Improve error message for statements used as expressions

[mcoblenz]

The code below has superfluous semicolons in the branches of the if expression:

fn test() -> i32 {
    let x = if (true) {
        3;
    }
    else {
        4;
    };
    x
}

The error message that results is:

error[E0308]: mismatched types
  --> src/lib.rs:11:5
   |
4  | fn test() -> i32 {
   |              --- expected `i32` because of return type
...
11 |     x
   |     ^ expected `i32`, found `()`

This can be confusing to beginners, who may not understand unit type. However, since the expressions 3 and 4 can be shown to have no side effects, the code (as is) can't be what the user intended. Could the compiler produce a more targeted error message that mentions the superfluous semicolon?

[fmease]

@rustbot label T-compiler A-diagnostics D-newcomer-roadblock D-terse

[cassaundra]

@rustbot claim

[cassaundra]

Thinking more about this, I don't think we should extend the "remove semicolon to make tail expression" suggestion to cover this case, since we don't know the expected type of x at the point at which the suggestion would be made. For example, we don't make the suggestion to change "a" to 'a' here:

fn test() -> char {
    let x = "a";
    x
}

But that's just a guess based on my limited knowledge of type mismatch suggestions. Maybe someone else could weigh in?