxt>::trait_impls_of::<rustc_span::def_id::DefId>
             at ./compiler/rustc_middle/src/query/plumbing.rs:406:35
  28: <rustc_middle::ty::context::TyCtxt>::all_impls
             at ./compiler/rustc_middle/src/ty/trait_def.rs:194:68

mu001999 · 2026-01-09T18:11:01Z

I think at least it's more difficult than I expected. Do you have any suggestions?

mu001999 · 2026-02-02T14:15:04Z

@rustbot ready

lcnr · 2026-02-10T12:58:45Z

another alternative would be to actually lower where-bounds on binders to the HIR, or to already error on them when creating the AST before we collect the DefIds?

I personally feel like erroring pre def-collection might be better 🤔 haven't interacted with this code much so I am not sure :>

mu001999 · 2026-02-14T14:32:01Z

to already error on them when creating the AST

Make sense, I think I could move the check in check_late_bound_lifetime_defs into parse_higher_ranked_binder, and report it as a parse error and then return empty bounds.

rustbot · 2026-02-14T16:06:02Z

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.

rustbot · 2026-02-15T03:13:54Z

Some changes occurred in src/tools/rustfmt

cc @rust-lang/rustfmt

jdonszelmann · 2026-02-26T14:15:02Z

@mu001999 is this ready for review? I think doing it like this is quite reasonable, and we don't error fatally which is nice.

mu001999 · 2026-02-26T14:18:10Z

is this ready for review

yep, it's ready :)

jdonszelmann · 2026-02-28T08:31:03Z

r=me after that

jdonszelmann · 2026-02-28T08:29:19Z

compiler/rustc_ast_passes/src/feature_gate.rs


        for param in params {
-            if !param.bounds.is_empty() {
+            if !matches!(param.kind, ast::GenericParamKind::Type { .. }) && !param.bounds.is_empty()


This check is the same as one below

No, they are different. This is !matches!(param.kind, ast::GenericParamKind::Type { .. }), the below is matches!(param.kind, ast::GenericParamKind::Type { .. })

jdonszelmann · 2026-02-28T08:29:36Z

compiler/rustc_parse/src/parser/ty.rs

+            let params = params
+                .into_iter()
+                .map(|mut param| {
+                    if matches!(param.kind, ast::GenericParamKind::Type { .. })


This one. Might be nice to split out?

Into a .is_forbidden_bound or whatever

As the above, .is_forbidden_bound will only check !param.bounds.is_empty() in fact

rustbot assigned wesleywiser Dec 7, 2025

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mu001999 force-pushed the fix/149695 branch from 888f87a to 84a0f1e Compare December 7, 2025 10:36

mu001999 changed the title ~~Emit ForbiddenBound fatally~~ Emit ForbiddenBound fatally if meeting complex bounds Dec 7, 2025

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mu001999 marked this pull request as draft January 9, 2026 15:56

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mu001999 force-pushed the fix/149695 branch from f06b317 to 4a6ed27 Compare February 2, 2026 13:51

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mu001999 changed the title ~~Collect hirless def ids and skip them when iterating parents~~ Emit ForbiddenBound fatally if meeting complex bound Feb 2, 2026

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mu001999 force-pushed the fix/149695 branch from e9ffcdd to 4158ee8 Compare February 14, 2026 16:05

mu001999 changed the title ~~Emit ForbiddenBound fatally if meeting complex bound~~ Emit ForbiddenBound when parsing higher ranked binders Feb 14, 2026

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mu001999 force-pushed the fix/149695 branch from 4158ee8 to 5781fda Compare February 15, 2026 01:10

mu001999 changed the title ~~Emit ForbiddenBound when parsing higher ranked binders~~ Emit ForbiddenBound for bounds of type params when parsing higher ranked binders Feb 15, 2026

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mu001999 force-pushed the fix/149695 branch from 5781fda to 151537e Compare February 15, 2026 03:13

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Emit ForbiddenBound for bounds of type params when parsing

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mu001999 force-pushed the fix/149695 branch from 151537e to ca6cc5d Compare February 15, 2026 04:27

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