C2Rust fails to translate `__builtin_nans` with “Unimplemented builtin __builtin_nans”

[lynae99]

C2Rust fails when translating a C program that uses the compiler builtin __builtin_nans. The translation stops during main and reports that this builtin is unimplemented.
This appears to be an unsupported builtin handling issue for signaling NaN generation.
To Reproduce

#include <stdio.h>

int main(void) {
    double x = __builtin_nans("");

    printf("%f\n", x);
    return 0;
}

Run C2Rust translation on the file:

c2rust transpile test.rs

Observed Behavior
C2Rust fails during translation:

Transpiling test.c
error: Failed to translate main: Unimplemented builtin __builtin_nans
--> test.c:4:16

Expected Behavior
C2Rust should either translate __builtin_nans to an equivalent Rust representation, or emit a more graceful unsupported-feature diagnostic without failing the whole function translation unexpectedly.
Environment

• c2rust version: v0.21.0
• platform: Ubuntu 24.04
• Rust: nightly-2022-08-08
• Clang version: 17.0.6