std::path::Display Nightly regression with precision

[jorendorff]

Code

(See previous issue #152804 and fix #152865.)

I tried this code (playground):

#[test]
fn format_path() {
    use std::path::Path;
    assert_eq!(format!("{:10.4}",  Path::new("/bin/ls").display()), "/bin      ");
    assert_eq!(format!("{:>10.4}", Path::new("/bin/ls").display()), "      /bin");
    assert_eq!(format!("{:<10.4}", Path::new("/bin/ls").display()), "/bin      ");
    assert_eq!(format!("{:^10.4}", Path::new("/bin/ls").display()), "   /bin   ");
}

I expected this test to pass; in Stable, specifying a precision for a string argument causes it to be truncated before padding is applied.

Instead, the test fails with:

---- format_path stdout ----

thread 'format_path' (47) panicked at src/lib.rs:12:5:
assertion `left == right` failed
  left: "/bin/ls   "
 right: "/bin      "
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Version it worked on

It most recently worked on: 1.94.0-beta.3

Version with regression

rustc --version --verbose:

rustc 1.95.0-nightly (c78a29473 2026-02-22)
binary: rustc
commit-hash: c78a29473a68f07012904af11c92ecffa68fcc75
commit-date: 2026-02-22
host: x86_64-unknown-linux-gnu
release: 1.95.0-nightly
LLVM version: 22.1.0

Note this version contains the previous fix in #152865.

Backtrace

Backtrace

$ RUST_BACKTRACE=1 cargo +nightly test
   Compiling fmt-test v0.1.0 (/home/jorendorff/src/fmt-test)
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.42s
     Running unittests src/main.rs (target/debug/deps/fmt_test-d2bdd0cffa5ebb82)

running 2 tests
test format_str ... ok
test format_path ... FAILED

failures:

---- format_path stdout ----

thread 'format_path' (135186) panicked at src/main.rs:12:5:
assertion `left == right` failed
  left: "/bin/ls   "
 right: "/bin      "
stack backtrace:
   0: __rustc::rust_begin_unwind
             at /rustc/c78a29473a68f07012904af11c92ecffa68fcc75/library/std/src/panicking.rs:689:5
   1: core::panicking::panic_fmt
             at /rustc/c78a29473a68f07012904af11c92ecffa68fcc75/library/core/src/panicking.rs:80:14
   2: core::panicking::assert_failed_inner
             at /rustc/c78a29473a68f07012904af11c92ecffa68fcc75/library/core/src/panicking.rs:439:17
   3: core::panicking::assert_failed::<alloc::string::String, &str>
             at /home/jorendorff/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/panicking.rs:394:5
   4: fmt_test::format_path
             at ./src/main.rs:12:5
   5: fmt_test::format_path::{closure#0}
             at ./src/main.rs:10:17
   6: <fmt_test::format_path::{closure#0} as core::ops::function::FnOnce<()>>::call_once
             at /home/jorendorff/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5
   7: <fn() -> core::result::Result<(), alloc::string::String> as core::ops::function::FnOnce<()>>::call_once
             at /rustc/c78a29473a68f07012904af11c92ecffa68fcc75/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.


failures:
    format_path

test result: FAILED. 1 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.27s

error: test failed, to rerun pass `--bin fmt-test`

@rustbot modify labels: +regression-from-stable-to-nightly -regression-untriaged

[jorendorff]

@asder8215 @joboet

[joboet]

I should note that this only worked correctly for UTF-8-only strings before #151010. We should obviously fix this for all strings.

[asder8215]

I'll try and fix this, and see what other cases that needs to be resolved with formatted strings in ByteStr. Thanks for showing these bugs @jorendorff!

@rustbot claim

[joboet]

Sorry @asder8215, I just read your comment but have already filed #153025 to fix this.