Incomplete `__float128` support: cast from `long double` fails.

[lynae99]

C2Rust appears to support __float128 / Float128 at the type-mapping level (e.g., mapping LongDouble | Float128 to an f128 type), but initialization via cast from long double fails with a translation error.
Reproducer

#include <float.h>

int main(void) {
    __float128 f128_1 = 1.0L;
    return 0;
}

Observed
C2Rust fails to translate main with an error similar to:

Failed to translate main: Tried casting long double to unsupported type: Float128

Environment

• c2rust version: v0.21.0
• platform：Ubuntu 24.04

[lynae99]

I found additional reproducers that appear to belong to the same root-cause family: casts from long double to integer types are also not fully supported.
For example, C2Rust fails on a valid cast from long double to uint32_t:

#include <stdio.h>
#include <stdint.h>

int main(void) {
    long double ld = 100.1234567890123456789L;
    uint32_t val = (uint32_t)ld;  // valid C, truncates to 100
    printf("%u\n", val);
    return 0;
}

Observed error:

Failed to translate main: Tried casting long double to unsupported type: UInt32

I also observed the same pattern for other integer targets such as:

• int32_t (Int32)

• uint64_t (UInt64)

So this seems broader than just one missing cast case, and may indicate incomplete support for long double -> integer conversions in general.
Additionally, even when translation fails, C2Rust still emits an incomplete .rs file with the translated function body missing, which then fails to compile.

[lynae99]

Another related reproducer: C2Rust also fails on a valid cast from long double to size_t.

#include <stdlib.h>

int main(void) {
    long double ld = 3.141592653589793238462643383279502884L;
    size_t n = (size_t)(ld * 2.0L);  // valid C
    return 0;
}

Observed error:

Failed to translate main: Tried casting long double to unsupported type: Size

This looks closely related to the unsupported long double cast cases above, but affects the platform-sized integer category specifically (size_t / Rust usize).
Together with the UInt32/Int32/UInt64 cases, this suggests that casts from long double to several integer-like destination types are not currently handled.
As with the other reproducers, C2Rust still emits an incomplete Rust output file after the translation error, leaving a missing function body and causing a follow-up Rust compilation failure.