fix: Check the fallback map before queueing child in `visible_parent_map` breadth-first search by zalanlevai · Pull Request #160464 · rust-lang/rust

zalanlevai · 2026-08-03T20:11:04Z

The fix in #159881 extended the visible_parent_map breadth-first search (BFS) to include items nested within "fallback" items, such as #[doc(hidden)] modules and re-exports. However, the fix resulted in fallback entries not being checked before enqueueing new items into the BFS, resulting in fallback items being explored in the BFS multiple times, once for each fallback parent, up until a non-fallback visible parent is found instead, if any.

This PR adds a check before new fallback items are recorded and the BFS queue is extended that ensures that the item has not already been searched through a fallback parent item.

Fixes the libc performance regression described in #160439.

Unfortunately, I could not think of a way to write a test case that covers the regression that this PR fixes, as it would require a massive crate with a very large number of "fallback" parents, and the test would be performance-based.

…map` BFS

rustbot · 2026-08-03T20:11:09Z

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zalanlevai · 2026-08-03T20:34:27Z

@rustbot label +perf-regression

zalanlevai · 2026-08-03T20:39:14Z

Actually, after looking at PRs labeled perf-regression, I think I used the label incorrectly as this PR fixes a performance regression, rather than introducing one.

@rustbot label -perf-regression

tgross35 · 2026-08-03T21:04:05Z

That makes me think, maybe we could have a perf job that runs rustdoc on libc or any other larger crate? That would have caught this.

Cc @Kobzol

zalanlevai · 2026-08-03T21:31:35Z

Huh, those CI failures on x86_64-gnu-gcc seem strange to me. All seven test failures are caused by the following unexpected warning on stderr:

libgccjit.so: warning: : ‘retain’ attribute ignored [-Wattributes]

Is my reading correct that the CI failure seems unrelated to this change and could be caused by something else?

tgross35 · 2026-08-03T21:35:13Z

Looks like that job may be having some issues #t-infra > Tree ops @ 💬

rust-log-analyzer · 2026-08-03T21:47:04Z

The job x86_64-gnu-gcc 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 linkage-attr/unreferenced-used-static-issue-127052.rs`

error: 1 errors occurred comparing output.
status: exit status: 0
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/linkage-attr/unreferenced-used-static-issue-127052.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/x86_64-unknown-linux-gnu/stage2" "-Zcodegen-backend=gcc" "--target=x86_64-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" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/linkage-attr/unreferenced-used-static-issue-127052" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers"
stdout: none
--- stderr -------------------------------
libgccjit.so: warning: : ‘retain’ attribute ignored [-Wattributes]
------------------------------------------

---
To only update this specific test, also pass `--test-args proc-macro/crt-static.rs`

error: 1 errors occurred comparing output.
status: exit status: 0
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/proc-macro/crt-static.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/x86_64-unknown-linux-gnu/stage2" "-Zcodegen-backend=gcc" "--target=x86_64-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" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/proc-macro/crt-static" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "--crate-type" "proc-macro" "-Ctarget-feature="
stdout: none
--- stderr -------------------------------
libgccjit.so: warning: : ‘retain’ attribute ignored [-Wattributes]
------------------------------------------

---
To only update this specific test, also pass `--test-args proc-macro/no-mangle-in-proc-macro-issue-111888.rs`

error: 1 errors occurred comparing output.
status: exit status: 0
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/proc-macro/no-mangle-in-proc-macro-issue-111888.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/x86_64-unknown-linux-gnu/stage2" "-Zcodegen-backend=gcc" "--target=x86_64-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" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/proc-macro/no-mangle-in-proc-macro-issue-111888" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/proc-macro/no-mangle-in-proc-macro-issue-111888/auxiliary"
stdout: none
--- stderr -------------------------------
libgccjit.so: warning: : ‘retain’ attribute ignored [-Wattributes]
------------------------------------------

---
To only update this specific test, also pass `--test-args proc-macro/no-missing-docs.rs`

error: 1 errors occurred comparing output.
status: exit status: 0
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/proc-macro/no-missing-docs.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/x86_64-unknown-linux-gnu/stage2" "-Zcodegen-backend=gcc" "--target=x86_64-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" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/proc-macro/no-missing-docs" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers"
stdout: none
--- stderr -------------------------------
libgccjit.so: warning: : ‘retain’ attribute ignored [-Wattributes]
------------------------------------------

---
To only update this specific test, also pass `--test-args runtime/stdout-before-main.rs`

error: 1 errors occurred comparing output.
status: exit status: 0
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/runtime/stdout-before-main.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/x86_64-unknown-linux-gnu/stage2" "-Zcodegen-backend=gcc" "--target=x86_64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "-O" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "-C" "prefer-dynamic" "-o" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/runtime/stdout-before-main/a" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers"
stdout: none
--- stderr -------------------------------
libgccjit.so: warning: : ‘retain’ attribute ignored [-Wattributes]
------------------------------------------

---
To only update this specific test, also pass `--test-args rust-2018/proc-macro-crate-in-paths.rs`

error: 1 errors occurred comparing output.
status: exit status: 0
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/rust-2018/proc-macro-crate-in-paths.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/x86_64-unknown-linux-gnu/stage2" "-Zcodegen-backend=gcc" "--target=x86_64-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" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/rust-2018/proc-macro-crate-in-paths" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers"
stdout: none
--- stderr -------------------------------
libgccjit.so: warning: : ‘retain’ attribute ignored [-Wattributes]
------------------------------------------

---
To only update this specific test, also pass `--test-args traits/dyn-drop-principal.rs`

error: 1 errors occurred comparing output.
status: exit status: 0
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/traits/dyn-drop-principal.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/x86_64-unknown-linux-gnu/stage2" "-Zcodegen-backend=gcc" "--target=x86_64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "-O" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "-C" "prefer-dynamic" "-o" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/traits/dyn-drop-principal/a" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers"
stdout: none
--- stderr -------------------------------
libgccjit.so: warning: : ‘retain’ attribute ignored [-Wattributes]
libgccjit.so: warning: : ‘retain’ attribute ignored [-Wattributes]
------------------------------------------

Important

For more information how to resolve CI failures of this job, visit this link.

LusterSourav · 2026-08-04T05:45:07Z

Validation with a minimal (no crates.io) reproducer, since this PR currently lacks performance evidence:

Structure: dep crate with a nested fan of #[doc(hidden)] modules — D hidden containers, each with B hidden modules that all pub use the next container (every level reachable only via fallback parents, so it's never recorded in visible_parent_map and is re-enqueued (B+1) times per level). See cargo doc freezes on libc with nightly-2026-08-03 #160439 for the full listing.
B=8, D=12: nightly-2026-08-02 runs rustdoc in 0.39s; nightly-2026-08-03 (first nightly containing fix: Do not stop visible_parent_map breadth-first search reaching children of #[doc(hidden)] modules #159881) hangs (>60s).
This PR's change applied locally (stage1 build): 0.15s.

I also added a regression test at tests/run-make/rustdoc/visible-parent-map-hang (via #160439). It passes on a fixed stage1; since the compiletest run-make harness has no per-test kill-timeout, the test enforces its own 60s watchdog and fails fast with a clear message if the exponential behavior returns.

fix: Check the fallback map before queueing child in `visible_parent_…

464edf2

…map` BFS

rustbot assigned nnethercote Aug 3, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Aug 3, 2026

zalanlevai mentioned this pull request Aug 3, 2026

cargo doc freezes on libc with nightly-2026-08-03 #160439

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zalanlevai changed the title ~~fix: Check the fallback map before queueing child in visible_parent_map breadth-first-search~~ fix: Check the fallback map before queueing child in visible_parent_map breadth-first search Aug 3, 2026

rustbot added the perf-regression Performance regression. label Aug 3, 2026

rustbot removed the perf-regression Performance regression. label Aug 3, 2026

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fix: Check the fallback map before queueing child in `visible_parent_map` breadth-first search - #160464

fix: Check the fallback map before queueing child in `visible_parent_map` breadth-first search#160464
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zalanlevai:fix-visible-parent-map-fallback-entry-check

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