Write RFC for adding an expect intrinsic

Author: Aatch

text/0000-expect-intrinsic.md

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+- Feature Name: expect_intrinsic
+- Start Date: 2015-05-20
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+
+Provide an intrinsic function for hinting the likelyhood of branches being taken.
+
+# Motivation
+
+Branch prediction can have significant effects on the running time of some code. Especially tight
+inner loops which may be run millions of times. While in general programmers aren't able to
+effectively provide hints to the compiler, there are cases where the likelyhood of some branch
+being taken can be known.
+
+For example: in arbitrary-precision arithmetic, operations are often performed in a base that is
+equal to `2^word_size`. The most basic division algorithm, "Schoolbook Division", has a step that
+will be taken in `2/B` cases (where `B` is the base the numbers are in), given random input. On a
+32-bit processor that is approximately one in two billion cases, for 64-bit it's one in 18
+quintillion cases.
+
+# Detailed design
+
+Implement an `expect` intrinsic with the signature: `fn(bool, bool) -> bool`. The first argument is
+the condition being tested, the second argument is the expected result. The return value is the
+same as the first argument, meaning that `if foo == bar { .. }` can be simply replaced with
+`if expect(foo == bar, false) { .. }`.
+
+The expected value is required to be a constant value.
+
+# Drawbacks
+
+The second argument is required to be a constant value, which can't be easily expressed.
+
+# Alternatives
+
+Provide a pair of intrinsics `likely` and `unlikely`, these are the same as `expect` just with
+`true` and `false` substituted in for the expected value, respectively.
+
+# Unresolved questions
+
+None.