Subpar diagnostics: unreachable_code lint fires before match arm coverage check

[pnkfelix]

When you do #![deny(unreachable_code)] (which is implied by #![deny(warnings)]), the corresponding lint runs and aborts compilation before the match arm coverage check gets a chance to report.

This leads to somewhat confusing messages.

• at least, they are confusing depending on what the user's mental model is of how match expressions should hypothetically behave if they were not forced to cover the domain of the input expression completely

Example:

#![deny(unreachable_code)]

fn main() {
    let v = vec![1, 2, 3];
    let mut i = v.iter().peekable();
    'label: while let Some(e1) = i.next() {
        
        match e1 {
            &1 => {
                continue 'label;
            }
        }
        println!("e1: {:?}", e1);
    }
}

yields the error message (playpen)

error: unreachable statement
  --> src/main.rs:13:9
   |
13 |         println!("e1: {:?}", e1);
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: lint level defined here
  --> src/main.rs:1:9
   |
1  | #![deny(unreachable_code)]
   |         ^^^^^^^^^^^^^^^^
   = note: this error originates in a macro outside of the current crate

error: aborting due to previous error

which, when I first look at the code, I think "that println! is clearly reachable", because in my head, execution is going to continue past the match expression after that first arm is not matched.

If I change the deny to warn, I get both the warning as above as well as an error about the non-exhaustive patterns in the match expression, saying that &_ is not covered.

[estebank]

Current output:

error: unreachable statement
  --> src/main.rs:13:9
   |
8  | /         match e1 {
9  | |             &1 => {
10 | |                 continue 'label;
11 | |             }
12 | |         }
   | |_________- any code following this `match` expression is unreachable, as all arms diverge
13 |           println!("e1: {:?}", e1);
   |           ^^^^^^^^^^^^^^^^^^^^^^^^^ unreachable statement
   |
note: lint level defined here
  --> src/main.rs:1:9
   |
1  | #![deny(unreachable_code)]
   |         ^^^^^^^^^^^^^^^^
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

[cjgillot]

Current output:

error: unreachable statement
  --> src/main.rs:13:9
   |
8  | /         match e1 {
9  | |             &1 => {
10 | |                 continue 'label;
11 | |             }
12 | |         }
   | |_________- any code following this `match` expression is unreachable, as all arms diverge
13 |           println!("e1: {:?}", e1);
   |           ^^^^^^^^^^^^^^^^^^^^^^^^ unreachable statement
   |
note: the lint level is defined here
  --> src/main.rs:1:9
   |
1  | #![deny(unreachable_code)]
   |         ^^^^^^^^^^^^^^^^
   = note: this error originates in the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0004]: non-exhaustive patterns: `&i32::MIN..=0_i32` and `&2_i32..=i32::MAX` not covered
 --> src/main.rs:8:15
  |
8 |         match e1 {
  |               ^^ patterns `&i32::MIN..=0_i32` and `&2_i32..=i32::MAX` not covered
  |
  = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
  = note: the matched value is of type `&i32`