What causes "only the `env!` macro is expected here" in diesel-rs/diesel?

Replace the macro call with `env!(YOUR_VAR)`.

only the `env!` macro is expected here

Error message

only the `env!` macro is expected here

What it means

In the `diesel_for_each_tuple!` macro invocation, the second positional argument for the maximum tuple size may be given either as an integer literal or as `env!(VAR_NAME)`. If a macro-like token appears but its identifier is not `env`, the parser rejects it with this compile error.

Solutions

Replace the macro call with `env!(YOUR_VAR)`.
Pass the max tuple size directly as an integer literal instead.
Fix the capitalization/typo so the identifier is exactly `env`.

Example fix

// before
diesel_for_each_tuple!(my_table, getenv!(MAX_TUPLE_SIZE));

// after
diesel_for_each_tuple!(my_table, env!(MAX_TUPLE_SIZE));

Defensive patterns

Strategy: validation

Validate before calling

// Only env! is accepted as a macro form for the tuple-size argument
fn validate_size_arg(arg: &str) -> Result<(), String> {
    if arg.ends_with('!') && !arg.starts_with("env!") {
        return Err(format!("{arg} is not allowed; use env! or a literal"));
    }
    Ok(())
}

Prevention

Use the literal-integer form unless a build-time env override is genuinely needed
Copy the exact `env!(VAR)` spelling from the macro docs
Avoid abstractions like custom wrappers around the size argument

When it happens

Trigger: Writing `diesel_for_each_tuple!(..., other_macro!(SOME_VAR));` — any macro name other than `env` in the max-size position.

Common situations: Typo like `envn!` or `ENV!`, or attempting to reuse a custom macro/environment helper where the grammar only allows `env!`.

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

AI-assisted analysis of diesel-rs/diesel@6fa6ed01b2 (2026-09-07). Data as JSON: /api/errors/57f8a1061f9d1538. Report an issue: GitHub.

Appendix: source

Thrown at diesel_derives/src/diesel_for_each_tuple.rs:60

        #input! {
            #(#out)*
        }
    }
}

pub struct ForEachTupleInput {
    inner: Ident,
    max_size: u16,
}

impl syn::parse::Parse for ForEachTupleInput {
    fn parse(input: syn::parse::ParseStream) -> syn::Result<Self> {
        let inner = input.parse()?;
        input.parse::<syn::Token![,]>()?;
        let max_size = if input.peek(syn::Ident) {
            let macro_ident = input.parse::<syn::Ident>()?;
            if macro_ident != "env" {
                return Err(syn::Error::new(
                    macro_ident.span(),
                    "only the `env!` macro is expected here",
                ));
            }
            let _bang = input.parse::<syn::Token![!]>()?;
            let name;
            syn::parenthesized!(name in input);
            let s = name.parse::<syn::LitStr>()?;
            std::env::var(s.value())
                .map_err(|_| {
                    syn::Error::new(
                        s.span(),
                        format!("Expected `{}` to be set as environment variable", s.value()),
                    )
                })?
                .parse::<u16>()
                .map_err(|_| {
                    syn::Error::new(

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