Do not use desugared ident when suggesting adding a type

src/librustc/hir/lowering.rs

@@ -4011,8 +4011,10 @@ impl<'a> LoweringContext<'a> {
                 let iter = self.str_to_ident("iter");
 
                 let next_ident = self.str_to_ident("__next");
+                let sp = self.allow_internal_unstable(CompilerDesugaringKind::ForLoop,
+                                                      pat.span);
                 let next_pat = self.pat_ident_binding_mode(
-                    pat.span,
+                    sp,
                     next_ident,
                     hir::BindingAnnotation::Mutable,
                 );

src/librustc/ich/impls_syntax.rs

@@ -412,6 +412,7 @@ impl_stable_hash_for!(enum ::syntax_pos::hygiene::CompilerDesugaringKind {
     DotFill,
     QuestionMark,
     ExistentialReturnType,
+    ForLoop,
     Catch
 });
 

src/librustc/infer/error_reporting/need_type_info.rs

@@ -132,7 +132,12 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
             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_ident) = pattern.simple_ident() {
-                labels.push((pattern.span, format!("consider giving `{}` a type", simple_ident)));
+                labels.push((
+                    pattern.span,
+                    match pattern.span.compiler_desugaring_kind() {
+                        None => format!("consider giving `{}` a type", simple_ident),
+                        _ => "consider giving this a type".to_string(),
+                    }));
             } else {
                 labels.push((pattern.span, format!("consider giving the pattern a type")));
             }

src/libsyntax_pos/hygiene.rs

@@ -602,6 +602,7 @@ pub enum CompilerDesugaringKind {
     /// `impl Trait` with `Foo`.
     ExistentialReturnType,
     Async,
+    ForLoop,
 }
 
 impl CompilerDesugaringKind {
@@ -612,6 +613,7 @@ impl CompilerDesugaringKind {
             CompilerDesugaringKind::QuestionMark => "?",
             CompilerDesugaringKind::Catch => "do catch",
             CompilerDesugaringKind::ExistentialReturnType => "existential type",
+            CompilerDesugaringKind::ForLoop => "for loop",
         })
     }
 }

src/test/ui/issue-51116.rs

@@ -0,0 +1,24 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {
+    let tiles = Default::default();
+    for row in &mut tiles {
+        for tile in row {
+            //~^ NOTE consider giving this a type
+            *tile = 0;
+            //~^ ERROR type annotations needed
+            //~| NOTE cannot infer type for `_`
+            //~| NOTE type must be known at this point
+        }
+    }
+    
+    let tiles: [[usize; 3]; 3] = tiles;
+}

src/test/ui/issue-51116.stderr

@@ -0,0 +1,14 @@
+error[E0282]: type annotations needed
+  --> $DIR/issue-51116.rs:16:13
+   |
+LL |         for tile in row {
+   |             ---- consider giving this a type
+LL |             //~^ NOTE consider giving this a type
+LL |             *tile = 0;
+   |             ^^^^^ cannot infer type for `_`
+   |
+   = note: type must be known at this point
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.