Rust error E0798: arguments for `"cmse-nonsecure-call"` function too large to pass via registers

The compiler team's explanation

Functions marked as cmse-nonsecure-call place restrictions on their inputs and outputs.

is relevant.

• inputs must fit in the 4 available 32-bit argument registers. Alignment

size 8 (i64, u64, f64).

• outputs must either fit in 4 bytes, or be a foundational type of

• no generics can be used in the signature

For more information, see arm's aapcs32.

Erroneous code example:

#![feature(abi_cmse_nonsecure_call)]

#[no_mangle]
pub fn test(
    f: extern "cmse-nonsecure-call" fn(u32, u32, u32, u32, u32) -> u32,
) -> u32 {
    f(1, 2, 3, 4, 5)
}

Arguments' alignment is respected. In the example below, padding is inserted so that the u64 argument is passed in registers r2 and r3. There is then no room left for the final f32 argument

#![feature(abi_cmse_nonsecure_call)]

#[no_mangle]
pub fn test(
    f: extern "cmse-nonsecure-call" fn(u32, u64, f32) -> u32,
) -> u32 {
    f(1, 2, 3.0)
}

From rustc --explain E0798 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.