rust-lang/rust · error · io::Error

IO error creating MIR dump directory

Error message

IO error creating MIR dump directory: {parent:?}; {e}

What it means

Returned by create_dump_file in rustc_middle::mir::pretty when fs::create_dir_all fails while creating the parent directory for a MIR dump file. The directory path comes from the -Z dump-mir-dir unstable flag (default mir_dump/). This function is invoked when -Z dump-mir or -Z dump-mir-excluded passes are enabled, causing the compiler to write MIR state to disk for debugging.

Solutions

1. Ensure the dump_mir_dir path (default: mir_dump/) is writable. Check with: ls -la . && mkdir -p mir_dump.
2. Set -Z dump-mir-dir to an absolute path you control, e.g., -Z dump-mir-dir=/tmp/mir-dumps.
3. If you don't need MIR dumps, remove -Z dump-mir from your RUSTFLAGS.
4. Check for a file vs directory collision — if 'mir_dump' exists as a file, remove or rename it.

Example fix

# before: dump dir is read-only
RUSTFLAGS="-Zdump-mir -Zdump-mir-dir=/opt/mir" cargo build
# after: use a writable directory
RUSTFLAGS="-Zdump-mir -Zdump-mir-dir=/tmp/mir-dumps" cargo build

// Before compiling with -Zdump-mir, verify the output directory is creatable.
use std::path::Path;

fn verify_dump_dir(dir: &str) -> Result<(), String> {
    let path = Path::new(dir);
    // Check parent is writable or path can be created
    std::fs::create_dir_all(path).map_err(|e| format!("cannot create dump dir '{}': {}", dir, e))?;
    // Clean up test
    if std::fs::read_dir(path).map(|mut d| d.next().is_none()).unwrap_or(true) {
        std::fs::remove_dir(path).ok();
    }
    Ok(())
}

// create_dump_file returns io::Result. In internal rustc code:
match body.create_dump_file("mir", &body) {
    Ok(file) => { /* write MIR */ },
    Err(e) => {
        tcx.sess.dcx().warn(format!("failed to create MIR dump file: {e}"));
        // continue compilation without the dump
    }
}

When it happens

Trigger: Compiling with RUSTFLAGS="-Zdump-mir" or -Z dump-mir-dir=<path> where the specified directory cannot be created. The parent path is derived from the dump_mir_dir option joined with the output filename.

Common situations: Setting dump_mir_dir to a path inside a read-only directory; permissions issues on the working directory; invalid path characters; the path component is a file rather than a directory; running the compiler in a sandboxed/containerized environment with restricted filesystem access.

Related errors

• IO error creating MIR dump file
• archive member at offset
• {context}: {err}
• expected monomorphic const in codegen
• Non terminating drop_in_place_real???

AI-assisted analysis of rust-lang/rust@7088e4b63a (2026-08-10). Data as JSON: /api/errors/009344fca158926d. Report an issue: GitHub.

Appendix: source

Thrown at compiler/rustc_middle/src/mir/pretty.rs:298

        file_path.push(&file_name);

        file_path
    }

    /// Attempts to open a file where we should dump a given MIR or other
    /// bit of MIR-related data. Used by `mir-dump`, but also by other
    /// bits of code (e.g., NLL inference) that dump graphviz data or
    /// other things, and hence takes the extension as an argument.
    pub fn create_dump_file(
        &self,
        extension: &str,
        body: &Body<'tcx>,
    ) -> io::Result<io::BufWriter<fs::File>> {
        let file_path = self.dump_path(extension, body);
        if let Some(parent) = file_path.parent() {
            fs::create_dir_all(parent).map_err(|e| {
                io::Error::new(
                    e.kind(),
                    format!("IO error creating MIR dump directory: {parent:?}; {e}"),
                )
            })?;
        }
        fs::File::create_buffered(&file_path).map_err(|e| {
            io::Error::new(e.kind(), format!("IO error creating MIR dump file: {file_path:?}; {e}"))
        })
    }
}

///////////////////////////////////////////////////////////////////////////
// Whole MIR bodies

/// Write out a human-readable textual representation of this crate's MIR,
/// with the default [`PrettyPrintMirOptions`].
pub fn write_mir_pretty<'tcx>(tcx: TyCtxt<'tcx>, w: &mut dyn io::Write) -> io::Result<()> {
    let writer = MirWriter::new(tcx);

View on GitHub (pinned to 7088e4b63a)