Unflatten `use` statements in HIR by oli-obk · Pull Request #161349 · rust-lang/rust

oli-obk · 2026-08-19T10:20:04Z

The HIR and AST representations of use statements are now mirrored, instead of having the HIR flatten the AST representation and duplicating early parts of nested paths:

use a::b::{c, d};

used to get flattened in HIR to

use a::b::c;
use a::b::d;
use a::b::{};

which duplicated the a::b part several times (and many more if you have more nesting).

This alone is fine, it made some parts of the compiler simpler and some parts harder, but it also meant we were generating more items (which are also owners) and had to duplicate resolver information across the flattened items. All that is gone with this PR.

This PR is the minimal version we can land, but there are obvious further avenues for cleanups and improvements. I did some refactorings in rustdoc, but I think the import logic can generally be improved by not handling imports together with other items and instead having them in separate tables.

Similarly clippy can probably benefit from a check_use_tree method on LateLintPass, but they often need to track separate information anyway, so it may be better to just always manually recurse.

cc @cjgillot

r? @petrochenkov (or reassign)

should unblock #159760

see related discussion on #t-compiler > partial_res_map vs per-owner tables @ 💬

part of rust-lang/goals#620

first commit is from #161299

everything up to 6936535 can land in a separate PR first

oli-obk · 2026-08-19T10:20:20Z

@bors try @rust-timer queue

Unflatten `use` statements in HIR

rust-bors · 2026-08-19T10:29:13Z

💔 Test for bb18a8a failed: CI. Failed job:

try - dist-x86_64-linux-quick (web logs, enhanced plaintext logs)

oli-obk · 2026-08-19T12:51:57Z

The CI failure makes no sense, the only way that assert is reachable is by there being UB somewhere afaict:

rust/compiler/rustc_metadata/src/rmeta/decoder.rs

Lines 1447 to 1452 in b588ef3

    
           match def_key.disambiguated_data.data { 
        
               DefPathData::Ctor => { 
        
                   // Structure and variant constructors don't have any attributes encoded for them, 
        
                   // but we assume that someone passing a constructor ID actually wants to look at 
        
                   // the attributes on the corresponding struct or variant. 
        
                   assert_eq!(def_key.disambiguated_data.data, DefPathData::Ctor);

Also I can't reproduce locally 😨

WaffleLapkin · 2026-08-19T14:57:44Z

The backtrace doesn't make sense either:

  12:     0x7f7d92df5b99 - core[8ec35bcd8078f253]::panicking::assert_failed::<rustc_hir[5841a66ff274cc3a]::definitions::DefPathData, rustc_hir[5841a66ff274cc3a]::definitions::DefPathData>
  13:     0x7f7d946407c1 - rustc_metadata[69b7fa7d7ff41b69]::rmeta::decoder::cstore_impl::provide_extern::attrs_for_def

The assert is in CrateMetadata::get_item_attrs, but the backtrace skips it, even though it isn't marked with #[track_caller]. 🤔

And attrs_for_def shouldn't be able to panic like that on its own:

rust/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs

Line 332 in f7d782a

    
           attrs_for_def => { tcx.arena.alloc_from_iter(cdata.get_item_attrs(tcx, def_id.index)) }

rust/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs

Lines 138 to 163 in f7d782a

    
           ($tcx:ident, $def_id:ident, $other:ident, $cdata:ident, $name:ident => $compute:block) => { 
        
               fn $name<'tcx>( 
        
                   $tcx: TyCtxt<'tcx>, 
        
                   def_id_arg: rustc_middle::queries::$name::Key<'tcx>, 
        
               ) -> rustc_middle::queries::$name::ProvidedValue<'tcx> { 
        
                   let _prof_timer = 
        
                       $tcx.prof.generic_activity(concat!("metadata_decode_entry_", stringify!($name))); 
        
                   #[allow(unused_variables)] 
        
                   let ($def_id, $other) = def_id_arg.into_args(); 
        
                   assert!(!$def_id.is_local()); 
        
                   // External query providers call `crate_hash` in order to register a dependency 
        
                   // on the crate metadata. The exception is `crate_hash` itself, which obviously 
        
                   // doesn't need to do this (and can't, as it would cause a query cycle). 
        
                   use rustc_middle::dep_graph::DepKind; 
        
                   if DepKind::$name != DepKind::crate_hash && $tcx.dep_graph.is_fully_enabled() { 
        
                       $tcx.ensure_ok().crate_hash($def_id.krate); 
        
                   } 
        
                   let cstore = CStore::from_tcx($tcx); 
        
                   let $cdata = cstore.get_crate_data($def_id.krate); 
        
                   $compute 
        
               } 
        
           };

oli-obk · 2026-08-19T15:09:37Z

The assert is in CrateMetadata::get_item_attrs, but the backtrace skips it, even though it isn't marked with #[track_caller]. 🤔

probably got inlined. but the assert message says it's in a different line

compiler/rustc_metadata/src/rmeta/decoder.rs:1404

I freshly rebased, so I assumed it should be fine, very odd

oli-obk · 2026-08-21T15:06:41Z

@bors try @rust-timer queue

Unflatten `use` statements in HIR

rust-bors · 2026-08-21T16:06:58Z

☀️ Try build successful (CI)
Build commit: 00e1020 (00e1020c22d36a6629c7d764b1967a735a100e74)
Base parent: 526c36b (526c36b4c477994b7905ec6a084d6150e49076f5)

rust-timer · 2026-08-21T16:55:36Z

Finished benchmarking commit (00e1020): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never rustc-perf
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.4%	[0.2%, 0.9%]	45
Regressions ❌ (secondary)	0.7%	[0.0%, 1.8%]	47
Improvements ✅ (primary)	-1.3%	[-18.7%, -0.1%]	80
Improvements ✅ (secondary)	-0.5%	[-1.4%, -0.1%]	12
All ❌✅ (primary)	-0.7%	[-18.7%, 0.9%]	125

Max RSS (memory usage)

Results (primary 1.0%, secondary 3.8%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.7%	[0.4%, 4.1%]	20
Regressions ❌ (secondary)	4.8%	[2.1%, 11.1%]	18
Improvements ✅ (primary)	-2.0%	[-11.4%, -0.4%]	12
Improvements ✅ (secondary)	-5.2%	[-7.7%, -2.7%]	2
All ❌✅ (primary)	1.0%	[-11.4%, 4.1%]	32

Cycles

Results (primary -5.9%, secondary 3.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.8%	[2.3%, 3.2%]	2
Regressions ❌ (secondary)	3.2%	[2.2%, 3.8%]	11
Improvements ✅ (primary)	-8.0%	[-16.0%, -2.4%]	8
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-5.9%	[-16.0%, 3.2%]	10

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 470.08s -> 468.116s (-0.42%)
Artifact size: 400.00 MiB -> 400.03 MiB (0.01%)

rustbot · 2026-09-07T16:01:26Z

clippy is developed in its own repository. If possible, consider making this change to rust-lang/rust-clippy instead.

cc @rust-lang/clippy

Some changes occurred in match checking

cc @Nadrieril

ada4a · 2026-09-07T16:11:55Z

+                (UseKind::Single(l_ident), UseKind::Single(r_ident)) => l_ident.name == r_ident.name,
+                (UseKind::Glob, UseKind::Glob) => true,
+                (UseKind::Nested { items: l_items }, UseKind::Nested { items: r_items }) => {
+                    l_items.len() == r_items.len()


nit: you should be able to use over here

View changes since the review

petrochenkov · 2026-09-08T11:26:49Z

+                PerNS { type_ns: Some(self), value_ns: None, macro_ns: None }
+            }
+            Res::Def(DefKind::Enum, _) => {
+                PerNS { type_ns: None, value_ns: Some(self), macro_ns: None }


This value namespace for DefKind::Enum is pre-existing, but clearly incorrect.
It probably doesn't cause issues because in HIR nobody actually inspects the namespace parts of PerNS.

View changes since the review

petrochenkov · 2026-09-08T11:36:09Z

+
+impl UseTree<'_> {
+    pub fn resolutions(&self) -> impl Iterator<Item = PerNS<Option<Res>>> {
+        Box::new(std::iter::iter!(|| {


Is the box necessary here?

View changes since the review

yes, the generator is recursively calling resolutions, which we can only handle with boxing rn

petrochenkov · 2026-09-08T12:10:34Z

+                    let hir::UsePath { segments, span, .. } = *tree.prefix;
+                    let path = hir::UsePath {
+                        segments,
+                        res: PerNS { value_ns: Some(*res), type_ns: None, macro_ns: None },


Suggested change

res: PerNS { value_ns: Some(*res), type_ns: None, macro_ns: None },

res: *res,

Does something break if this is not changed?

View changes since the review

The old code used to directly invoke clean_use_statement_inner. The logic for that is now in clean_use_statement_leaf, but this situation here can have nesting. The other sites used to invoke clean_use_statement with a UsePath, and then internally loop over all the namespaces. I could try splitting it up more, but just creating a fake PerNS has the same behaviour as previously. If in the future the namespace becomes actually relevant for rustdoc, we can always preserve the namespace information from the logic that inlines it.

But I did just realize that I broke the inlining logic where those inlined_foreigns are created. rustdoc processes hir items twice in different ways, and I didn't make one of the ways handle nested items

…ding scope instead

rust-log-analyzer · 2026-09-09T15:21:11Z

The job test-aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

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

To only update this specific test, also pass `--test-args internal-lints/default_hash_types.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui-fulldeps/internal-lints/default_hash_types.rs" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui-fulldeps/internal-lints/default_hash_types" "-Znext-solver=coherence" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Z" "unstable-options"
stdout: none
--- stderr -------------------------------
error: prefer `FxHashMap` over `HashMap`, it has better performance
##[error]  --> /checkout/tests/ui-fulldeps/internal-lints/default_hash_types.rs:9:24
   |
---
To only update this specific test, also pass `--test-args internal-lints/non_glob_import_of_type_ir_inherent.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui-fulldeps/internal-lints/non_glob_import_of_type_ir_inherent.rs" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui-fulldeps/internal-lints/non_glob_import_of_type_ir_inherent" "-Znext-solver=coherence" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Z" "unstable-options"
stdout: none
--- stderr -------------------------------
error: non-glob import of `rustc_type_ir::inherent`
##[error]  --> /checkout/tests/ui-fulldeps/internal-lints/non_glob_import_of_type_ir_inherent.rs:16:9
   |
LL |     use rustc_type_ir::inherent::Predicate; //~ ERROR non-glob import of `rustc_type_ir::inherent`
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^---------
   |                                  |
   |                                  help: try using a glob import instead: `*`
   |
note: the lint level is defined here
---

error: non-glob import of `rustc_type_ir::inherent`
##[error]  --> /checkout/tests/ui-fulldeps/internal-lints/non_glob_import_of_type_ir_inherent.rs:20:9
   |
LL |     use rustc_type_ir::inherent::ParamEnv as _; //~ ERROR non-glob import of `rustc_type_ir::inherent`
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^-------------
   |                                  |
   |                                  help: try using a glob import instead: `*`

error: non-glob import of `rustc_type_ir::inherent`
##[error]  --> /checkout/tests/ui-fulldeps/internal-lints/non_glob_import_of_type_ir_inherent.rs:24:9
   |
LL |     use rustc_type_ir::inherent; //~ ERROR non-glob import of `rustc_type_ir::inherent`
   |         ^^^^^^^^^^^^^^^^^^^^^^^- help: try using a glob import instead: `::*`

error: non-glob import of `rustc_type_ir::inherent`
##[error]  --> /checkout/tests/ui-fulldeps/internal-lints/non_glob_import_of_type_ir_inherent.rs:25:9
   |
LL |     use rustc_type_ir::inherent as inh; //~ ERROR non-glob import of `rustc_type_ir::inherent`
   |         ^^^^^^^^^^^^^^^^^^^^^^^------- help: try using a glob import instead: `::*`

error: aborting due to 4 previous errors
------------------------------------------

---- [ui] tests/ui-fulldeps/internal-lints/non_glob_import_of_type_ir_inherent.rs stdout end ----

rustbot assigned petrochenkov Aug 19, 2026

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rust-bors Bot pushed a commit that referenced this pull request Aug 19, 2026

Auto merge of #161349 - oli-obk:unflatten-use, r=<try>

bb18a8a

Unflatten `use` statements in HIR

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oli-obk force-pushed the unflatten-use branch from ab9b7ef to 324077c Compare August 19, 2026 15:21

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oli-obk force-pushed the unflatten-use branch from 637ff05 to 2839b85 Compare August 21, 2026 14:07

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rust-bors Bot pushed a commit that referenced this pull request Aug 21, 2026

Auto merge of #161349 - oli-obk:unflatten-use, r=<try>

00e1020

Unflatten `use` statements in HIR

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rustbot added the perf-regression Performance regression. label Aug 21, 2026

rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Aug 21, 2026

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mejrs mentioned this pull request Sep 2, 2026

add suggestion for rustc_allowed_through_unstable_modules attribute #162155

Merged

oli-obk added 4 commits September 7, 2026 15:36

Remove an unnecessary derive

697dcfb

Pull nested import printing out into a method

6cc12f7

Extract res -> PerNS logic into a method

7ca57c4

Remove an argument that is the same everywhere

4947bcb

oli-obk force-pushed the unflatten-use branch 2 times, most recently from 3733fc3 to 0fe41d8 Compare September 7, 2026 16:01

oli-obk marked this pull request as ready for review September 7, 2026 16:01

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Sep 7, 2026

ada4a reviewed Sep 7, 2026

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petrochenkov reviewed Sep 8, 2026

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petrochenkov added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 8, 2026

oli-obk added 10 commits September 9, 2026 16:39

Remove a level of indentation and make later filtering easier

a4a00e2

Split path segment stability checking out into a separate method

3eb8714

Handle use item collection in the late resolution visitor

1df8683

Don't create an owner per import, lower them as part of their surroun…

59ea8de

…ding scope instead

Stop special casing Use items in ast indexing

ff94f83

Make use trees in HIR be a tree again

f5dba9f

Preserve nested use tree DefIds to HIR

f1977ef

Make clippy handle HIR use trees

40f56f2

Make rustdoc handle HIR use trees

fd54650

Remove redundant assert

12cd64d

oli-obk force-pushed the unflatten-use branch from 0fe41d8 to 12cd64d Compare September 9, 2026 14:39

	res: PerNS { value_ns: Some(*res), type_ns: None, macro_ns: None },
	res: *res,

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rust-timer commented Aug 21, 2026

Overall result: ❌✅ regressions and improvements - please read:

Instruction count

Max RSS (memory usage)

Cycles

Binary size

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