Emit a type error for integer literals where the expected type is char

For example, in let x: char = 42; This was an ICE and is now a
proper type error, as per rust-lang#3477

Author: catamorphism

src/librustc/middle/ty.rs

@@ -144,10 +144,11 @@ export kind_is_durable;
 export meta_kind, kind_lteq, type_kind, type_kind_ext;
 export operators;
 export type_err, terr_vstore_kind;
-export terr_mismatch, terr_onceness_mismatch;
+export terr_integer_as_char, terr_mismatch, terr_onceness_mismatch;
 export type_err_to_str, note_and_explain_type_err;
 export expected_found;
 export type_needs_drop;
+export type_is_char;
 export type_is_empty;
 export type_is_integral;
 export type_is_numeric;
@@ -696,6 +697,7 @@ enum type_err {
     terr_in_field(@type_err, ast::ident),
     terr_sorts(expected_found<t>),
     terr_self_substs,
+    terr_integer_as_char,
     terr_no_integral_type,
     terr_no_floating_point_type,
 }
@@ -2520,6 +2522,13 @@ fn type_is_integral(ty: t) -> bool {
     }
 }
 
+fn type_is_char(ty: t) -> bool {
+    match get(ty).sty {
+        ty_int(ty_char) => true,
+        _ => false
+    }
+}
+
 fn type_is_fp(ty: t) -> bool {
     match get(ty).sty {
       ty_infer(FloatVar(_)) | ty_float(_) => true,
@@ -3489,6 +3498,10 @@ fn type_err_to_str(cx: ctxt, err: &type_err) -> ~str {
             ~"couldn't determine an appropriate integral type for integer \
               literal"
         }
+        terr_integer_as_char => {
+            ~"integer literals can't be inferred to char type \
+              (try an explicit cast)"
+        }
         terr_no_floating_point_type => {
             ~"couldn't determine an appropriate floating point type for \
               floating point literal"

src/librustc/middle/typeck/infer/unify.rs

@@ -362,6 +362,10 @@ impl infer_ctxt {
     }
 
     fn int_var_sub_t(a_id: ty::IntVid, b: ty::t) -> ures {
+        if ty::type_is_char(b) {
+            return Err(ty::terr_integer_as_char);
+        }
+
         assert ty::type_is_integral(b);
 
         let vb = &self.int_var_bindings;

src/test/compile-fail/issue-3477.rs

@@ -0,0 +1,3 @@
+fn main() {
+    let _p: char = 100; //~ ERROR mismatched types: expected `char` but found
+}