Show better warning for trying to cast non-u8 scalar to char

src/librustc_lint/types.rs

@@ -11,6 +11,7 @@
 #![allow(non_snake_case)]
 
 use rustc::hir::def_id::DefId;
+use rustc::hir::map as hir_map;
 use rustc::ty::subst::Substs;
 use rustc::ty::{self, AdtKind, Ty, TyCtxt};
 use rustc::ty::layout::{self, LayoutOf};
@@ -176,6 +177,22 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for TypeLimits {
                             _ => bug!(),
                         };
                         if lit_val < min || lit_val > max {
+                            let parent_id = cx.tcx.hir.get_parent_node(e.id);
+                            if let hir_map::NodeExpr(parent_expr) = cx.tcx.hir.get(parent_id) {
+                                if let hir::ExprCast(..) = parent_expr.node {
+                                    if let ty::TyChar = cx.tables.expr_ty(parent_expr).sty {
+                                        let mut err = cx.struct_span_lint(
+                                                             OVERFLOWING_LITERALS,
+                                                             parent_expr.span,
+                                                             "only u8 can be casted into char");
+                                        err.span_suggestion(parent_expr.span,
+                                                            &"use a char literal instead",
+                                                            format!("'\\u{{{:X}}}'", lit_val));
+                                        err.emit();
+                                        return
+                                    }
+                                }
+                            }
                             cx.span_lint(OVERFLOWING_LITERALS,
                                          e.span,
                                          &format!("literal out of range for {:?}", t));

src/test/ui/cast_char.rs

@@ -0,0 +1,20 @@
+// 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.
+
+#![deny(overflowing_literals)]
+
+fn main() {
+    const XYZ: char = 0x1F888 as char;
+    //~^ ERROR only u8 can be casted into char
+    const XY: char = 129160 as char;
+    //~^ ERROR only u8 can be casted into char
+    const ZYX: char = '\u{01F888}';
+    println!("{}", XYZ);
+}

src/test/ui/cast_char.stderr

@@ -0,0 +1,20 @@
+error: only u8 can be casted into char
+  --> $DIR/cast_char.rs:14:23
+   |
+14 |     const XYZ: char = 0x1F888 as char;
+   |                       ^^^^^^^^^^^^^^^ help: use a char literal instead: `'/u{1F888}'`
+   |
+note: lint level defined here
+  --> $DIR/cast_char.rs:11:9
+   |
+11 | #![deny(overflowing_literals)]
+   |         ^^^^^^^^^^^^^^^^^^^^
+
+error: only u8 can be casted into char
+  --> $DIR/cast_char.rs:16:22
+   |
+16 |     const XY: char = 129160 as char;
+   |                      ^^^^^^^^^^^^^^ help: use a char literal instead: `'/u{1F888}'`
+
+error: aborting due to 2 previous errors
+