syntax: Copy unstable str::slice_shift_char into libsyntax

Author: erickt

src/libsyntax/ext/asm.rs

@@ -22,6 +22,7 @@ use feature_gate;
 use parse::token::InternedString;
 use parse::token;
 use ptr::P;
+use str::slice_shift_char;
 
 enum State {
     Asm,
@@ -109,7 +110,7 @@ pub fn expand_asm<'cx>(cx: &'cx mut ExtCtxt, sp: Span, tts: &[ast::TokenTree])
                     // It's the opposite of '=&' which means that the memory
                     // cannot be shared with any other operand (usually when
                     // a register is clobbered early.)
-                    let output = match constraint.slice_shift_char() {
+                    let output = match slice_shift_char(&constraint) {
                         Some(('=', _)) => None,
                         Some(('+', operand)) => {
                             Some(token::intern_and_get_ident(&format!(

src/libsyntax/lib.rs

@@ -31,7 +31,6 @@
 #![feature(rustc_private)]
 #![feature(staged_api)]
 #![feature(unicode)]
-#![feature(str_char)]
 
 extern crate arena;
 extern crate fmt_macros;

src/libsyntax/print/pprust.rs

@@ -27,6 +27,7 @@ use print::pp::{Breaks, eof};
 use print::pp::Breaks::{Consistent, Inconsistent};
 use ptr::P;
 use std_inject;
+use str::slice_shift_char;
 
 use std::ascii;
 use std::io::{self, Write, Read};
@@ -1873,7 +1874,7 @@ impl<'a> State<'a> {
 
                 try!(self.commasep(Inconsistent, &a.outputs,
                                    |s, &(ref co, ref o, is_rw)| {
-                    match co.slice_shift_char() {
+                    match slice_shift_char(co) {
                         Some(('=', operand)) if is_rw => {
                             try!(s.print_string(&format!("+{}", operand),
                                                 ast::CookedStr))

src/libsyntax/str.rs

@@ -8,6 +8,18 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// FIXME: This was copied from core/str/mod.rs because it is currently unstable.
 pub fn char_at(s: &str, byte: usize) -> char {
     s[byte..].chars().next().unwrap()
 }
+
+// FIXME: This was copied from core/str/mod.rs because it is currently unstable.
+#[inline]
+pub fn slice_shift_char(s: &str) -> Option<(char, &str)> {
+    if s.is_empty() {
+        None
+    } else {
+        let ch = char_at(s, 0);
+        Some((ch, &s[ch.len_utf8()..]))
+    }
+}