slint-ui/slint · error

an identifier

Error message

an identifier

What it means

This panic fires in `point_span` when the string `p<id>` (e.g. `p3`) fails to lex as a Slint identifier. The function fabricates a fallback `Span` by parsing a synthetic token, and the compiler treats its own ability to parse that token as an invariant.

Solutions

1. Ensure the generator runs inside the compiler where `str::parse::<TokenStream>()` is supported (proc-macro runtime available)
2. Update proc-macro2/proc-macro dependency; a broken token source usually means a bad proc-macro2 runtime
3. Report as a compiler bug — the invariant `p<id>` is always an identifier should hold for every usize

assert!(!format!("p{id}").is_empty(), "point_span id must produce a token");

When it happens

Trigger: Calling `point_span` with an id such that `format!("p{id}")` is not a valid identifier token — practically impossible since every usize yields `p` + digits, so a failure would indicate a broken lexer or `proc_macro2` token stream in a non-compiler context (e.g. invoking the generator outside `proc-macro` mode where `parse::<TokenStream>()` on strings is unavailable).

Common situations: Hitting this while porting the slint-sc generator to a context without proc-macro token source support, or a regression in the lexer rejecting digit-suffixed identifiers.

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
• default value requested for unknown struct field
• element without a size
• Global was just valid
• invalid parent reference

AI-assisted analysis of slint-ui/slint@bb937076de (2026-09-16). Data as JSON: /api/errors/3f2aad5b9a0da7b3. Report an issue: GitHub.

Appendix: source

Thrown at internal/compiler/generator/slint_sc.rs:191

    let written = elem.debug.first().and_then(|d| d.node.QualifiedName());
    match written {
        Some(name) => name.text().to_string().trim().to_string(),
        None => elem.base_type.to_string(),
    }
}

/// The name of a property as the source writes it: `move_declarations` hoists
/// a declaration onto the root element under a name of its own making.
fn source_name(elem: &Element, name: &SmolStr) -> SmolStr {
    let moved_from = elem.property_declarations.get(name).and_then(|d| d.moved_from.clone());
    moved_from.unwrap_or_else(|| name.clone())
}

/// A span naming point `id`: the span of a token parsed from the text
/// `p<id>`, which `source_text` gives back. Fallback spans can be made in no
/// other way, and the printer is the only reader.
fn point_span(id: usize) -> Span {
    let tokens: TokenStream = format!("p{id}").parse().expect("an identifier");
    tokens.into_iter().next().expect("one token").span()
}

/// The point a span names, if any.
fn point_id(span: Span) -> Option<usize> {
    span.source_text()?.strip_prefix('p')?.parse().ok()
}

/// The tokens as the range of the point, when they are one. See `stamped`.
fn stamped_at(point: Option<Span>, tokens: TokenStream) -> TokenStream {
    match point {
        Some(span) => stamped(span, tokens),
        None => tokens,
    }
}

/// The tokens as a point's range: a group without delimiters, which prints
/// as the tokens alone, stamped with the point's span.

View on GitHub (pinned to bb937076de)