Using a macro as the tail expression in a function that doesn't return a value should prompt to add a semicolon

[shepmaster]

Given the following code:

fn demo() -> u8 {
    1
}

fn good() {
    demo()
}

fn subpar() {
    dbg!(demo())
}

The current output is:

error[E0308]: mismatched types
 --> src/lib.rs:6:5
  |
6 |     demo()
  |     ^^^^^^ expected `()`, found `u8`
  |
help: consider using a semicolon here
  |
6 |     demo();
  |           +
help: try adding a return type
  |
5 | fn good() -> u8 {
  |           +++++

error[E0308]: mismatched types
  --> src/lib.rs:10:5
   |
9  | fn subpar() {
   |             - help: try adding a return type: `-> u8`
10 |     dbg!(demo())
   |     ^^^^^^^^^^^^ expected `()`, found `u8`
   |
   = note: this error originates in the macro `dbg` (in Nightly builds, run with -Z macro-backtrace for more info)

Note that the subpar function does not have help for adding a semicolon.

Ideally the output should look like:

error[E0308]: mismatched types
 --> src/lib.rs:6:5
  |
6 |     demo()
  |     ^^^^^^ expected `()`, found `u8`
  |
help: consider using a semicolon here
  |
6 |     demo();
  |           +
help: try adding a return type
  |
5 | fn good() -> u8 {
  |           +++++

error[E0308]: mismatched types
  --> src/lib.rs:10:5
   |
help: consider using a semicolon here
   |
10 |     dbg!(demo());
   |                 +
9  | fn subpar() {
   |             - help: try adding a return type: `-> u8`
10 |     dbg!(demo())
   |     ^^^^^^^^^^^^ expected `()`, found `u8`
   |
   = note: this error originates in the macro `dbg` (in Nightly builds, run with -Z macro-backtrace for more info)

Note that the subpar function now has help for adding a semicolon.

[vincenzopalazzo]

@rustbot claim

[rustbot]

Error: Parsing assign command in comment failed: ...'bot assign' | error: specify user to assign to at >| ''...

Please file an issue on GitHub at triagebot if there's a problem with this bot, or reach out on #t-infra on Zulip.

[gimbling-away]

Note that this also happens on simple expressions, there is no "add semicolon" diagnostic here either

fn demo() {
    1
}

error[[E0308]](https://doc.rust-lang.org/stable/error-index.html#E0308): mismatched types
 --> src/lib.rs:2:5
  |
1 | fn demo() {
  |           - help: try adding a return type: `-> i32`
2 |     1
  |     ^ expected `()`, found integer

Also, ideally ideally we should really show that the different help suggestions can not be used together, rather the developer can pick one of them, making an issue for this :)