Switch to begin_panic again

In https://github.com/rust-lang/rust/pull/42938 we made the compiler
emit a call to begin_panic_new in order to pass column info to it. Now
with stage0 updated (https://github.com/rust-lang/rust/pull/43320),
we can safely change begin_panic and start emitting calls for it again.

src/libstd/macros.rs

@@ -46,7 +46,7 @@ macro_rules! panic {
         panic!("explicit panic")
     });
     ($msg:expr) => ({
-        $crate::rt::begin_panic_new($msg, {
+        $crate::rt::begin_panic($msg, {
             // static requires less code at runtime, more constant data
             static _FILE_LINE_COL: (&'static str, u32, u32) = (file!(), line!(), column!());
             &_FILE_LINE_COL

src/libstd/panicking.rs

@@ -518,7 +518,7 @@ pub fn begin_panic_fmt(msg: &fmt::Arguments,
 
     let mut s = String::new();
     let _ = s.write_fmt(*msg);
-    begin_panic_new(s, file_line_col)
+    begin_panic(s, file_line_col)
 }
 
 // FIXME: In PR #42938, we have added the column as info passed to the panic
@@ -529,15 +529,17 @@ pub fn begin_panic_fmt(msg: &fmt::Arguments,
 // By changing the compiler source, we can only affect behaviour of higher
 // stages. We need to perform the switch over two stage0 replacements, using
 // a temporary function begin_panic_new while performing the switch:
-// 0. Right now, we tell stage1 onward to emit a call to begin_panic_new.
-// 1. In the first SNAP, stage0 calls begin_panic_new with the new ABI,
-//    begin_panic stops being used. Now we can change begin_panic to
-//    the new ABI, and start emitting calls to begin_panic in higher
+// 0. Before the current switch, we told stage1 onward to emit a call
+//    to begin_panic_new.
+// 1. Right now, stage0 calls begin_panic_new with the new ABI,
+//    begin_panic stops being used. We have changed begin_panic to
+//    the new ABI, and started to emit calls to begin_panic in higher
 //    stages again, this time with the new ABI.
 // 2. After the second SNAP, stage0 calls begin_panic with the new ABI,
 //    and we can remove the temporary begin_panic_new function.
 
 /// This is the entry point of panicking for panic!() and assert!().
+#[cfg(stage0)]
 #[unstable(feature = "libstd_sys_internals",
            reason = "used by the panic! macro",
            issue = "0")]
@@ -558,7 +560,7 @@ pub fn begin_panic_new<M: Any + Send>(msg: M, file_line_col: &(&'static str, u32
            reason = "used by the panic! macro",
            issue = "0")]
 #[inline(never)] #[cold] // avoid code bloat at the call sites as much as possible
-pub fn begin_panic<M: Any + Send>(msg: M, file_line: &(&'static str, u32)) -> ! {
+pub fn begin_panic<M: Any + Send>(msg: M, file_line_col: &(&'static str, u32, u32)) -> ! {
     // Note that this should be the only allocation performed in this code path.
     // Currently this means that panic!() on OOM will invoke this code path,
     // but then again we're not really ready for panic on OOM anyway. If
@@ -566,10 +568,7 @@ pub fn begin_panic<M: Any + Send>(msg: M, file_line: &(&'static str, u32)) -> !
     // be performed in the parent of this thread instead of the thread that's
     // panicking.
 
-    let (file, line) = *file_line;
-    let file_line_col = (file, line, 0);
-
-    rust_panic_with_hook(Box::new(msg), &file_line_col)
+    rust_panic_with_hook(Box::new(msg), file_line_col)
 }
 
 /// Executes the primary logic for a panic, including checking for recursive

src/libstd/rt.rs

@@ -25,7 +25,9 @@
 
 
 // Reexport some of our utilities which are expected by other crates.
-pub use panicking::{begin_panic_new, begin_panic, begin_panic_fmt, update_panic_count};
+#[cfg(stage0)]
+pub use panicking::begin_panic_new;
+pub use panicking::{begin_panic, begin_panic_fmt, update_panic_count};
 
 #[cfg(not(test))]
 #[lang = "start"]

src/libsyntax/ext/build.rs

@@ -774,7 +774,7 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
         let expr_loc_ptr = self.expr_addr_of(span, expr_loc_tuple);
         self.expr_call_global(
             span,
-            self.std_path(&["rt", "begin_panic_new"]),
+            self.std_path(&["rt", "begin_panic"]),
             vec![
                 self.expr_str(span, msg),
                 expr_loc_ptr])