[robobun]

Behaviour change: none

Problem

• clippy.toml:31-32 tells an author with bytes to use bstr::BStr "for display". Display for BStr writes U+FFFD (EF BF BD) for each invalid UTF-8 sequence. String::from_utf8_lossy writes the same, and line 32 bans it because it "silently corrupts non-UTF-8 bytes".
• Bytes formatted this way were lost in failed lifecycle script output and in bunx cache paths. install: more fixes after the package manager consolidation #45710 fixed both. On the script output: "The bytes are now printed as-is, like in 1.3.14".
• src/runtime/cli/mod.rs:510 says Display for BStr "writes raw bytes and ignores formatter width/alignment". BStr writes U+FFFD, and it pads for the {:<16} spec in question.

Fix

• Both reasons say that BStr Display writes U+FFFD and is for a diagnostic. For everything else they name each sink's byte API: Vec::extend_from_slice, write_all, bun_core::output::print_bytes (stdout), bun_core::strings::concat_buf_t (a fixed buffer).
• The comment keeps its true part: the tables are &str, so {:<16} pads.
• Verified: cargo clippy -p bun_core --no-deps passes. A scratch crate linted with this clippy.toml prints both new notes.

Background

• A disallowed-methods entry bans a function. clippy prints its reason under the error, so an author reads it when the lint stops them.
• bstr::BStr wraps &[u8] so that {} compiles. bstr 1.12.1 (Cargo.lock) writes each invalid chunk as "\u{FFFD}". It pads for {:<16} (an alignment flag), not for a bare {:16}.
• A lint cannot replace the text. disallowed-methods can ban bstr::BStr::new, but it cannot tell a diagnostic from another sink. src/ has 2,663 BStr::new( calls.

Notes

bstr 1.12.1 (src/impls.rs:459-519 of the crate). Probe results:

spec and input	BStr	&str or from_utf8_lossy
{} on a\xFFb	61 EF BF BD 62	61 EF BF BD 62
{:<8} on abc	[abc ]	[abc ]
{:8} on abc	[abc]	[abc ]
{:.2} on abc	[abc]	[ab]

format!("{}", BStr::new(b)) equals String::from_utf8_lossy(b) for 5,000,000 random byte strings (4,542,371 of them not valid UTF-8) and for 5,701,632 strings of 1 to 3 bytes. String::from_utf8_lossy returns Cow::Borrowed for valid UTF-8. It allocates only when it writes U+FFFD.

The lint output with this change (clippy 0.1.100, nightly-2026-09-15):

error: use of a disallowed method `alloc::string::String::from_utf8_lossy`
  = note: silently corrupts non-UTF-8 bytes, and allocates to do it; keep data as bytes: in a diagnostic format the bytes with bstr::BStr (its Display writes the same U+FFFD, without the allocation), everywhere else (child process output, a path to open or to print for another program, hash input) use the sink's byte API (Vec::extend_from_slice, write_all, bun_core::output::print_bytes for stdout, bun_core::strings::concat_buf_t for a fixed buffer)

Why these APIs.

• bun_core::output::print_bytes (src/bun_core/output.rs:1331) writes to stdout through the writer that Output::print uses. test/internal/source-lints/no-std-stdio.test.ts bans std::io::stdout().
• bun_core::strings::concat_buf_t (src/bun_core/string/immutable.rs:1330) returns NoSpaceLeft when the parts do not fit. bun_core::concat_into panics in that case, so the reasons do not name it.
• install: more fixes after the package manager consolidation #45710 used Output::error_writer().write_all(..) for the script output (src/install/lifecycle_script_runner.rs:901) and [..].concat() for the bunx paths.

The code already says this in other places. src/bun_core/fmt.rs:761, src/install/resolution.rs:668, src/bundler/bundled_ast.rs:386, src/ptr/CowSlice.rs:353, src/standalone_graph/StandaloneModuleGraph.rs:1592.

The comment. "Writes raw bytes and ignores formatter width" describes bun_core::fmt::Raw (fmt::s, src/bun_core/fmt.rs:770), which calls f.write_str. It does not describe BStr.

Not changed. The two lint entries still ban the same functions. No Rust code changes, only one comment.

Related. #31668 added the two entries. #46157 (open) converts four more sinks from BStr formatting to byte appends.

Checks run. cargo clippy -p bun_core --no-deps. rustfmt --check src/runtime/cli/mod.rs. The source lints port-era-markers, pre-port-identifiers, byte-search, workspace-lints and comment-cop in test/internal/source-lints/ (30 pass). The full bun run rust:clippy did not run locally.

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no test proof · iteration 0 · the description declares no behaviour change, so there is no failing test to prove; the existing suite in CI is the check

[robobun]

Status: ready for review.

How the claims were checked:

• bstr 1.12.1 is the version in Cargo.lock. format!("{}", BStr::new(b"a\xFFb")) gives the bytes 61 EF BF BD 62. String::from_utf8_lossy gives the same bytes.
• format!("{:<8}", BStr::new(b"abc")) gives abc and five spaces. So the {:<16} spec in src/runtime/cli/mod.rs pads a BStr too.
• A scratch crate that calls String::from_utf8 and String::from_utf8_lossy, linted with this clippy.toml, prints both new notes.

The Notes block of the PR body has the full numbers.

[robobun]

^{Updated 7:45 AM PT - Oct 22nd, 2026}

✅ @robobun, your commit 826fbf21d4f084a608e1efe084dae5177ffe48b7 passed in Build #130917! 🎉

---

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bunx bun-pr 46377

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bun-46377 --bun

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Walkthrough

This change updates Clippy guidance for String::from_utf8 and String::from_utf8_lossy. It also clarifies the CLI filler-table comment about &str formatting and width padding.

Changes

Guidance Updates

Layer / File(s)	Summary
Clarify conversion and formatting guidance clippy.toml, src/runtime/cli/mod.rs	The Clippy reasons direct diagnostic formatting to bstr::BStr, JavaScript-visible strings to bun_core::String::clone_utf8, and other data flows to byte-oriented sink APIs. The CLI comment explains the use of &str and width formatting for filler tables.

Suggested reviewers: jarred-sumner

Priority: ⬇️ Low

Merge Risk: 🔵 Low · up to 826fb

The lint guidance still overstates allocation for valid UTF-8, so it should be corrected before merge; runtime behavior is unchanged.

Pre-merge checks: 4

Check name	Status	Explanation
Title check		The title clearly identifies the main change: updating clippy guidance to recommend bstr::BStr Display only for diagnostics.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Description check		The description clearly explains the problem, fix, scope, and verification steps. It does not use the template headings exactly, but it provides the required information and is complete enough for rev…

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The description clearly explains the problem, fix, scope, and verification steps. It does not use the template headings exactly, but it provides the required information and is complete enough for review.

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Actionable comments posted: 1

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• 🪄 Fix comments: Fix CodeRabbit comments on this PR

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Inline comments:
Review comments at @clippy.toml:
- Line 32: Update the reason for the alloc::string::String::from_utf8_lossy lint
entry to state that invalid UTF-8 is replaced with U+FFFD and allocation occurs
only when replacement is needed, while preserving the existing guidance about
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[claude]

Code review found no issues

No high-confidence issues detected in this change.

[claude]

Code review found no issues

No high-confidence issues detected in this change.