A simple C debug macro (needs a version of gcc compiler which supports the C11 standard)

dbg.h

/* Copyright 2019 Ramakrishnan Muthukrishnan
 *
 * To the extent possible under law, the author(s) have dedicated 
 * all copyright and related and neighboring rights to this software 
 * to the public domain worldwide. This software is distributed without
 * any warranty.
 *
 * You should have received a copy of the CC0 Public Domain Dedication 
 * along with this software. 
 * If not, see <http://creativecommons.org/publicdomain/zero/1.0/>. 
 */

#ifndef DBG_H_INCLUDED

#define DBG_H_INCLUDED 1

#if defined (__GNUC__)
#if defined (__STDC_VERSION__)

#if __STDC_VERSION__ >= 201112L

#include <stdio.h>

#define PRINTF_FMT_STR(x) _Generic((x), \
    char: "%c", \
    signed char: "%hhd", \
    unsigned char: "%hhu", \
    signed short: "%hd", \
    unsigned short: "%hu", \
    signed int: "%d", \
    unsigned int: "%u", \
    long int: "%ld", \
    unsigned long int: "%lu", \
    long long int: "%lld", \
    unsigned long long int: "%llu", \
    float: "%f", \
    double: "%f", \
    long double: "%Lf", \
    char *: "%s", \
    void *: "%p", \
    bool: "%s" \
)

#define DBG(x) \
    ({  typeof (x) x_;                                             \
        x_ = (x);                                                  \
        fprintf(stderr, "[%s:%d] %s = " , __FILE__, __LINE__, #x); \
        fprintf(stderr, PRINTF_FMT_STR(x), x_); \
        fprintf(stderr, "\n"); \
        x_; \
    })

#endif /* __STDC_VERSION >= XXXX */
#endif /* __STDC_VERSION__ */
#endif /* __GNUC__ */

#endif

Here is the version of factorial program example given in the Rust blog post:

fn factorial(n: u32) -> u32 {
    if dbg!(n <= 1) {
        dbg!(1)
    } else {
        dbg!(n * factorial(n - 1))
    }
}

And the output:

[src/main.rs:3] n <= 1 = false
[src/main.rs:3] n <= 1 = false
[src/main.rs:3] n <= 1 = false
[src/main.rs:3] n <= 1 = true
[src/main.rs:4] 1 = 1
[src/main.rs:5] n * factorial(n - 1) = 2
[src/main.rs:5] n * factorial(n - 1) = 6
[src/main.rs:5] n * factorial(n - 1) = 24
[src/main.rs:11] factorial(4) = 24

Here is the equivalent C version:

#include <stdio.h>
#include <stdbool.h>

#include "dbg.h"

int factorial(int n)
{
    if (DBG(n <= 1))
        return DBG(1);
    else
        return DBG(n * factorial(n - 1));
}

int main(void)
{
    int x = 4;
    
    DBG(factorial(x));
    return 0;
}

and the corresponding output:

$ gcc -Wall  foo.c && ./a.out
[foo.c:8] n <= 1 = 0
[foo.c:8] n <= 1 = 0
[foo.c:8] n <= 1 = 0
[foo.c:8] n <= 1 = 1
[foo.c:9] 1 = 1
[foo.c:11] n * factorial(n - 1) = 2
[foo.c:11] n * factorial(n - 1) = 6
[foo.c:11] n * factorial(n - 1) = 24
[foo.c:18] factorial(x) = 24

Thanks to Sajith for the inspiration and Olve Maudal for many suggestions.