What causes "invalid identifier" in rwf2/Rocket?

Rename the parameter to a valid snake_case Rust identifier: instead of

rwf2/Rocket · error · rocket_codegen::param::Error

BadIdent

BadIdent

Error message

invalid identifier

What it means

Compile-time error from Rocket's codegen: the name inside a dynamic route parameter <name> is not a valid Rust identifier (is_valid_ident failed), so Rocket cannot generate a binding for it. The name must be a valid identifier because it is used to match against a function argument in the generated code. The error span covers just the offending name inside the brackets.

Source

Thrown at core/codegen/src/attribute/param/parse.rs:63

        segment: &str,
        source_span: Span,
    ) -> Result<Self, Error<'_>>  {
        let mut trailing = false;

        // Check if this is a dynamic param. If so, check its well-formedness.
        let lint = Lint::SegmentChars;
        if segment.starts_with('<') && segment.ends_with('>') {
            let mut name = &segment[1..(segment.len() - 1)];
            if name.ends_with("..") {
                trailing = true;
                name = &name[..(name.len() - 2)];
            }

            let span = subspan(name, segment, source_span);
            if name.is_empty() {
                return Err(Error::new(name, source_span, ErrorKind::Empty));
            } else if !is_valid_ident(name) {
                return Err(Error::new(name, span, ErrorKind::BadIdent));
            }

            let dynamic = Dynamic { name: Name::new(name, span), trailing, index: 0 };
            if dynamic.is_wild() && P::KIND != Kind::Path {
                return Err(Error::new(name, span, ErrorKind::Ignored));
            } else if dynamic.is_wild() {
                return Ok(Parameter::Ignored(dynamic));
            } else {
                return Ok(Parameter::Dynamic(dynamic));
            }
        } else if segment.is_empty() {
            return Err(Error::new(segment, source_span, ErrorKind::Empty));
        } else if segment.starts_with('<') && lint.enabled(source_span) {
            let candidate = candidate_from_malformed(segment);
            source_span.warning("`segment` starts with `<` but does not end with `>`")
                .help(format!("perhaps you meant the dynamic parameter `<{}>`?", candidate))
                .note(lint.how_to_suppress())
                .emit_as_item_tokens();

View on GitHub (pinned to 3a54d079ae)

Solutions

Rename the parameter to a valid snake_case Rust identifier: <user_id> instead of <user-id>
Keep the pretty URL only if you don't need the value; otherwise map it in the handler, since the parameter name must equal the function argument name
Check for stray characters (spaces, unicode dashes) if the name looks valid

Example fix

// before
#[get("/users/<user-id>")]
fn user(user_id: &str) { }

// after
#[get("/users/<user_id>")]
fn user(user_id: &str) { }

Defensive patterns

Strategy: validation

Prevention

Use snake_case for route parameter names; keep kebab-case only in static path text
Parameter name must equal the handler argument name
cargo check after renaming path segments

When it happens

Trigger: Route parameters containing hyphens, leading digits, spaces, or other non-ident characters: #[get("/<user-id>")], #[get("/<0id>")], #[get("/<user id..>")].

Common situations: Mirroring URL naming (kebab-case) into the parameter name instead of the function argument; renaming a path segment for SEO without updating the parameter; typos inserting invisible characters or spaces.

Related errors

AI-assisted analysis of rwf2/Rocket@3a54d079ae (2026-08-16). Data as JSON: /api/errors/e5fa9c3521ccc498. Report an issue: GitHub.