But I think its a GNU extension. It'd be amazing if it were ISO standard.

I found about this __INCLUDE_LEVEL__ some time ago, but I had something better in mind. Unfortunately this requires some parser that could at least understand some basic syntax tricks and can at least identify C (and maybe also C++) entities, where they begin and end and how to transform the "definition" into "declaration". This would be then the original source version from which this presented in the front page would be only generated:

module mtvec;

// Only import: contents of these header files will not be provided to those
// who include THIS one.
import <stdio.h>;
import <stdint.h>;

export typedef struct mtvec_t mtvec_t;

struct mtvec_t
{
    void** data;
    uint32_t length;
    uint32_t length_real;
};

export mtvec_t* mtvec_alloc( )
{
    mtvec_t* vector = mtmem_calloc( sizeof( mtvec_t ) , mtvec_dealloc );
    vector->data = mtmem_calloc( sizeof( void* ) * 10 , NULL );
    vector->length = 0;
    vector->length_real = 10;
    return vector;
}

export void mtvec_dealloc( void* pointer )
{
    mtvec_t* vector = pointer;

    for ( uint32_t index = 0 ; index < vector->length ; index++ ) {
    mtmem_release( vector->data[index] );
    }
    mtmem_release( vector->data );
}

export void mtvec_reset( mtvec_t* vector )
{
    for ( uint32_t index = 0 ; index < vector->length ; index++ ) mtmem_release( vector->data[index] );

    vector->length = 0;
}

All type definitions (typedefs and structured) with export will be "swallowed whole" into the "header part". Functions will get only the header extracted and put into the generated header part, unless they have inline modifier, in which case they will be also put into the header part as whole.

Structures that are not marked with export will get only the incomplete type definition in the header, if it is used anywhere in the function signatures, otherwise it will be not present in the header part at all.

The generated single source file for this one would have to have an appropriate name with specific extension so that if you use import mtvec; statement in another file it knows what to look for (let's say, the generated file will have a name mtvec.chi, so this instruction will be equivalent to import "mtvec.chi";. Simultanously, import <stdio.h>; would simply reach out to the exact header file.