Rust: Type inference suggests correction to non-existent variable `__next`

Created on 28 May 2018 · 3Comments · Source: rust-lang/rust

fn main() {
    let tiles = Default::default();
    for row in &mut tiles {
        for tile in row {
            *tile = 0;
        }
    }

    let tiles: [[usize; 3]; 3] = tiles;
}

Fails with this error:

error[E0282]: type annotations needed
 --> src/main.rs:5:13
  |
4 |         for tile in row {
  |             ---- consider giving `__next` a type
5 |             *tile = 0;
  |             ^^^^^ cannot infer type for `_`

Type inference (rightfully) fails on this code. However, the variable that it suggests fixing is called __next and does not actually exist in my code. It is probably some internal variable generated by the compiler.

A-diagnostics C-bug

Source

sunjay

😄1

All 3 comments

Well, declining to name the variable in this case would be simple enough—

diff --git a/src/librustc/infer/error_reporting/need_type_info.rs b/src/librustc/infer/error_reporting/need_type_info.rs
index 7352c14..25bfb3e 100644
--- a/src/librustc/infer/error_reporting/need_type_info.rs
+++ b/src/librustc/infer/error_reporting/need_type_info.rs
@@ -131,7 +131,10 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
             labels.clear();
             labels.push((pattern.span, format!("consider giving this closure parameter a type")));
         } else if let Some(pattern) = local_visitor.found_local_pattern {
-            if let Some(simple_name) = pattern.simple_name() {
+            // don't put internal desugared-loop identifier in user-facing
+            // message (Issue #51116)
+            let simple_name = pattern.simple_name().filter(|n| n.as_str() != "__next");
+            if let Some(simple_name) = simple_name {
                 labels.push((pattern.span, format!("consider giving `{}` a type", simple_name)));
             } else {
                 labels.push((pattern.span, format!("consider giving the pattern a type")));

But more fundamentally, this isn't a good label to set on loops: we don't have syntax to type-annotate loop variables.