[FIXED] Why does a match expression not report an error for a catch-all arm (_) prior to other arms?

Issue

Rust has a construct called match which looks very similar to switch in other languages. However I observed a very peculiar behavior of match.

let some_u32_value = 3;
match some_u32_value {
    _ => (),
    3 => println!("three"),
}

match respects the order in which the cases/patterns are mentioned. Why does it not report an error if the default (_) case is at the top? It does give a warning though:

warning: unreachable pattern
 --> src/main.rs:5:9
  |
5 |         3 => println!("three"),
  |         ^
  |
  = note: #[warn(unreachable_patterns)] on by default

A similar construct in Java, the switch, does not preserve any order, so having a default prior to other cases is not an error (ignoring the fall through behavior).

int x = 0;

switch (x) {
  default:
    System.out.println("default");
    break;
  case 0:
      System.out.println("Zero");
} 

Is there some purpose for doing this explicitly?

Solution

An unreachable pattern is not strictly an error, I mean: it does not prevent the compiler from "understanding" the code nor does it make the code unsafe.

Similarly, in C, for example, you can return a reference to a local variable without triggering an error (at least with gcc):

#include <stdio.h>

int* foo() {
    int x = 0;

    return &x;
}

int main() {
    printf("%d", *foo());

    return 0;
}

Generally, you should not consider a warning as "oh, that's only a warning, I don't care". A warning is an actual useful advice/information given by the compiler.

I like the definition given in warnings:

A warning is often issued on recognizing a potential high-risk situation, a probable misunderstanding, degraded service or imminent failure.

because it helps to understand the difference between an error and a warning:

• An error is an error,
• A warning is a potential/probable error or a problematic thing.

In this situation, the last arm of the match is some dead code, so the compiler reports it accordingly.

Answered By - Boiethios

Answer Checked By - Candace Johnson (PHPFixing Volunteer)