I want to implement this. My code is in C.

How does this help me check my implementation? I guess I could ask ChatGPT to convert your tests to my code, but that seems the long way around.

https://github.com/timbray/RFC9839/blob/main/unichars.go

I don't know rust at all but I can pretty quickly understand:

    var unicodeAssignables = []runePair{
     {0x20, 0x7E},       // ASCII
     {0xA, 0xA},         // newline
     {0xA0, 0xD7FF},     // most of the BMP
     {0xE000, 0xFDCF},   // BMP after surrogates
     {0xFDF0, 0xFFFD},   // BMP after noncharacters block
     {0x9, 0x9},         // Tab
     {0xD, 0xD},         // CR
     {0x10000, 0x1FFFD}, // astral planes from here down
     {0x20000, 0x2FFFD},
     {0x30000, 0x3FFFD},
     {0x40000, 0x4FFFD},
     {0x50000, 0x5FFFD},
     {0x60000, 0x6FFFD},
     {0x70000, 0x7FFFD},
     {0x80000, 0x8FFFD},
     {0x90000, 0x9FFFD},
     {0xA0000, 0xAFFFD},
     {0xB0000, 0xBFFFD},
     {0xC0000, 0xCFFFD},
     {0xD0000, 0xDFFFD},
     {0xE0000, 0xEFFFD},
     {0xF0000, 0xFFFFD},
     {0x100000, 0x10FFFD},
    }