Improper str AddAssign produces confusing error on otherwise-correct match statement.

Code

// Code written by a beginner
pub fn foo(byte: u8) {
    let mut s = "";
    s += match byte {
        0 => '1',
        1 => '2',
        _ => panic!(),
    };
}

Current output

error[E0308]: `match` arms have incompatible types
 --> src/lib.rs:6:14
  |
3 |       s += match byte {
  |  __________-
4 | |         0 => '1',
  | |              --- this is found to be of type `char`
5 | |         1 => '2',
  | |              --- this is found to be of type `char`
6 | |         _ => panic!(),
  | |              ^^^^^^^^ expected `char`, found `!`
7 | |     };
  | |_____- `match` arms have incompatible types
  |
  = note: expected type `char`
             found type `!`

Desired output

error[E0368]: binary assignment operation `+=` cannot be applied to type `&str`
...
...
  |     -^^^^^
  |     |
  |     cannot use `+=` on type `&str`

Rationale and extra context

This error was very confusing to the programmer, because they reasoned (correctly) that the never type can be coerced into a char.

Neither of us understand why this error was emitted. It was the only error emitted: the compiler was silent about the actual problem (can't AddAssign to str).

Other cases

// Similar code that produces the expected error
pub fn foo(byte: u8) {
    let mut s = "";
    let c = match byte {
        0 => '1',
        1 => '2',
        _ => panic!(),
    };
    s += c;
}

Rust Version

rustc 1.98.0 (88d9e12ae 2026-08-18)
binary: rustc
commit-hash: 88d9e12ae178fab0fb5cc050a94da85685d449ea
commit-date: 2026-08-18
host: x86_64-unknown-linux-gnu
release: 1.98.0
LLVM version: 22.1.8

Anything else?

Also happens on rustc 1.100.0-nightly (e7769602a 2026-08-24).