let_underscore_drop: weird "+" in output?

[matthiaskrgr]

Code

#![allow(unused_must_use)]
use std::thread;

macro_rules! expr { ($e: expr) => { $e } }

macro_rules! spawn {
    ($($code: tt)*) => {
        expr!(thread::spawn(move|| {$($code)*}).join())
    }
}

pub fn main() {
    spawn! {
        println!("expr");
    };
    let _ = spawn! {
        println!("expr");
    };
}

Current output

warning: non-binding let on a type that implements `Drop`
  --> 2B8C38DE2265366FC40C9872C2EB2C8B37BDA99B72B47C43B03BF5A58936ABCC.rs:16:5
   |
16 | /     let _ = spawn! {
17 | |         println!("expr");
18 | |     };
   | |______^
   |
   = note: requested on the command line with `-W let-underscore-drop`
help: consider binding to an unused variable to avoid immediately dropping the value
   |
16 |     let _unused = spawn! {
   |         ~~~~~~~
help: consider immediately dropping the value
   |
8  |         expr!(drop())
   |               ~~~~~                                   +

warning: 1 warning emitted

This free-standing "+" looks very weird, not sure what this is supposed to point at?
Maybe the macros messed something up here?

[clubby789]

While reducing this

#![allow(unused_must_use)]
#![warn(let_underscore_drop)]
use std::thread;

struct DropMe;
impl Drop for DropMe {
    fn drop(&mut self) {}
}

macro_rules! spawn {
    ($code: tt) => {
        $code
    }
}

pub fn main() {
    let _ = spawn!(DropMe);
}

ICEs with

thread 'rustc' panicked at 'assertion failed: `(left == right)`
  left: `Some([SubstitutionPart { span: poc.rs:12:9: 17:5 (#0), snippet: "drop(" }, SubstitutionPart { span: poc.rs:12:14: 12:14 (#0), snippet: ")" }])`,
 right: `None`: suggestion must not have overlapping parts'

Filing that here as I suspect it's part of the same issue

EDIT: The issue here is that the span associated with the drop( suggestion is from the start of local.span (which covers the whole let _ = ... line) up until the start of init.span, which is the span of $code inside the macro definition.

[chenyukang]

While reducing this

#![allow(unused_must_use)]
#![warn(let_underscore_drop)]
use std::thread;

struct DropMe;
impl Drop for DropMe {
    fn drop(&mut self) {}
}

macro_rules! spawn {
    ($code: tt) => {
        $code
    }
}

pub fn main() {
    let _ = spawn!(DropMe);
}

ICEs with

thread 'rustc' panicked at 'assertion failed: `(left == right)`
  left: `Some([SubstitutionPart { span: poc.rs:12:9: 17:5 (#0), snippet: "drop(" }, SubstitutionPart { span: poc.rs:12:14: 12:14 (#0), snippet: ")" }])`,
 right: `None`: suggestion must not have overlapping parts'

Filing that here as I suspect it's part of the same issue

EDIT: The issue here is that the span associated with the drop( suggestion is from the start of local.span (which covers the whole let _ = ... line) up until the start of init.span, which is the span of $code inside the macro definition.

The ice dispear now:

warning: unused import: `std::thread`
 --> ./t/t4.rs:3:5
  |
3 | use std::thread;
  |     ^^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

warning: non-binding let on a type that implements `Drop`
  --> ./t/t4.rs:17:5
   |
17 |     let _ = spawn!(DropMe);
   |     ^^^^^^^^^^^^^^^^^^^^^^^
   |
note: the lint level is defined here
  --> ./t/t4.rs:2:9
   |
2  | #![warn(let_underscore_drop)]
   |         ^^^^^^^^^^^^^^^^^^^
help: consider binding to an unused variable to avoid immediately dropping the value
   |
17 |     let _unused = spawn!(DropMe);
   |         ~~~~~~~
help: consider immediately dropping the value
   |
17 |     drop(DropMe));
   |     ~~~~~      +

warning: 2 warnings emitted