apply mark-i-m's suggestions

Author: nikomatsakis

Diff for: src/SUMMARY.md

@@ -24,7 +24,7 @@
 - [Type checking](./type-checking.md)
 - [The MIR (Mid-level IR)](./mir.md)
     - [MIR construction](./mir-construction.md)
-    - [MIR visitor](./mir-visitor.md)
+    - [MIR visitor and traversal](./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)

Diff for: src/background.md

@@ -17,9 +17,9 @@ 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**):
+all the remainder. Only at the end of the block is there the
+possibility of branching to more than one place (in MIR, we call that
+final statement the **terminator**):
 
 ```
 bb0: {
@@ -88,7 +88,8 @@ cycle.
 
 ## What is co- and contra-variance?
 
-*to be written*
+Check out the subtyping chapter from the
+[Rust Nomicon](https://doc.rust-lang.org/nomicon/subtyping.html).
 
 <a name=free-vs-bound>
 
@@ -97,18 +98,17 @@ cycle.
 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,
+- 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).
+- 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 (i.e., they are **free**, not **bound** (tied up)).
 
 So there you have it: a variable "appears free" in some
 expression/statement/whatever if it refers to something defined

Diff for: src/glossary.md

@@ -6,22 +6,30 @@ The compiler uses a number of...idiosyncratic abbreviations and things. This glo
 Term                    | Meaning
 ------------------------|--------
 AST                     |  the abstract syntax tree produced by the syntax crate; reflects user syntax very closely.
+binder                  |  a "binder" is a place where a variable or type is declared; for example, the `<T>` is a binder for the generic type parameter `T` in `fn foo<T>(..)`, and `|a| ...` is a binder for the parameter `a`. See [the background chapter for more](./background.html#free-vs-bound)
+bound variable          |  a "bound variable" is one that is declared within an expression/term. For example, the variable `a` is bound within the closure expession `|a| a * 2`. See [the background chapter for more](./background.html#free-vs-bound)
 codegen unit            |  when we produce LLVM IR, we group the Rust code into a number of codegen units. Each of these units is processed by LLVM independently from one another, enabling parallelism. They are also the unit of incremental re-use.
+control-flow graph      |  a representation of the control-flow of a program; see [the background chapter for more](./background.html#cfg)  
 completeness            |  completeness is a technical term in type theory. Completeness means that every type-safe program also type-checks. Having both soundness and completeness is very hard, and usually soundness is more important. (see "soundness").
 cx                      |  we tend to use "cx" as an abbrevation for context. See also `tcx`, `infcx`, etc.
 DAG                     |  a directed acyclic graph is used during compilation to keep track of dependencies between queries. ([see more](incremental-compilation.html))
+data-flow analysis      |  a static analysis that figures out what properties are true at each point in the control-flow of a program; see [the background chapter for more](./background.html#dataflow)  
 DefId                   |  an index identifying a definition (see `librustc/hir/def_id.rs`). Uniquely identifies a `DefPath`.
 HIR                     |  the High-level IR, created by lowering and desugaring the AST ([see more](hir.html))
 HirId                   |  identifies a particular node in the HIR by combining a def-id with an "intra-definition offset".
 HIR Map                 |  The HIR map, accessible via tcx.hir, allows you to quickly navigate the HIR and convert between various forms of identifiers.
+free variable           |  a "free variable" is one that is not bound within an expression or term; see [the background chapter for more](./background.html#free-vs-bound)
 'gcx                    |  the lifetime of the global arena ([see more](ty.html))
 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.
+inference variable      |  when doing type or region inference, an "inference variable" is a kind of special type/region that represents what you are trying to infer. Think of X in algebra. For example, if we are trying to infer the type of a variable in a program, we create an inference variable to represent that unknown type.
 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))
+NLL                     | [non-lexical lifetimes](./mir-regionck.html), an extension to Rust's borrowing system to make it be based on the control-flow graph.
+promoted constants      |  constants extracted from a function and lifted to static scope; see [this section](./mir.html#promoted) for more details.
+quantified              |  in math or logic, existential and universal quantification are used to ask questions like "is there any type T for which is true?" or "is this true for all types T?"; see [the background chapter for more](./background.html#quantified)  
 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))
@@ -33,6 +41,7 @@ query                   |  perhaps some sub-computation during compilation ([see
 region                  |  another term for "lifetime" often used in the literature and in the borrow checker.
 sess                    |  the compiler session, which stores global data used throughout compilation
 side tables             |  because the AST and HIR are immutable once created, we often carry extra information about them in the form of hashtables, indexed by the id of a particular node.
+skolemization           |  a way of handling subtyping around "for-all" types (e.g., `for<'a> fn(&'a u32)` as well as solving higher-ranked trait bounds (e.g., `for<'a> T: Trait<'a>`). See [the chapter on skolemization and universes](./mir-regionck.html#skol) for more details.
 sigil                   |  like a keyword but composed entirely of non-alphanumeric tokens. For example, `&` is a sigil for references.
 soundness               |  soundness is a technical term in type theory. Roughly, if a type system is sound, then if a program type-checks, it is type-safe; i.e. I can never (in safe rust) force a value into a variable of the wrong type. (see "completeness").
 span                    |  a location in the user's source code, used for error reporting primarily. These are like a file-name/line-number/column tuple on steroids: they carry a start/end point, and also track macro expansions and compiler desugaring. All while being packed into a few bytes (really, it's an index into a table). See the Span datatype for more.
@@ -43,3 +52,4 @@ token                   |  the smallest unit of parsing. Tokens are produced aft
 trans                   |  the code to translate MIR into LLVM IR.
 trait reference         |  a trait and values for its type parameters ([see more](ty.html)).
 ty                      |  the internal representation of a type ([see more](ty.html)).
+variance                |  variance determines how changes to a generic type/lifetime parameter affect subtyping; for example, if `T` is a subtype of `U`, then `Vec<T>` is a subtype `Vec<U>` because `Vec` is *covariant* in its generic parameter. See [the background chapter for more](./background.html#variance).

Diff for: src/mir-background.md

@@ -37,9 +37,9 @@ in several modes, but this text will describe only the mode when NLL is enabled
 
 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 first create a **local copy** C of the MIR. In the coming steps,
+  we will modify 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).
@@ -51,6 +51,6 @@ The overall flow of the borrow checker is as follows:
   - (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
+  checker. This allows us to figure out which loans are still in scope
   at any particular point.
 

Diff for: src/mir-borrowck.md

@@ -4,9 +4,9 @@ If you would like to get the MIR for a function (or constant, etc),
 you can use the `optimized_mir(def_id)` query. This will give you back
 the final, optimized MIR. For foreign def-ids, we simply read the MIR
 from the other crate's metadata. But for local def-ids, the 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.
+construct the MIR and then iteratively optimize it by applying a
+series of passes. This section describes how those passes work and how
+you can extend them.
 
 To produce the `optimized_mir(D)` for a given def-id `D`, the MIR
 passes through several suites of optimizations, each represented by a
@@ -97,18 +97,19 @@ that appeared within the `main` function.)
 ### Implementing and registering a pass
 
 A `MirPass` is some bit of code that processes the MIR, typically --
-but not always -- transforming it along the way in some way. For
-example, it might perform an optimization. The `MirPass` trait itself
-is found in in [the `rustc_mir::transform` module][mirtransform], and
-it basically consists of one method, `run_pass`, that simply gets an
+but not always -- transforming it along the way somehow. For example,
+it might perform an optimization. The `MirPass` trait itself is found
+in in [the `rustc_mir::transform` module][mirtransform], and it
+basically consists of one method, `run_pass`, that simply gets an
 `&mut Mir` (along with the tcx and some information about where it
-came from).
+came from). The MIR is therefore modified in place (which helps to
+keep things efficient).
 
-A good example of a basic MIR pass is [`NoLandingPads`], which walks the
-MIR and removes all edges that are due to unwinding -- this is used
-with when configured with `panic=abort`, which never unwinds. As you can see
-from its source, a MIR pass is defined by first defining a dummy type, a struct
-with no fields, something like:
+A good example of a basic MIR pass is [`NoLandingPads`], which walks
+the MIR and removes all edges that are due to unwinding -- this is
+used when configured with `panic=abort`, which never unwinds. As you
+can see from its source, a MIR pass is defined by first defining a
+dummy type, a struct with no fields, something like:
 
 ```rust
 struct MyPass;
@@ -120,8 +121,9 @@ this pass into the appropriate list of passes found in a query like
 should go into the `optimized_mir` list.)
 
 If you are writing a pass, there's a good chance that you are going to
-want to use a [MIR visitor] too -- those are a handy visitor that
-walks the MIR for you and lets you make small edits here and there.
+want to use a [MIR visitor]. MIR visitors are a handy way to walk all
+the parts of the MIR, either to search for something or to make small
+edits.
 
 ### Stealing
 
@@ -149,7 +151,9 @@ be **stolen** by the `mir_validated()` suite. If nothing was done,
 then `mir_const_qualif(D)` would succeed if it came before
 `mir_validated(D)`, but fail otherwise. Therefore, `mir_validated(D)`
 will **force** `mir_const_qualif` before it actually steals, thus
-ensuring that the reads have already happened:
+ensuring that the reads have already happened (remember that
+[queries are memoized](./query.html), so executing a query twice
+simply loads from a cache the second time):
 
 ```
 mir_const(D) --read-by--> mir_const_qualif(D)