Suggest &[_] instead of &[_; N] on inference error

Author: estebank

compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs

@@ -366,12 +366,21 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 None
             };
             printer.name_resolver = Some(Box::new(&getter));
-            let _ = if let ty::FnDef(..) = ty.kind() {
+            let _ = match ty.kind() {
                 // We don't want the regular output for `fn`s because it includes its path in
                 // invalid pseudo-syntax, we want the `fn`-pointer output instead.
-                ty.fn_sig(self.tcx).print(printer)
-            } else {
-                ty.print(printer)
+                ty::FnDef(..) => ty.fn_sig(self.tcx).print(printer),
+                ty::Ref(_, ref_ty, mutability) => {
+                    match ref_ty.kind() {
+                        ty::Array(ty, _) => {
+                            // Do not suggest `&[_; 0]`, instead suggest `&[_]`.
+                            let _ = ty.print(printer);
+                            return format!("&{}[{}]", mutability.prefix_str(), s);
+                        }
+                        _ => ty.print(printer),
+                    }
+                }
+                _ => ty.print(printer),
             };
             s
         };

src/test/ui/issues/issue-7813.stderr

@@ -1,13 +1,13 @@
-error[E0282]: type annotations needed for `&[_; 0]`
+error[E0282]: type annotations needed for `&[_]`
   --> $DIR/issue-7813.rs:2:13
    |
 LL |     let v = &[];
    |             ^^^ cannot infer type
    |
-help: consider giving this binding the explicit type `&[_; 0]`, with the type parameters specified
+help: consider giving this binding the explicit type `&[_]`, with the type parameters specified
    |
-LL |     let v: &[_; 0] = &[];
-   |          ^^^^^^^^^
+LL |     let v: &[_] = &[];
+   |          ^^^^^^
 
 error: aborting due to previous error