slint-ui/slint · error

Cannot create output directory

Error message

Cannot create output directory {out_class_dir:?} - {e}

What it means

The android-activity backend build script fails when it cannot create the directory into which the compiled Java class files go (OUT_DIR plus the slint class path). Any create_dir_all error (permissions, a conflicting file, disk space) panics with this message and aborts the Android backend build.

Solutions

1. Make the build output directory (OUT_DIR/target) writable by the build user
2. Delete any non-directory file occupying the class output path and rebuild
3. Free disk space if the error is ENOSPC
4. Run `cargo clean -p slint-backend-android-activity` and rebuild

Example fix

// before: permission denied
# chmod -R u+w target/ && cargo build
// after
build succeeds creating the class directory

import os
out_class_dir = os.path.join(os.environ["OUT_DIR"], "slint")
assert os.access(os.path.dirname(out_class_dir), os.W_OK), "OUT_DIR not writable"
assert not os.path.isfile(out_class_dir), f"{out_class_dir} is a file, not a directory"

When it happens

Trigger: Building the android-activity crate where `std::fs::create_dir_all(&out_class_dir)` fails — read-only OUT_DIR, stale file at the class dir path, or ENOSPC.

Common situations: CI runners with restricted write permissions; a previous build left a file (not directory) at that path; disk quota exceeded during Android toolchain builds.

Understand the failure class

Background: mkdir permission denied (EACCES): failed to create directory errors explained — this error's family across 32 libraries.

Related errors

• Could not generate the java compiler command
• Could not run the java compiler
• Java compilation failed
• create
• write

AI-assisted analysis of slint-ui/slint@bb937076de (2026-09-16). Data as JSON: /api/errors/3f5c87c887fb66c0. Report an issue: GitHub.

Appendix: source

Thrown at internal/backends/android-activity/build.rs:32

    }
    let release_mode = env::var("PROFILE").as_ref().map(|s| s.as_str()) == Ok("release");

    // This is the only Java source file
    let java_src = "SlintAndroidJavaHelper.java";
    let java_src_path = format!("java/{java_src}");

    let slint_path: PathBuf = ["dev", "slint", "android-activity"].iter().collect();

    let out_dir: PathBuf = env::var_os("OUT_DIR").unwrap().into();
    let mut out_class_dir = out_dir.clone();
    out_class_dir.push("java");
    out_class_dir.push(slint_path);

    if out_class_dir.try_exists().unwrap_or(false) {
        let _ = std::fs::remove_dir_all(&out_class_dir);
    }
    std::fs::create_dir_all(&out_class_dir)
        .unwrap_or_else(|e| panic!("Cannot create output directory {out_class_dir:?} - {e}"));

    let android_jar = android_build::android_jar(None).expect("No Android platforms found");

    // Compile the Java file into .class files
    let o = JavaBuild::new()
        .file(&java_src_path)
        .class_path(&android_jar)
        .classes_out_dir(&out_class_dir)
        .java_source_version(8)
        .java_target_version(8)
        .debug_info(android_build::DebugInfo {
            line_numbers: !release_mode,
            variables: !release_mode,
            source_files: !release_mode,
        })
        .command()
        .unwrap_or_else(|e| panic!("Could not generate the java compiler command: {e}"))
        .args(["-encoding", "UTF-8"])

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