"return ..." vs "result = ..."

return_result.c

// ...

N_NIMCALL(NimStringDesc*, test1_82003)(NimStringDesc* a) {
	NimStringDesc* result;
	NimStringDesc* LOC1;
	result = 0;
	LOC1 = 0;
	LOC1 = rawNewString(a->Sup.len + 5);
appendString(LOC1, a);
appendString(LOC1, ((NimStringDesc*) &TMP5));
	result = LOC1;
	goto BeforeRet;
	BeforeRet: ;
	return result;
}

N_NIMCALL(NimStringDesc*, test2_82009)(NimStringDesc* a) {
	NimStringDesc* result;
	NimStringDesc* LOC1;
	result = 0;
	LOC1 = 0;
	LOC1 = rawNewString(a->Sup.len + 5);
appendString(LOC1, a);
appendString(LOC1, ((NimStringDesc*) &TMP5));
	result = LOC1;
	return result;
}

N_NIMCALL(NimStringDesc*, test3_82017)(NimStringDesc* a, NI b) {
	NimStringDesc* result;
	NimStringDesc* res;
	NI i_82044;
	NI HEX3Atmp_82045;
	NI res_82047;
	result = 0;
	res = copyString(((NimStringDesc*) &TMP8));
	i_82044 = 0;
	HEX3Atmp_82045 = 0;
	HEX3Atmp_82045 = b - 1;
	res_82047 = 0;
	while (1) {
		NimStringDesc* LOC2;
		NimStringDesc* LOC3;
		if (!(res_82047 <= HEX3Atmp_82045)) goto LA1;
		i_82044 = res_82047;
		LOC2 = 0;
		LOC3 = 0;
		LOC3 = nimIntToStr(i_82044);
		LOC2 = rawNewString(LOC3->Sup.len + a->Sup.len + 0);
appendString(LOC2, LOC3);
appendString(LOC2, a);
		res = resizeString(res, LOC2->Sup.len + 0);
appendString(res, LOC2);
		res_82047 += 1;
	} LA1: ;
	result = copyString(res);
	goto BeforeRet;
	BeforeRet: ;
	return result;
}

N_NIMCALL(NimStringDesc*, test4_82052)(NimStringDesc* a, NI b) {
	NimStringDesc* result;
	NI i_82078;
	NI HEX3Atmp_82079;
	NI res_82081;
	result = 0;
	result = copyString(((NimStringDesc*) &TMP8));
	i_82078 = 0;
	HEX3Atmp_82079 = 0;
	HEX3Atmp_82079 = b - 1;
	res_82081 = 0;
	while (1) {
		NimStringDesc* LOC2;
		NimStringDesc* LOC3;
		if (!(res_82081 <= HEX3Atmp_82079)) goto LA1;
		i_82078 = res_82081;
		LOC2 = 0;
		LOC3 = 0;
		LOC3 = nimIntToStr(i_82078);
		LOC2 = rawNewString(LOC3->Sup.len + a->Sup.len + 0);
appendString(LOC2, LOC3);
appendString(LOC2, a);
		result = resizeString(result, LOC2->Sup.len + 0);
appendString(result, LOC2);
		res_82081 += 1;
	} LA1: ;
	return result;
}

// ...

return_result.nim

proc test_1(a: string): string =
    return a & "asdfg"

proc test_2(a: string): string =
    result = a & "asdfg"

echo test_1("abc")
echo test_2("def")

proc test_3(a: string, b: int): string =
    var res = ""
    for i in 0.. < b:
        res.add($i & a)
    return res

proc test_4(a: string, b: int): string =
    result = ""
    for i in 0.. < b:
        result.add($i & a)

echo test_3("abc", 3)
echo test_4("def", 3)

Using the result implicit variable is better than using return when control flow is not affected by the use of return.

As you can see between the test_3 and test_4 procs, it can avoid copying the returned variable (result = copyString(res); in test3_82017 when using return res in test_3).