diesel-rs/diesel · error

` ` must be in the form `#[ ="something"]

Error message

`{}` must be in the form `#[{}="something"]`

What it means

Helper `str_value_of_meta_item` in diesel's migrations macros expects an attribute in the exact string-literal form `#[name = "something"]`. If the attribute exists but is not a `Meta::NameValue` with a `Lit::Str` (e.g. a path/flag-style attribute or a non-string literal), the macro panics with this message naming the expected attribute.

Solutions

1. Quote the attribute value: `#[migration_name = "my_migration"]`.
2. Ensure the attribute is a key = "string literal" pair, not a bare flag or path segment.
3. Remove non-string literals (numbers, bools) and wrap them in string form if the option accepts them.
4. Check the diesel version's documented attribute names for the macro you use.

Example fix

// before
#[migration_name = create_users]
mod migrations;
// after
#[migration_name = "create_users"]
mod migrations;

// attribute values must be string literals
// bad: #[migration_name = foo] or #[migration_name]
// good:
#[migration_name = "create_users"]
mod migrations {}

When it happens

Trigger: Writing `#[migration_name = something]` (non-string literal), `#[migration_name]` (flag without value), or `#[migration_name = 42]` on a module used with `embed_migrations!` / `import_migrations!` style attributes (e.g. `migration_name`, `migration_version`).

Common situations: Copy-pasting attribute syntax from other crates; forgetting quotes around the value; using `=` with an identifier instead of a string literal; typos creating duplicate attributes.

Understand the failure class

Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.

Related errors

• unexpected token after positional `#[variadic(..)]`
• expect `last_arguments`
• Can't get file name from path
• Invalid migration directory: the directory's name should be
• unexpected end of input, expected `=` help: the correct…

AI-assisted analysis of diesel-rs/diesel@6fa6ed01b2 (2026-09-07). Data as JSON: /api/errors/ca6c57de60cf89af. Report an issue: GitHub.

Appendix: source

Thrown at diesel_migrations/migrations_macros/src/util.rs:9

use syn::*;

pub fn str_value_of_meta_item(item: &Meta, name: &str) -> String {
    match *item {
        Meta::NameValue(MetaNameValue {
            lit: Lit::Str(ref value),
            ..
        }) => value.value(),
        _ => panic!(
            r#"`{}` must be in the form `#[{}="something"]`"#,
            name, name
        ),
    }
}

pub fn get_options_from_input(
    name: &Path,
    attrs: &[Attribute],
    on_bug: fn() -> !,
) -> Option<Vec<Meta>> {
    let options = attrs
        .iter()
        .find(|a| &a.path == name)
        .map(Attribute::parse_meta);
    match options {
        Some(Ok(Meta::List(MetaList { ref nested, .. }))) => Some(
            nested

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