Rollup merge of rust-lang#28874 - GuillaumeGomez:error_code, r=Manishearth

r? @Manishearth

Author: steveklabnik

src/librustc_trans/diagnostics.rs

@@ -12,6 +12,21 @@
 
 register_long_diagnostics! {
 
+E0515: r##"
+A constant index expression was out of bounds. Erroneous code example:
+
+```
+let x = &[0, 1, 2][7]; // error: const index-expr is out of bounds
+```
+
+Please specify a valid index (not inferior to 0 or superior to array length).
+Example:
+
+```
+let x = &[0, 1, 2][2]; // ok!
+```
+"##,
+
 }
 
 register_diagnostics! {

src/librustc_trans/trans/consts.rs

@@ -628,8 +628,8 @@ fn const_expr_unadjusted<'a, 'tcx>(cx: &CrateContext<'a, 'tcx>,
             if iv >= len {
                 // FIXME #3170: report this earlier on in the const-eval
                 // pass. Reporting here is a bit late.
-                cx.sess().span_err(e.span,
-                                   "const index-expr is out of bounds");
+                span_err!(cx.sess(), e.span, E0515,
+                          "const index-expr is out of bounds");
                 C_undef(val_ty(arr).element_type())
             } else {
                 const_get_elt(cx, arr, &[iv as c_uint])