rustc: Add a debug_assertions #[cfg] directive

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]

1 files changed, 14 insertions, 10 deletions

diff --git a/src/libcore/macros.rs b/src/libcore/macros.rs
index 94ca9ec37b4..96c9beece1c 100644
--- a/src/libcore/macros.rs
+++ b/src/libcore/macros.rs
@@ -100,10 +100,12 @@ macro_rules! assert_eq {
 /// This will invoke the `panic!` macro if the provided expression cannot be
 /// evaluated to `true` at runtime.
 ///
-/// Unlike `assert!`, `debug_assert!` statements can be disabled by passing
-/// `--cfg ndebug` to the compiler. This makes `debug_assert!` useful for
-/// checks that are too expensive to be present in a release build but may be
-/// helpful during development.
+/// Unlike `assert!`, `debug_assert!` statements are only enabled in non
+/// optimized builds by default. An optimized build will omit all
+/// `debug_assert!` statements unless `-C debug-assertions` is passed to the
+/// compiler. This makes `debug_assert!` useful for checks that are too
+/// expensive to be present in a release build but may be helpful during
+/// development.
 ///
 /// # Example
 ///
@@ -125,7 +127,7 @@ macro_rules! assert_eq {
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]
 macro_rules! debug_assert {
-    ($($arg:tt)*) => (if cfg!(not(ndebug)) { assert!($($arg)*); })
+    ($($arg:tt)*) => (if cfg!(debug_assertions) { assert!($($arg)*); })
 }
 
 /// Asserts that two expressions are equal to each other, testing equality in
@@ -133,10 +135,12 @@ macro_rules! debug_assert {
 ///
 /// On panic, this macro will print the values of the expressions.
 ///
-/// Unlike `assert_eq!`, `debug_assert_eq!` statements can be disabled by
-/// passing `--cfg ndebug` to the compiler. This makes `debug_assert_eq!`
-/// useful for checks that are too expensive to be present in a release build
-/// but may be helpful during development.
+/// Unlike `assert_eq!`, `debug_assert_eq!` statements are only enabled in non
+/// optimized builds by default. An optimized build will omit all
+/// `debug_assert_eq!` statements unless `-C debug-assertions` is passed to the
+/// compiler. This makes `debug_assert_eq!` useful for checks that are too
+/// expensive to be present in a release build but may be helpful during
+/// development.
 ///
 /// # Example
 ///
@@ -147,7 +151,7 @@ macro_rules! debug_assert {
 /// ```
 #[macro_export]
 macro_rules! debug_assert_eq {
-    ($($arg:tt)*) => (if cfg!(not(ndebug)) { assert_eq!($($arg)*); })
+    ($($arg:tt)*) => (if cfg!(debug_assertions) { assert_eq!($($arg)*); })
 }
 
 /// Short circuiting evaluation on Err