slint-ui/slint · error

We just added spaces

Error message

We just added spaces

What it means

`extract_rust_macro` masks everything outside the `slint!{...}` macro with spaces, then converts the bytes back to a String. Since the function only ever replaces ASCII bytes with spaces in valid UTF-8 input, `from_utf8` cannot fail — the panic guards that invariant.

Solutions

1. Verify your change to the masking logic preserves UTF-8 boundaries (only whole characters overwritten)
2. Confirm the input path to the proc-macro is valid UTF-8 Rust source
3. Operate on char boundaries instead of raw byte indices when masking

Example fix

// before
Some(String::from_utf8(bytes).expect("We just added spaces"))
// after
Some(String::from_utf8(bytes).unwrap_or_else(|e| {
    panic!("extract_rust_macro produced invalid UTF-8 at byte {}: masking logic bug", e.valid_up_to())
}))

assert!(src.is_char_boundary(end + 1), "mask index must fall on a char boundary");

When it happens

Trigger: Only if the masking code corrupted multi-byte UTF-8 sequences (an off-by-one in the `end` index) would `String::from_utf8` fail. In practice unreachable for correct input.

Common situations: A developer modifies the byte-masking loop (e.g. the newline-preserving logic) and introduces an off-by-one that splits a multi-byte character; then the slint! macro proc-macro panics while parsing a Rust file.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

• a setter belongs to a field
• an identifier
• binding was of the wrong type
• Cannot get property
• Cannot invoke callback

AI-assisted analysis of slint-ui/slint@bb937076de (2026-09-16). Data as JSON: /api/errors/b4ae23e7894717c2. Report an issue: GitHub.

Appendix: source

Thrown at internal/compiler/lexer.rs:504

    for c in &mut bytes[..start] {
        if *c != b'\n' {
            *c = b' '
        }
    }

    if start > 0 {
        bytes[start - 1] = 2;
    }
    if end < bytes.len() {
        bytes[end] = 3;

        for c in &mut bytes[end + 1..] {
            if *c != b'\n' {
                *c = b' '
            }
        }
    }
    Some(String::from_utf8(bytes).expect("We just added spaces"))
}

#[test]
fn test_extract_rust_macro() {
    assert_eq!(extract_rust_macro("\nslint{!{}}".into()), None);
    assert_eq!(
        extract_rust_macro(
            "abc\n€\nslint !  {x \" \\\" }🦀\" { () {}\n {} }xx =}-  ;}\n xxx \n yyy {}\n".into(),
        ),
        Some(
            "   \n   \n         \u{2}x \" \\\" }🦀\" { () {}\n {} }xx =\u{3}     \n     \n       \n".into(),
        )
    );

    assert_eq!(
        extract_rust_macro("xx\nabcd::slint!{abc{}efg".into()),
        Some("  \n            \u{2}abc{}efg".into())
    );

View on GitHub (pinned to bb937076de)