anchor build panics with ParseIntError { kind: InvalidDigit } in idl/parse/mod.rs because the IDL generator cannot handle const generics

Symptom: anchor build dies before producing an IDL with a panic that mentions nothing you wrote:

thread 'main' panicked at lang/syn/src/idl/parse/mod.rs:17:76:
called `Result::unwrap()` on an `Err` value: ParseIntError { kind: InvalidDigit }

Replacing the generic parameter with a literal makes it build, e.g. struct MyStruct<const N: usize> { vals: [u32; N] } fails but vals: [u32; 100] succeeds.

Cause: Anchor's IDL generator parses your source with syn and, when it hits an array type, tries to parse::<usize>() the array length token. A const-generic parameter is an identifier, not a digit, so the parse fails and the unwrap panics. The compiler itself is fine with the code — this is purely the IDL step, which is why the panic path is inside Anchor rather than rustc, and why the message contains no file or type name from your program.

Fix options: remove the const generic and use concrete sizes or a macro that expands to concrete-sized structs; or upgrade Anchor — const generics support landed for 0.30.0 (coral-xyz/anchor#2824), so on 0.30 or later this specific panic should be gone. If you are stuck on an older Anchor for other reasons, moving the generic struct out of any type reachable from #[derive(Accounts)]/#[account] also keeps it out of the IDL walk.

Transferable lesson: an unwrap panic inside lang/syn/src/idl/... means the IDL parser choked on a type it cannot express, not that your Rust is invalid — try building with a literal type to isolate which field it is.

Source: adapted from a Stack Exchange answer by McBain (question by McBain), https://solana.stackexchange.com/questions/12430/does-anchor-support-const-generics — CC BY-SA 4.0