crate: expose `cfg` for general 64-bit `time_t` functionality by dybucc · Pull Request #5411 · rust-lang/libc

dybucc · 2026-08-14T14:00:37Z

Description

This patch adds support for a new cfg that should allow easily
toggling support for 64-bit time_t in supported platforms. This
should make testing of this unstable feature flag in downstream crates
easier than having to manually set up the equivalent cfgs for any
one of linux_time_bits64, gnu_time_bits64 or uclibc_time64.

Note support for the equivalent flag in musl has not been included
because we already have set-up automatic detection and toggling of the
corresponding cfg under supported targets ¹.

Checklist

Relevant tests in libc-test/semver have been updated
Commit messages permalink to headers for added or changed API
Placeholder or unstable values like *LAST or *MAX have the
standard doc comment
Tested locally (cargo test -p libc-test --target mytarget);
especially relevant for platforms that may not be checked in CI

@rustbot label +stable-nominated

https://github.com/rust-lang/libc/blob/1a8e71f33b1d6ea1e072210e7fc994417bbb2e34/build.rs#L181-L190 ↩

tgross35

Could you also update CI to use this cfg rather than the current glibc/musl cfg? Since this is what we're most likely to ship, we should make sure it works.

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tgross35 · 2026-08-25T08:28:42Z

@@ -190,7 +196,7 @@ fn main() {
    }


This should also set the musl_v1_2 flag, since that's pretty much all it's gating

I don't think I understand. You mention the musl_v1_2 flag, but that
is already set in that particular code block. Further, you also mention
in another review comment that I shouldn't need the time64 flag, so I
don't think there's anything to change in this particular area.

tgross35 · 2026-08-25T08:30:04Z

+    // Global flag to enable one of `linux_time_bits64` or `gnu_time_bits64`.
+    // The musl flags are enabled by default on supported platforms.
+    "time64",


I think this isn't actually needed, this list is only for what gets sent to check-cfg and we won't use time64 directly (at least for now)

rustbot · 2026-08-25T08:31:11Z

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

dybucc · 2026-08-29T10:00:07Z

I think it's done now. While checking through the CI workflow file, I
noticed that we even though we set the "updated but deprecated"
RUST_LIBC_UNSTABLE_MUSL_V1_2 environment variable for certain targets
¹, we still check for RUSTC_LIBC_UNSTABLE_MUSL_V1_2_3 in build.rs
².

I was wondering what's our stance on that change for stable releases,
and whether we should even set the deprecated environment variable in
CI. It seems wrong.

@rustbot ready

tgross35

Two small things then LGTM

View changes since this review

tgross35 · 2026-08-31T09:10:25Z

    steps:
-    - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
-      with:
-        persist-credentials: false
-    - run: |
-        msrv="$(
-          cargo metadata --format-version 1 |
-          jq -r --arg CRATE_NAME ctest '.packages | map(select((.name == $CRATE_NAME) and (.id | startswith("path+file")))) | first | .rust_version'
-        )"
-        echo "MSRV: $msrv"
-        echo "MSRV=$msrv" >> "$GITHUB_ENV"
-    - name: Install Rust
-      run: rustup update "$MSRV" --no-self-update && rustup default "$MSRV"
-    - uses: Swatinem/rust-cache@6323deb102c322ba6fcbdcafc7e3dddab59af2b6 # v2.9.2
-    - run: cargo build -p ctest
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          persist-credentials: false
+      - run: |
+          msrv="$(
+            cargo metadata --format-version 1 |
+            jq -r --arg CRATE_NAME ctest '.packages | map(select((.name == $CRATE_NAME) and (.id | startswith("path+file")))) | first | .rust_version'
+          )"
+          echo "MSRV: $msrv"
+          echo "MSRV=$msrv" >> "$GITHUB_ENV"
+      - name: Install Rust
+        run: rustup update "$MSRV" --no-self-update && rustup default "$MSRV"
+      - uses: Swatinem/rust-cache@6323deb102c322ba6fcbdcafc7e3dddab59af2b6 # v2.9.2
+      - run: cargo build -p ctest


It's fine to reformat this if it makes things more consistent, but please put it in a separate commit

Done. I didn't even notice this. Must have been format on save while
editing the CI file.

tgross35 · 2026-08-31T09:11:20Z

-    if "musl" in target_env:
-        # Check with breaking changes from musl, including 64-bit time_t on 32-bit
-        run(cmd, rustflags=f"{rustflags} --cfg=libc_unstable_musl_v1_2")
+    if "gnu" in target_env and target_bits == "32" or "musl" in target_env:


Use parens to make the order of operations clear

Add `cfg` enabling `time64` functionality across all supported targets. This ensures users have a simple entry point to the crate functionality gated behind one of `linux_time_bits64`, `uclibc_time64` and `gnu_time_bits64`.

Switch as many uses of other target-specific `cfg`s with the `time64` `cfg` introduced in the prior patch.

Change indentation of two YAML arrays to consistently appear as nested within the key that corresponds with the array. Change one inconsistent use of single quotes with double quotes for YAML strings. The changes were not manual; They were automatically applied by the YAML extension for VSCode.

rustbot · 2026-09-01T14:54:56Z

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.

dybucc · 2026-09-01T14:57:17Z

Wondering if you saw ¹, @tgross35?

@rustbot ready

https://github.com/rust-lang/libc/pull/5411#issuecomment-5461679894 ↩

rustbot added S-waiting-on-review stable-nominated This PR should be considered for cherry-pick to libc's stable release branch labels Aug 14, 2026

tgross35 requested changes Aug 25, 2026

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dybucc force-pushed the time64-global-cfg branch from 98e48b9 to 1ab4287 Compare August 29, 2026 06:37

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dybucc force-pushed the time64-global-cfg branch from 4c3b4f1 to 1b1d2bb Compare August 29, 2026 09:53

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dybucc force-pushed the time64-global-cfg branch from 1b1d2bb to a89083f Compare August 30, 2026 13:44

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dybucc added 3 commits September 1, 2026 16:43

crate: expose cfg for general time64

18295e0

Add `cfg` enabling `time64` functionality across all supported targets. This ensures users have a simple entry point to the crate functionality gated behind one of `linux_time_bits64`, `uclibc_time64` and `gnu_time_bits64`.

crate(ci): make prevalent use of time64 cfg

4db8b17

Switch as many uses of other target-specific `cfg`s with the `time64` `cfg` introduced in the prior patch.

dybucc force-pushed the time64-global-cfg branch from 36137bc to 3198d46 Compare September 1, 2026 14:54

rustbot added S-waiting-on-review and removed S-waiting-on-author labels Sep 1, 2026

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dybucc commented Aug 14, 2026

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rustbot commented Aug 25, 2026

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dybucc commented Aug 29, 2026

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rustbot commented Sep 1, 2026

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dybucc commented Sep 1, 2026

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