Bug: Function mutates argument when it is also the result location

[ghost]

Noticed by sinitax on Discord.

Consider the following code:

const std = @import("std");
const Pos = struct { x: i64, y: i64 };

pub fn modify(p: Pos) Pos {
    return Pos{ .x = 1, .y = p.x };
}

pub fn main() !void {
    p = modify(p);
    std.debug.print("{} {}\n", .{ p.x, p.y });
}

The expected output is 1 5, but the actual output is 1 1. This is because p is passed by reference, but also set as the result location; so, the first field assignment mutates it in-place. This is not expected or desired behaviour. To avoid this, when the result location would overlap any argument, we should instead allocate a temporary.

[Vexu]

Duplicate of #3696