slint-ui/slint · error

Missing tag

Error message

Missing tag

What it means

check_file_tags throws "Missing tag" when a source file has no license header tag at all and the check runs without --fix. The check expects every source file to begin with the standard Slint SPDX/license header comment.

Solutions

1. Run `cargo xtask check --fix` which prepends the expected license header automatically
2. Copy the standard license header from another file at the top of the file
3. Confirm the file isn't excluded from the check if it's generated or vendored

Example fix

// before (top of file)
fn main() {}
// after
// Copyright © SixtyFPS GmbH <info@slint.dev>
// SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-Royalty-free-2.0 OR LicenseRef-Slint-commercial

fn main() {}

const HEADER: &str = "// Copyright © SixtyFPS GmbH <info@slint.dev>\n// SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-Royalty-free-2.0 OR LicenseRef-Slint-commercial";
let src = std::fs::read_to_string(path)?;
if !src.starts_with("// Copyright") {
    panic!("{path}: missing license header");
}

fn has_license_header(src: &str) -> bool {
    src.trim_start().starts_with("// Copyright")
        && src.contains("SPDX-License-Identifier:")
}

When it happens

Trigger: Adding a new .rs/.slint/.cpp file without copying the license header; running `cargo xtask check` (no --fix) so the auto-fix branch (which would insert EXPECTED_HEADER) is skipped.

Common situations: Newly created files by IDE templates; generated files copied into the repo; contributors unaware of the header requirement.

Understand the failure class

Background: "--flag is required" and "must specify" CLI errors: how missing-required-flag validation works and how to fix it — this error's family across 20 libraries.

Related errors

• unexpected header. expected
• License ` ` of crate is not in the accepted list
• Missing LICENSES directory for published crate
• No license file found in root LICENSES/ for SPDX id
• Encountered one or multiple errors. See above for details.

AI-assisted analysis of slint-ui/slint@3a7e700487 (2026-09-16). Data as JSON: /api/errors/e1dd475f363446ce. Report an issue: GitHub.

Appendix: source

Thrown at xtask/src/license_headers_check.rs:982

            Err(anyhow!("Encountered one or multiple errors. See above for details.").into())
        } else {
            println!("All files are ok.");
            Ok(())
        }
    }

    fn check_file_tags(&self, path: &Path, style: &LicenseTagStyle, license: &str) -> Result<()> {
        let source = &std::fs::read_to_string(path).context("Error reading file")?;

        let source = SourceFileWithTags::new(source, style);

        if !source.has_tag() {
            if self.fix_it {
                eprintln!("Fixing up {path:?} as instructed. It's missing a license header.");
                let source = source.replace_tag(&EXPECTED_HEADER, license);
                std::fs::write(path, source).context("Error writing source")
            } else {
                Err(anyhow!("Missing tag"))
            }
        } else if source.tag_matches(&EXPECTED_HEADER, license) {
            Ok(())
        } else if self.fix_it {
            eprintln!("Fixing up {path:?} as instructed. It has a wrong license header.");
            let source = source.replace_tag(&EXPECTED_HEADER, license);
            std::fs::write(path, source).context("Error writing new source")
        } else {
            Err(anyhow!(
                "unexpected header.\nexpected: {}\nfound: {}",
                EXPECTED_HEADER.to_string(style, license),
                source.found_tag()
            ))
        }
    }

    fn check_cargo_toml(&self, path: &Path, license: &str) -> Result<()> {
        let mut doc = CargoToml::new(path)?;

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