Add RFC for disablable assertions

Author: pcwalton

active/0000-assert.md

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+- Start Date: 2014-04-18
+- RFC PR #: (leave this empty)
+- Rust Issue #: (leave this empty)
+
+# Summary
+
+Asserts are too expensive for release builds and mess up inlining. There must be a way to turn them off. I propose macros `assert!` and `enforce!`. For test cases, `enforce!` should be used.
+
+# Motivation
+
+Asserts are too expensive in release builds.
+
+# Detailed design
+
+There should be two macros, `assert!(EXPR)` and `enforce!(EXPR)`. In debug builds (without `--cfg ndebug`), `assert!()` is the same as `enforce!()`. In release builds (with `--cfg ndebug`), `assert!()` compiles away to nothing. The definition of `enforce!()` is `if (!EXPR) { fail!("assertion failed ({}, {}): {}", file!(), line!(), stringify!(expr) }`
+
+# Alternatives
+
+Other designs that have been considered are using `assert!` in test cases and not providing `enforce!`, but this doesn't work with separate compilation.
+
+There has been an issue raised that `enforce!` is unintuitive for test cases, but I think all workarounds for this are worse because they add complexity.
+
+The impact of not doing this is that `assert!` will become expensive, prompting people will write their own local `assert!` macros, duplicating functionality that should have been in the standard library.
+
+# Unresolved questions
+
+None.