rust-lang/rust · error · anyhow::Error

{e:?}

Error message

{e:?}

What it means

Thrown by read_chunk_to_uncompressed_bytes when miniz_oxide fails to zlib-inflate a compressed coverage data chunk up to the recorded uncompressed length. The error is the Debug representation of miniz_oxide's DecompressError (e.g. TINFLStatus::Failed/AdlerMismatch). It signals the on-disk coverage record is internally inconsistent or corrupted.

Source

Thrown at src/tools/coverage-dump/src/llvm_utils.rs:79

    ///
    /// Returns the uncompressed bytes that were read directly or decompressed.
    pub(crate) fn read_chunk_to_uncompressed_bytes(&mut self) -> anyhow::Result<Cow<'a, [u8]>> {
        let uncompressed_len = self.read_uleb128_usize()?;
        let compressed_len = self.read_uleb128_usize()?;

        if compressed_len == 0 {
            // The bytes are uncompressed, so read them directly.
            let uncompressed_bytes = self.read_n_bytes(uncompressed_len)?;
            Ok(Cow::Borrowed(uncompressed_bytes))
        } else {
            // The bytes are compressed, so read and decompress them.
            let compressed_bytes = self.read_n_bytes(compressed_len)?;

            let uncompressed_bytes = miniz_oxide::inflate::decompress_to_vec_zlib_with_limit(
                compressed_bytes,
                uncompressed_len,
            )
            .map_err(|e| anyhow!("{e:?}"))?;
            ensure!(uncompressed_bytes.len() == uncompressed_len);

            Ok(Cow::Owned(uncompressed_bytes))
        }
    }
}

View on GitHub (pinned to 7088e4b63a)

Solutions

1. Regenerate the coverage artifact with the same toolchain that produced it (re-run the instrumented binary, then re-export).
2. Verify the file is fully written and not truncated (compare size against writer-side expectations).
3. Confirm the LLVM version embedded in the coverage data matches the llvm-tools/coverage-dump build; rebuild coverage-dump against the matching LLVM.
4. If decompressing manually, check the zlib adler32/CRC to localize corruption.

Example fix

// before: assume any compressed chunk is valid
let bytes = miniz_oxide::inflate::decompress_to_vec_zlib_with_limit(
    compressed_bytes, uncompressed_len,
).map_err(|e| anyhow!("{e:?}"))?;

// after: surface a richer, version-aware error
let bytes = miniz_oxide::inflate::decompress_to_vec_zlib_with_limit(
    compressed_bytes, uncompressed_len,
).map_err(|e| anyhow!(
    "failed to decompress coverage chunk: {e:?} \
     (uncompressed_len={uncompressed_len}, compressed_len={compressed_len}); \
     the artifact may be from an incompatible LLVM/rustc version"
))?

// Before invoking coverage-dump on an artifact, sanity-check it is a valid
// coverage file and fully written:
fn looks_like_complete_coverage(path: &Path) -> bool {
    let Ok(md) = std::fs::metadata(path) else { return false; };
    md.len() > 0 && !path.ends_with(".tmp")
}
// Prefer: only process artifacts produced by the same toolchain in one run.

null

// Wrap the coverage-dump invocation and report a clear upstream error:
match coverage_dump::dump(&path) {
    Ok(out) => { /* ... */ }
    Err(e) if e.to_string().contains("decompress") => {
        eprintln!("{path:?} appears corrupt or from an incompatible LLVM version: {e}");
        std::process::exit(2);
    }
    Err(e) => return Err(e),
}

When it happens

Trigger: Invoked when coverage-dump reads an LLVM coverage instrumentation chunk whose header declares a non-zero compressed_len, then decompress_to_vec_zlib_with_limit returns Err. Happens when the .profraw/.llvm-cov artifact was produced by a mismatched LLVM version, truncated by a crashed build, or read from a partially-written file.

Common situations: Mixing coverage artifacts between different rustc/LLVM versions; running coverage-dump against a file that is still being written by the compiler; bit-rot on cached coverage data; reading a non-coverage binary that happens to start with a plausible chunk header.

Related errors

• unexpected base kind for gap region: {kind:?}
• unknown mapping kind: {raw_mapping_kind:#x}
• unparsed bytes: 0x{:02x?}
• llvm-readelf failed for binary {binary} with output {stdout}
• Build ID not found for binary {binary}

AI-assisted analysis of rust-lang/rust@7088e4b63a (2026-08-10). Data as JSON: /api/errors/d9325d1540539ec9. Report an issue: GitHub.