add mir-opt section for optimization fuel

Author: cjkenn

src/mir/optimizations.md

@@ -60,3 +60,24 @@ want to query whether your optimization pass should run, you can check the
 current level using `tcx.sess.opts.debugging_opts.mir_opt_level`.
 
 [compiler MCP]: https://github.com/rust-lang/compiler-team/issues/319
+
+## Optimization fuel
+
+Optimization fuel is a compiler option (`-Z fuel=<crate>=<value>`) that allows for fine grained
+control over which optimizations can be applied during compilation: each optimization reduces
+fuel by 1, and when fuel reaches 0 no more optimizations are applied. This can help with debugging
+and identifying problems with optimizations.
+
+MIR optimizations respect fuel, and in general each pass should check fuel by calling
+[`tcx.consider_optimizing`][consideroptimizing] and skipping the optimization if fuel
+is empty. There are a few considerations:
+
+1. If the pass is considered "guaranteed" (for example, it should always be run because it is
+needed for correctness), then fuel should not be used. An example of this is `PromoteTemps`.
+2. In some cases, an initial pass is performed to gather candidates, which are then iterated to
+perform optimizations. In these situations, we should allow for the initial gathering pass
+and then check fuel as close to the mutation as possible. This allows for the best
+debugging experience, because we can determine where in the list of candidates an optimization
+may have been misapplied. Examples of this are `InstCombine` and `ConstantPropagation`.
+
+[consideroptimizing]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TyCtxt.html#method.consider_optimizing