Do not suggest .into() in consts

src/librustc_typeck/check/demand.rs

@@ -18,8 +18,9 @@ use syntax::ast;
 use syntax::util::parser::PREC_POSTFIX;
 use syntax_pos::{self, Span};
 use rustc::hir;
-use rustc::hir::print;
 use rustc::hir::def::Def;
+use rustc::hir::map::NodeItem;
+use rustc::hir::{Item, ItemConst, print};
 use rustc::ty::{self, Ty, AssociatedItem};
 use errors::{DiagnosticBuilder, CodeMapper};
 
@@ -318,6 +319,18 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                       checked_ty: Ty<'tcx>,
                       expected_ty: Ty<'tcx>)
                       -> bool {
+        let parent_id = self.tcx.hir.get_parent_node(expr.id);
+        match self.tcx.hir.find(parent_id) {
+            Some(parent) => {
+                // Shouldn't suggest `.into()` on `const`s.
+                if let NodeItem(Item { node: ItemConst(_, _), .. }) = parent {
+                    // FIXME(estebank): modify once we decide to suggest `as` casts
+                    return false;
+                }
+            }
+            None => {}
+        };
+
         let will_truncate = "will truncate the source value";
         let depending_on_isize = "will truncate or zero-extend depending on the bit width of \
                                   `isize`";

src/test/ui/suggestions/const-type-mismatch.rs

@@ -0,0 +1,21 @@
+// 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.
+
+// `const`s shouldn't suggest `.into()`
+
+const TEN: u8 = 10;
+const TWELVE: u16 = TEN + 2;
+//~^ ERROR mismatched types [E0308]
+
+fn main() {
+    const TEN: u8 = 10;
+    const ALSO_TEN: u16 = TEN;
+    //~^ ERROR mismatched types [E0308]
+}

src/test/ui/suggestions/const-type-mismatch.stderr

@@ -0,0 +1,15 @@
+error[E0308]: mismatched types
+  --> $DIR/const-type-mismatch.rs:14:21
+   |
+LL | const TWELVE: u16 = TEN + 2;
+   |                     ^^^^^^^ expected u16, found u8
+
+error[E0308]: mismatched types
+  --> $DIR/const-type-mismatch.rs:19:27
+   |
+LL |     const ALSO_TEN: u16 = TEN;
+   |                           ^^^ expected u16, found u8
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.