add a README describing the whole design

src/librustc/mir/README.md

@@ -0,0 +1,142 @@
+# MIR definition and pass system
+
+This file contains the definition of the MIR datatypes along with the
+various types for the "MIR Pass" system, which lets you easily
+register and define new MIR transformations and analyses.
+
+Most of the code that operates on MIR can be found in the
+`librustc_mir` crate or other crates. The code found here in
+`librustc` is just the datatype definitions, alonging the functions
+which operate on MIR to be placed everywhere else.
+
+## MIR Data Types and visitor
+
+The main MIR data type is `rustc::mir::Mir`, defined in `mod.rs`.
+There is also the MIR visitor (in `visit.rs`) which allows you to walk
+the MIR and override what actions will be taken at various points (you
+can visit in either shared or mutable mode; the latter allows changing
+the MIR in place). Finally `traverse.rs` contains various traversal
+routines for visiting the MIR CFG in [different standard orders][traversal]
+(e.g. pre-order, reverse post-order, and so forth).
+
+[traversal]: https://en.wikipedia.org/wiki/Tree_traversal
+
+## MIR pass suites and their integration into the query system
+
+As a MIR *consumer*, you are expected to use one of the queries that
+returns a "final MIR". As of the time of this writing, there is only
+one: `optimized_mir(def_id)`, but more are expected to come in the
+future. For foreign def-ids, we simply read the MIR from the other
+crate's metadata. But for local query, this query will construct the
+MIR and then iteratively optimize it by putting it through various
+pipeline stages. This section describes those pipeline stages and how
+you can extend them.
+
+Here is a diagram showing the various MIR queries involved in producing
+the final `optimized_mir()` for a single def-id `D`. The arrows here
+indicate how data flows from query to query.
+
+```
+mir_build(D)
+  -> mir_pass((0,0,D))              ---+ each suite consists of many passes
+    -> ...                             |
+      -> mir_pass((0,N,D))             |
+        -> mir_suite((0,D))         ---+ ---+ there are several suites
+          -> ...                            |
+            -> mir_suite((M,D))          ---+
+              -> mir_optimized(D)
+```
+
+The MIR transformation pipeline is organized into **suites**.  When
+you ask for `mir_optimized(D)`, it will turn around and request the
+result from the final **suite** of MIR passes
+(`mir_suite((M,D))`). This will in turn (eventually) trigger the MIR
+to be build and then passes through each of the optimization suites.
+Each suite internally triggers one query for each of its passes
+(`mir_pass(...)`).
+
+The reason for the suites is that they represent points in the MIR
+transformation pipeline where other bits of code are interested in
+observing. For example, the `MIR_CONST` suite defines the point where
+analysis for constant rvalues and expressions can take
+place. `MIR_OPTIMIZED` naturally represents the point where we
+actually generate machine code. Nobody should ever request the result
+of an individual *pass*, at least outside of the transformation
+pipeline: this allows us to add passes into the appropriate suite
+without having to modify anything else in the compiler.
+
+### Stealing
+
+Each of these intermediate queries yields up a `&'tcx
+Steal<Mir<'tcx>>`, allocated using `tcx.alloc_steal_mir()`. This
+indicates that the result may be **stolen** by the next pass -- this
+is an optimization to avoid cloning the MIR. Attempting to use a
+stolen result will cause a panic in the compiler. Therefore, it is
+important that you not read directly from these intermediate queries
+except as part of the MIR processing pipeline.
+
+Because of this stealing mechanism, some care must also be taken to
+ensure that, before the MIR at a particular phase in the processing
+pipeline is stolen, anyone who may want to read from it has already
+done so. Sometimes this requires **forcing** queries
+(`ty::queries::foo::force(...)`) during an optimization pass -- this
+will force a query to execute even though you don't directly require
+its result. The query can then read the MIR it needs, and -- once it
+is complete -- you can steal it.
+
+As an example, consider MIR const qualification. It wants to read the
+result produced by the `MIR_CONST` suite. However, that result will be
+**stolen** by the first pass in the next suite (that pass performs
+const promotion):
+
+```
+mir_suite((MIR_CONST,D)) --read-by--> mir_const_qualif(D)
+            |
+        stolen-by
+            |
+            v
+mir_pass((MIR_VALIDATED,0,D))
+```
+
+Therefore, the const promotion pass (the `mir_pass()` in the diagram)
+will **force** `mir_const_qualif` before it actually steals, thus
+ensuring that the reads have already happened (and the final result is
+cached).
+
+### Implementing and registering a pass
+
+To create a new MIR pass, you have to implement one of the MIR pass
+traits. There are several traits, and you want to pick the most
+specific one that applies to your pass. They are described here in
+order of preference. Once you have implemented a trait for your type
+`Foo`, you then have to insert `Foo` into one of the suites; this is
+done in `librustc_driver/driver.rs` by invoking `push_pass()` with the
+appropriate suite.
+
+**The `MirPass` trait.** For the most part, a MIR pass works by taking
+as input the MIR for a single function and mutating it imperatively to
+perform an optimization. To write such a pass, you can implement the
+`MirPass` trait, which has a single callback that takes an `&mut Mir`.
+
+**The `DefIdPass` trait.** When a `MirPass` trait is executed, the
+system will automatically steal the result of the previous pass and
+supply it to you. (See the section on queries and stealing below.)
+Sometimes you don't want to steal the result of the previous pass
+right away. In such cases, you can define a `DefIdPass`, which simply
+gets a callback and lets you decide when to steal the previous result.
+
+**The `Pass` trait.** The most primitive but flexible trait is `Pass`.
+Unlike the other pass types, it returns a `Multi` result, which means
+it scan be used for interprocedural passes which mutate more than one
+MIR at a time (e.g., `inline`).
+
+### The MIR Context
+
+All of the passes when invoked take a `MirCtxt` object. This contains
+various methods to find out (e.g.) the current pass suite and pass
+index, the def-id you are operating on, and so forth. You can also
+access the MIR for the current def-id using `read_previous_mir()`; the
+"previous" refers to the fact that this will be the MIR that was
+output by the previous pass. Finally, you can `steal_previous_mir()`
+to steal the output of the current pass (in which case you get
+ownership of the MIR).

src/librustc/mir/mod.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+//! MIR datatypes and passes. See [the README](README.md) for details.
+
 use graphviz::IntoCow;
 use middle::const_val::ConstVal;
 use rustc_const_math::{ConstUsize, ConstInt, ConstMathErr};

src/librustc/mir/transform.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+//! See [the README](README.md) for details on writing your own pass.
+
 use hir;
 use hir::def_id::DefId;
 use hir::map::DefPathData;

src/librustc/ty/maps.rs

@@ -801,17 +801,23 @@ define_maps! { <'tcx>
     /// Performs the initial MIR construction. You almost certainly do not
     /// want to use this query, because its output is intended to be stolen
     /// immediately by the MIR passes below. Consider `optimized_mir` instead.
+    ///
+    /// See the README for the `mir` module for details.
     [] mir_build: Mir(DefId) -> &'tcx Steal<mir::Mir<'tcx>>,
 
     /// Fetch the MIR for a given def-id after the given set of passes has ben
     /// applied to it. This is mostly an "intermediate" query. Normally, you would
     /// prefer to use `optimized_mir(def_id)`, which will fetch the MIR after all
     /// optimizations and so forth.
+    ///
+    /// See the README for the `mir` module for details.
     [] mir_suite: mir_suite((MirSuite, DefId)) -> &'tcx Steal<mir::Mir<'tcx>>,
 
     /// Fetch the MIR for a given def-id after a given pass has been executed. This is
     /// **only** intended to be used by the `mir_suite` provider -- if you are using it
     /// manually, you're doing it wrong.
+    ///
+    /// See the README for the `mir` module for details.
     [multi] mir_pass: mir_pass((MirSuite, MirPassIndex, DefId)) -> &'tcx Steal<mir::Mir<'tcx>>,
 
     /// MIR after our optimization passes have run. This is MIR that is ready