core/std: squash dead_code warnings from fail! invocations.

The fail macro defines some function/static items internally, which got
a dead_code warning when `fail!()` is used inside a dead function. This
is ugly and unnecessarily reveals implementation details, so the
warnings can be squashed.

Fixes #16192.

Author: huonw

src/libcore/macros.rs

@@ -31,12 +31,17 @@ macro_rules! fail(
         // because it's just a tiny wrapper. Small wins (156K to 149K in size)
         // were seen when forcing this to be inlined, and that number just goes
         // up with the number of calls to fail!()
+        //
+        // The leading _'s are to avoid dead code warnings if this is
+        // used inside a dead function. Just `#[allow(dead_code)]` is
+        // insufficient, since the user may have
+        // `#[forbid(dead_code)]` and which cannot be overridden.
         #[inline(always)]
-        fn run_fmt(fmt: &::std::fmt::Arguments) -> ! {
-            static FILE_LINE: (&'static str, uint) = (file!(), line!());
-            ::core::failure::begin_unwind(fmt, &FILE_LINE)
+        fn _run_fmt(fmt: &::std::fmt::Arguments) -> ! {
+            static _FILE_LINE: (&'static str, uint) = (file!(), line!());
+            ::core::failure::begin_unwind(fmt, &_FILE_LINE)
         }
-        format_args!(run_fmt, $fmt, $($arg)*)
+        format_args!(_run_fmt, $fmt, $($arg)*)
     });
 )
 

src/librustc/middle/dead.rs

@@ -26,7 +26,6 @@ use syntax::ast_util::{local_def, is_local, PostExpansionMethod};
 use syntax::attr::AttrMetaMethods;
 use syntax::attr;
 use syntax::codemap;
-use syntax::parse::token;
 use syntax::visit::Visitor;
 use syntax::visit;
 

src/libstd/macros.rs

@@ -43,8 +43,8 @@ macro_rules! fail(
     });
     ($msg:expr) => ({
         // static requires less code at runtime, more constant data
-        static FILE_LINE: (&'static str, uint) = (file!(), line!());
-        let (file, line) = FILE_LINE;
+        static _FILE_LINE: (&'static str, uint) = (file!(), line!());
+        let (file, line) = _FILE_LINE;
         ::std::rt::begin_unwind($msg, file, line)
     });
     ($fmt:expr, $($arg:tt)*) => ({
@@ -59,12 +59,17 @@ macro_rules! fail(
         // because it's just a tiny wrapper. Small wins (156K to 149K in size)
         // were seen when forcing this to be inlined, and that number just goes
         // up with the number of calls to fail!()
+        //
+        // The leading _'s are to avoid dead code warnings if this is
+        // used inside a dead function. Just `#[allow(dead_code)]` is
+        // insufficient, since the user may have
+        // `#[forbid(dead_code)]` and which cannot be overridden.
         #[inline(always)]
-        fn run_fmt(fmt: &::std::fmt::Arguments) -> ! {
-            static FILE_LINE: (&'static str, uint) = (file!(), line!());
-            ::std::rt::begin_unwind_fmt(fmt, &FILE_LINE)
+        fn _run_fmt(fmt: &::std::fmt::Arguments) -> ! {
+            static _FILE_LINE: (&'static str, uint) = (file!(), line!());
+            ::std::rt::begin_unwind_fmt(fmt, &_FILE_LINE)
         }
-        format_args!(run_fmt, $fmt, $($arg)*)
+        format_args!(_run_fmt, $fmt, $($arg)*)
     });
 )
 

src/test/compile-fail/fail-no-dead-code-core.rs

@@ -0,0 +1,29 @@
+// Copyright 2014 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.
+
+#![feature(phase)]
+#![deny(dead_code)]
+#![allow(unreachable_code)]
+
+#[phase(link, syntax)] extern crate core;
+
+
+fn foo() { //~ ERROR code is never used
+
+    // none of these should have any dead_code exposed to the user
+    fail!();
+
+    fail!("foo");
+
+    fail!("bar {}", "baz")
+}
+
+
+fn main() {}

src/test/compile-fail/fail-no-dead-code.rs

@@ -0,0 +1,25 @@
+// Copyright 2014 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(dead_code)]
+#![allow(unreachable_code)]
+
+fn foo() { //~ ERROR code is never used
+
+    // none of these should have any dead_code exposed to the user
+    fail!();
+
+    fail!("foo");
+
+    fail!("bar {}", "baz")
+}
+
+
+fn main() {}