Rollup merge of #22980 - alexcrichton:debug-assertions, r=pnkfelix

This commit is an implementation of [RFC 563][rfc] which adds a new
`cfg(debug_assertions)` directive which is specially recognized and calculated
by the compiler. The flag is turned off at any optimization level greater than 1
and may also be explicitly controlled through the `-C debug-assertions`
flag.

[rfc]: https://github.com/rust-lang/rfcs/pull/563

The `debug_assert!` and `debug_assert_eq!` macros now respect this instead of
the `ndebug` variable and `ndebug` no longer holds any meaning to the standard
library.

Code which was previously relying on `not(ndebug)` to gate expensive code should
be updated to rely on `debug_assertions` instead.

Closes #22492
[breaking-change]

src/test/run-make/debug-assertions/Makefile

@@ -0,0 +1,21 @@
+-include ../tools.mk
+
+all:
+	$(RUSTC) debug.rs -C debug-assertions=no
+	$(call RUN,debug) good
+	$(RUSTC) debug.rs -C opt-level=0
+	$(call RUN,debug) bad
+	$(RUSTC) debug.rs -C opt-level=1
+	$(call RUN,debug) good
+	$(RUSTC) debug.rs -C opt-level=2
+	$(call RUN,debug) good
+	$(RUSTC) debug.rs -C opt-level=3
+	$(call RUN,debug) good
+	$(RUSTC) debug.rs -O
+	$(call RUN,debug) good
+	$(RUSTC) debug.rs
+	$(call RUN,debug) bad
+	$(RUSTC) debug.rs -C debug-assertions=yes -O
+	$(call RUN,debug) bad
+	$(RUSTC) debug.rs -C debug-assertions=yes -C opt-level=1
+	$(call RUN,debug) bad

src/test/run-make/debug-assertions/debug.rs

@@ -0,0 +1,42 @@
+// Copyright 2015 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(warnings)]
+
+use std::env;
+use std::thread;
+
+fn main() {
+    let should_fail = env::args().nth(1) == Some("bad".to_string());
+
+    assert_eq!(thread::spawn(debug_assert_eq).join().is_err(), should_fail);
+    assert_eq!(thread::spawn(debug_assert).join().is_err(), should_fail);
+    assert_eq!(thread::spawn(overflow).join().is_err(), should_fail);
+}
+
+fn debug_assert_eq() {
+    let mut hit1 = false;
+    let mut hit2 = false;
+    debug_assert_eq!({ hit1 = true; 1 }, { hit2 = true; 2 });
+    assert!(!hit1);
+    assert!(!hit2);
+}
+
+fn debug_assert() {
+    let mut hit = false;
+    debug_assert!({ hit = true; false });
+    assert!(!hit);
+}
+
+fn overflow() {
+    fn add(a: u8, b: u8) -> u8 { a + b }
+
+    add(200u8, 200u8);
+}