rework the MIR intro section, breaking out passes and visitors

Author: nikomatsakis

src/SUMMARY.md

@@ -24,11 +24,15 @@
 - [Type checking](./type-checking.md)
 - [The MIR (Mid-level IR)](./mir.md)
     - [MIR construction](./mir-construction.md)
+    - [MIR visitor](./mir-visitor.md)
+    - [MIR passes: getting the MIR for a function](./mir-passes.md)
     - [MIR borrowck](./mir-borrowck.md)
+      - [MIR-based region checking (NLL)](./mir-regionck.md)
     - [MIR optimizations](./mir-optimizations.md)
 - [Constant evaluation](./const-eval.md)
     - [miri const evaluator](./miri.md)
 - [Parameter Environments](./param_env.md)
 - [Generating LLVM IR](./trans.md)
+- [Background material](./background.md)
 - [Glossary](./glossary.md)
 - [Code Index](./code-index.md)

src/background.md

@@ -0,0 +1,122 @@
+# MIR Background topics
+
+This section covers a numbers of common compiler terms that arise when
+talking about MIR and optimizations. We try to give the general
+definition while providing some Rust-specific context.
+
+<a name=cfg>
+
+## What is a control-flow graph?
+
+A control-flow graph is a common term from compilers. If you've ever
+used a flow-chart, then the concept of a control-flow graph will be
+pretty familiar to you. It's a representation of your program that
+exposes the underlying control flow in a very clear way.
+
+A control-flow graph is structured as a set of **basic blocks**
+connected by edges. The key idea of a basic block is that it is a set
+of statements that execute "together" -- that is, whenever you branch
+to a basic block, you start at the first statement and then execute
+all the remainder. Only at the end of the is there the possibility of
+branching to more than one place (in MIR, we call that final statement
+the **terminator**):
+
+```
+bb0: {
+    statement0;
+    statement1;
+    statement2;
+    ...
+    terminator;
+}
+```
+
+Many expressions that you are used to in Rust compile down to multiple
+basic blocks. For example, consider an if statement:
+
+```rust
+a = 1;
+if some_variable {
+    b = 1;
+} else {
+    c = 1;
+}
+d = 1;
+```
+
+This would compile into four basic blocks:
+
+```
+BB0: {
+    a = 1;
+    if some_variable { goto BB1 } else { goto BB2 }
+}
+
+BB1: {
+    b = 1;
+    goto BB3;
+}
+
+BB2: {
+    c = 1;
+    goto BB3;
+}
+
+BB3: {
+    d = 1;
+    ...;
+}
+```
+
+When using a control-flow graph, a loop simply appears as a cycle in
+the graph, and the `break` keyword translates into a path out of that
+cycle.
+
+<a name=dataflow>
+
+## What is a dataflow analysis?
+
+*to be written*
+
+<a name=quantified>
+
+## What is "universally quantified"? What about "existentially quantified"?
+
+*to be written*
+
+<a name=variance>
+
+## What is co- and contra-variance?
+
+*to be written*
+
+<a name=free-vs-bound>
+
+## What is a "free region" or a "free variable"? What about "bound region"?
+
+Let's describe the concepts of free vs bound in terms of program
+variables, since that's the thing we're most familiar with.
+
+- Consider this expression: `a + b`. In this expression, `a` and `b`
+  refer to local variables that are defined *outside* of the
+  expression. We say that those variables **appear free** in the
+  expression. To see why this term makes sense, consider the next
+  example.
+- In contrast, consider this expression, which creates a closure: `|a,
+  b| a + b`. Here, the `a` and `b` in `a + b` refer to the arguments
+  that the closure will be given when it is called. We say that the
+  `a` and `b` there are **bound** to the closure, and that the closure
+  signature `|a, b|` is a **binder** for the names `a` and `b`
+  (because any references to `a` or `b` within refer to the variables
+  that it introduces).
+
+So there you have it: a variable "appears free" in some
+expression/statement/whatever if it refers to something defined
+outside of that expressions/statement/whatever. Equivalently, we can
+then refer to the "free variables" of an expression -- which is just
+the set of variables that "appear free".
+
+So what does this have to do with regions? Well, we can apply the
+analogous concept to type and regions. For example, in the type `&'a
+u32`, `'a` appears free.  But in the type `for<'a> fn(&'a u32)`, it
+does not.

src/glossary.md

@@ -18,12 +18,14 @@ HIR Map                 |  The HIR map, accessible via tcx.hir, allows you to qu
 generics                |  the set of generic type parameters defined on a type or item
 ICE                     |  internal compiler error. When the compiler crashes.
 ICH                     |  incremental compilation hash. ICHs are used as fingerprints for things such as HIR and crate metadata, to check if changes have been made. This is useful in incremental compilation to see if part of a crate has changed and should be recompiled.
+inference variable      |  when doing type or region inference, an "inference variable" is a kind of special type/region that represents value you are trying to find. Think of `X` in algebra.
 infcx                   |  the inference context (see `librustc/infer`)
 MIR                     |  the Mid-level IR that is created after type-checking for use by borrowck and trans ([see more](./mir.html))
 miri                    |  an interpreter for MIR used for constant evaluation ([see more](./miri.html))
 obligation              |  something that must be proven by the trait system ([see more](trait-resolution.html))
 local crate             |  the crate currently being compiled.
 MIR                     |  the Mid-level IR that is created after type-checking for use by borrowck and trans ([see more](./mir.html))
+newtype                 |  a "newtype" is a wrapper around some other type (e.g., `struct Foo(T)` is a "newtype" for `T`). This is commonly used in Rust to give a stronger type for indices.
 node-id or NodeId       |  an index identifying a particular node in the AST or HIR; gradually being phased out and replaced with `HirId`.
 obligation              |  something that must be proven by the trait system ([see more](trait-resolution.html))
 provider                |  the function that executes a query ([see more](query.html))

src/mir-background.md

@@ -0,0 +1,122 @@
+# MIR Background topics
+
+This section covers a numbers of common compiler terms that arise when
+talking about MIR and optimizations. We try to give the general
+definition while providing some Rust-specific context.
+
+<a name=cfg>
+
+## What is a control-flow graph?
+
+A control-flow graph is a common term from compilers. If you've ever
+used a flow-chart, then the concept of a control-flow graph will be
+pretty familiar to you. It's a representation of your program that
+exposes the underlying control flow in a very clear way.
+
+A control-flow graph is structured as a set of **basic blocks**
+connected by edges. The key idea of a basic block is that it is a set
+of statements that execute "together" -- that is, whenever you branch
+to a basic block, you start at the first statement and then execute
+all the remainder. Only at the end of the is there the possibility of
+branching to more than one place (in MIR, we call that final statement
+the **terminator**):
+
+```
+bb0: {
+    statement0;
+    statement1;
+    statement2;
+    ...
+    terminator;
+}
+```
+
+Many expressions that you are used to in Rust compile down to multiple
+basic blocks. For example, consider an if statement:
+
+```rust
+a = 1;
+if some_variable {
+    b = 1;
+} else {
+    c = 1;
+}
+d = 1;
+```
+
+This would compile into four basic blocks:
+
+```
+BB0: {
+    a = 1;
+    if some_variable { goto BB1 } else { goto BB2 }
+}
+
+BB1: {
+    b = 1;
+    goto BB3;
+}
+
+BB2: {
+    c = 1;
+    goto BB3;
+}
+
+BB3: {
+    d = 1;
+    ...;
+}
+```
+
+When using a control-flow graph, a loop simply appears as a cycle in
+the graph, and the `break` keyword translates into a path out of that
+cycle.
+
+<a name=dataflow>
+
+## What is a dataflow analysis?
+
+*to be written*
+
+<a name=quantified>
+
+## What is "universally quantified"? What about "existentially quantified"?
+
+*to be written*
+
+<a name=variance>
+
+## What is co- and contra-variance?
+
+*to be written*
+
+<a name=free-vs-bound>
+
+## What is a "free region" or a "free variable"? What about "bound region"?
+
+Let's describe the concepts of free vs bound in terms of program
+variables, since that's the thing we're most familiar with.
+
+- Consider this expression: `a + b`. In this expression, `a` and `b`
+  refer to local variables that are defined *outside* of the
+  expression. We say that those variables **appear free** in the
+  expression. To see why this term makes sense, consider the next
+  example.
+- In contrast, consider this expression, which creates a closure: `|a,
+  b| a + b`. Here, the `a` and `b` in `a + b` refer to the arguments
+  that the closure will be given when it is called. We say that the
+  `a` and `b` there are **bound** to the closure, and that the closure
+  signature `|a, b|` is a **binder** for the names `a` and `b`
+  (because any references to `a` or `b` within refer to the variables
+  that it introduces).
+
+So there you have it: a variable "appears free" in some
+expression/statement/whatever if it refers to something defined
+outside of that expressions/statement/whatever. Equivalently, we can
+then refer to the "free variables" of an expression -- which is just
+the set of variables that "appear free".
+
+So what does this have to do with regions? Well, we can apply the
+analogous concept to type and regions. For example, in the type `&'a
+u32`, `'a` appears free.  But in the type `for<'a> fn(&'a u32)`, it
+does not.

src/mir-borrowck.md

@@ -1 +1,56 @@
-# MIR borrowck
+# MIR borrow check
+
+The borrow check is Rust's "secret sauce" -- it is tasked with
+enforcing a number of properties:
+
+- That all variables are initialized before they are used.
+- That you can't move the same value twice.
+- That you can't move a value while it is borrowed.
+- That you can't access a place while it is mutably borrowed (except through the reference).
+- That you can't mutate a place while it is shared borrowed.
+- etc
+
+At the time of this writing, the code is in a state of transition. The
+"main" borrow checker still works by processing [the HIR](hir.html),
+but that is being phased out in favor of the MIR-based borrow checker.
+Doing borrow checking on MIR has two key advantages:
+
+- The MIR is *far* less complex than the HIR; the radical desugaring
+  helps prevent bugs in the borrow checker. (If you're curious, you
+  can see
+  [a list of bugs that the MIR-based borrow checker fixes here][47366].)
+- Even more importantly, using the MIR enables ["non-lexical lifetimes"][nll],
+  which are regions derived from the control-flow graph. 
+
+[47366]: https://github.com/rust-lang/rust/issues/47366
+[nll]: http://rust-lang.github.io/rfcs/2094-nll.html
+
+### Major phases of the borrow checker
+
+The borrow checker source is found in
+[the `rustc_mir::borrow_check` module][b_c]. The main entry point is
+the `mir_borrowck` query. At the time of this writing, MIR borrowck can operate
+in several modes, but this text will describe only the mode when NLL is enabled
+(what you get with `#![feature(nll)]`).
+
+[b_c]: https://github.com/rust-lang/rust/tree/master/src/librustc_mir/borrow_check
+
+The overall flow of the borrow checker is as follows:
+
+- We first create a **local copy** C of the MIR. We will be modifying
+  this copy in place to modify the types and things to include
+  references to the new regions that we are computing.
+- We then invoke `nll::replace_regions_in_mir` to modify this copy C.
+  Among other things, this function will replace all of the regions in
+  the MIR with fresh [inference variables](glossary.html).
+  - (More details can be found in [the regionck section](./mir-regionck.html).)
+- Next, we perform a number of [dataflow analyses](./background.html#dataflow)
+  that compute what data is moved and when. The results of these analyses
+  are needed to do both borrow checking and region inference.
+- Using the move data, we can then compute the values of all the regions in the MIR.
+  - (More details can be found in [the NLL section](./mir-regionck.html).)
+- Finally, the borrow checker itself runs, taking as input (a) the
+  results of move analysis and (b) the regions computed by the region
+  checker. This allows is to figure out which loans are still in scope
+  at any particular point.
+