Poor Trailing Semicolon Error in -> impl Trait Function

[cramertj]

The diagnostics that check whether the concrete return type of an -> impl Trait function meets the : Trait bound should special-case the error when the concrete type is () to suggest removing the semicolon. Without this, the errors for a misplaced semicolon are misleading:

Without impl Trait:

trait Bar {}
impl Bar for u8 {}
fn foo() -> u8 {
    5;
}

error[E0308]: mismatched types
 --> src/main.rs:3:16
  |
3 |   fn foo() -> u8 {
  |  ________________^
4 | |     5;
  | |      - help: consider removing this semicolon
5 | | }
  | |_^ expected u8, found ()
  |
  = note: expected type `u8`
             found type `()`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.
error: Could not compile `playground`.

With impl Trait:

trait Bar {}
impl Bar for u8 {}
fn foo() -> impl Bar {
    5;
}

error[E0277]: the trait bound `(): Bar` is not satisfied
 --> src/main.rs:3:13
  |
3 | fn foo() -> impl Bar {
  |             ^^^^^^^^ the trait `Bar` is not implemented for `()`
  |
  = note: the return type of a function must have a statically known size

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.
error: Could not compile `playground`.

[cramertj]

Note: there's a similar issue in closure return types:

trait Bar {}
impl Bar for u8 {}

fn bar<R: Bar>(_: impl Fn() -> R) {}
fn main() {
    bar(|| { 5u8; })
}

gives

error[E0277]: the trait bound `(): Bar` is not satisfied
 --> src/main.rs:6:5
  |
6 |     bar(|| { 5u8; })
  |     ^^^ the trait `Bar` is not implemented for `()`
  |
note: required by `bar`
 --> src/main.rs:4:1
  |
4 | fn bar<R: Bar>(_: impl Fn() -> R) {}
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

[csmoe]

the impl-Trait <: T subtyping seems fail here:

rust/src/librustc_typeck/check/mod.rs

Lines 4917 to 4919 in 4cf1176

	if self.can_sub(self.param_env, last_expr_ty, expected_ty).is_err() {
	return;
	}

---

DEBUG LOG:

DEBUG 2018-10-03T09:41:07Z: rustc_typeck::check: >> typechecking: expr=expr(21: { 1u8; }) expected=ExpectHasType(_)
DEBUG 2018-10-03T09:41:07Z: rustc_typeck::check: >> typechecking: expr=expr(16: 1u8) expected=NoExpectation
DEBUG 2018-10-03T09:41:07Z: rustc_typeck::check: write_ty(HirId { owner: DefIndex(0:5), local_id: ItemLocalId(1) }, u8) in fcx 0x7ffef324d930
DEBUG 2018-10-03T09:41:07Z: rustc_typeck::check: type of expr 1u8 (id=16) is...
DEBUG 2018-10-03T09:41:07Z: rustc_typeck::check: ... u8, expected is NoExpectation
/// STOP!!!
DEBUG 2018-10-03T09:41:07Z: rustc_typeck::check: write_ty(HirId { owner: DefIndex(0:5), local_id: ItemLocalId(3) }, ()) in fcx 0x7ffef324d930
DEBUG 2018-10-03T09:41:07Z: rustc_typeck::check: write_ty(HirId { owner: DefIndex(0:5), local_id: ItemLocalId(4) }, ()) in fcx 0x7ffef324d930
DEBUG 2018-10-03T09:41:07Z: rustc_typeck::check: type of expr { 1u8; } (id=21) is...
DEBUG 2018-10-03T09:41:07Z: rustc_typeck::check: ... (), expected is ExpectHasType(_)
D

as it shows the checking should move to lint_remove_semi before the 2nd time of calling write_ty , or then expr { 1u8; } will be "parsed" as (), that's why we get (): Bar is not satisfied.

[estebank]

there's a similar issue in closure return types

@cramertj closure output is already problematic for the simple case there too:

error[E0308]: mismatched types
 --> src/main.rs:6:12
  |
6 |     bar(|| { 5u8; })
  |            ^^^^^^^^ expected u32, found ()
  |
  = note: expected type `u32`
             found type `()`

---

Disregard this, we actually do the right thing for this case:

error[E0308]: mismatched types
 --> file.rs:3:12
  |
3 |     bar(|| { 5; })
  |            ^^^-^^
  |            |  |
  |            |  help: consider removing this semicolon
  |            expected u8, found ()
  |
  = note: expected type `u8`
             found type `()`

[estebank]

Current output:

error[E0277]: the trait bound `(): Bar` is not satisfied
 --> src/lib.rs:3:13
  |
3 | fn foo() -> impl Bar {
  |             ^^^^^^^^ the trait `Bar` is not implemented for `()`
4 |     5;
  |      - consider removing this semicolon
  |
  = note: the return type of a function must have a statically known size

It still needs checking that the last statement's expr can actually conform to the trait, but the naïve behavior is there.

[edwardw]

This opaque the trait bound ... is not satisfied error message creeps up in other places too. E.g., this stackoverflow question regarding futures::stream::Stream::for_each. The more prevalent std::iter::Iterator::for_each expects a closure of unit type, but this one expects a closure that returns something. rustc ends up emitting mysterious error messages and the error also propagates to the next function call in the chain.

Ideally, the error message in this case should be something like:

...
socket
    .for_each(|message| {
        println!("recieved: {:?}", message) // error: mismatched types: expected `IntoFuture` but found `()`
    })
    .map_err(|e| println!("read error: {:?}", e))
...

But is this possible at all?

[ultrasaurus]

I also found this error message confusing and referred here via stackoverflow question

Setting aside whether it is possible, it would have been really helpful to see an error like:

expected return value to be a type that implements `IntoFuture` trait;  
however, return value has type `()` which does not implement the required trait
-- perhaps this code is missing an explicit return value?

That's kind of wordy, but thought another suggestion might help someone to come up with an improvement here.

The key thing is issue for me is that the annotation directs my attention to the top of the function (which is good since I would need to look that up in the docs), but I didn't have the experience to recognize that it was referring to an incorrect type of the return value and had forgotten that a function that ends with println! is actually returning ().