When seeing `x<-y` in a boolean expression, suggest `x < -y`.

[kennytm]

• Rationale: https://stackoverflow.com/q/47028390

Test case:

fn main() {
    let x = -5;
    if x<-1 {
        println!("ok");
    }
}

The error currently looks like:

error: placement-in expression syntax is experimental and subject to change. (see issue #27779)
 --> src/main.rs:3:8
  |
3 |     if x<-1 {
  |        ^^^^
  |
  = help: add #![feature(placement_in_syntax)] to the crate attributes to enable

but the intent was not to use the placement-in, the suggestion is just confusing the user. If we do add #![feature(placement_in_syntax)] the error message would be even worse:

error[E0277]: the trait bound `{integer}: std::ops::Placer<_>` is not satisfied
 --> src/main.rs:5:8
  |
5 |     if x<-1 {
  |        ----
  |        |
  |        the trait `std::ops::Placer<_>` is not implemented for `{integer}`
  |        in this macro invocation
  |
  = help: the following implementations were found:
            <std::collections::linked_list::BackPlace<'a, T> as std::ops::Placer<T>>
            <std::collections::vec_deque::PlaceBack<'a, T> as std::ops::Placer<T>>
            <std::collections::hash_map::Entry<'a, K, V> as std::ops::Placer<V>>
            <&'a mut std::collections::BinaryHeap<T> as std::ops::Placer<T>>
          and 4 others
  = note: required by `std::ops::Placer::make_place`

The suggestion should actually mention inserting a space between < and - if a comparison with a negative number is intended, e.g.

error: placement-in expression syntax is experimental and subject to change. (see issue #27779)
 --> src/main.rs:3:8
  |
3 |     if x<-1 {
  |        ^^^^
  |        hint: for less-than comparison, try to add some spaces: `x < -1`
  |
  = help: add #![feature(placement_in_syntax)] to the crate attributes to enable

[estebank]

Should expand the following code:

rust/src/libsyntax/feature_gate.rs

Lines 1518 to 1519 in 442b7bd

	ast::ExprKind::InPlace(..) => {
	gate_feature_post!(&self, placement_in_syntax, e.span, EXPLAIN_PLACEMENT_IN);

to have a version gate_feature_post! that accepts extra notes, and add a note that if you meant to compare x with 1 you should add a space between < and - (you can get the spans from the two fields in InPlace(expr, expr).

The same should be done for the case where the feature flag has been enabled:

error[E0277]: the trait bound `{integer}: std::ops::Placer<_>` is not satisfied
 --> src/main.rs:4:8
  |
4 |     if x<-1 {
  |        ----
  |        |
  |        the trait `std::ops::Placer<_>` is not implemented for `{integer}`
  |        in this macro invocation
  |
  = help: the following implementations were found:
            <std::vec::PlaceBack<'a, T> as std::ops::Placer<T>>
            <std::collections::linked_list::FrontPlace<'a, T> as std::ops::Placer<T>>
            <std::collections::vec_deque::PlaceFront<'a, T> as std::ops::Placer<T>>
            <std::collections::vec_deque::PlaceBack<'a, T> as std::ops::Placer<T>>
          and 4 others
  = note: required by `std::ops::Placer::make_place`

I believe it is somewhere under:

rust/src/librustc/hir/lowering.rs

Line 2454 in 442b7bd

ExprKind::InPlace(ref placer, ref value_expr) => {

The second case can also be dealt with by using rustc_on_unimplemented to add a special note/label for all integer types.

[arshiamufti]

Can I take this on? I have some time in the latter half of this month.

[estebank]

@arshiamufti feel free to do so and to ask for help!

[PramodBisht]

@estebank Hi, Can I take this one? if yes, then I will need your guidance.

[arshiamufti]

hi @PramodBisht -- i'm actually going to work on this sometime in the near future :)

[PramodBisht]

@arshiamufti sure!