replace bound region with placeholder

Author: csmoe

src/appendix/glossary.md

@@ -53,7 +53,7 @@ rib                     |  a data structure in the name resolver that keeps trac
 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.
 sigil                   |  like a keyword but composed entirely of non-alphanumeric tokens. For example, `&` is a sigil for references.
-placeholder             |  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 placeholder and universes](../borrow_check/region_inference.html#placeholder) for more details.
+placeholder            |  **NOTE: skolemization is deprecated by placeholder** 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 placeholder and universes](../borrow_check/region_inference.html#placeholder) for more details.
 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.
 substs                  |  the substitutions for a given generic type or item (e.g. the `i32`, `u32` in `HashMap<i32, u32>`)

src/borrow_check/region_inference.md

@@ -80,8 +80,8 @@ The kinds of region elements are as follows:
   to the remainder of program execution after this function returns.
 - There is an element `!1` for each placeholder region `!1`. This
   corresponds (intuitively) to some unknown set of other elements –
-  for details on placeholder, see the section
-  [placeholder and universes](#placeholder).
+  for details on placeholders, see the section
+  [placeholders and universes](#placeholder).
 
 ## Causal tracking
 
@@ -117,7 +117,7 @@ for its argument, and `bar` wants to be given a function that that
 accepts **any** reference (so it can call it with something on its
 stack, for example). But *how* do we reject it and *why*?
 
-### Subtyping and Placeholder
+### Subtyping and Placeholders
 
 When we type-check `main`, and in particular the call `bar(foo)`, we
 are going to wind up with a subtyping relationship like this one:
@@ -129,8 +129,7 @@ the type of `foo`   the type `bar` expects
 ```
 
 We handle this sort of subtyping by taking the variables that are
-bound in the supertype and **placeholder** them: this means that we
-replace them with
+bound in the supertype and replace them with
 [universally quantified](../appendix/background.html#quantified)
 representatives, written like `!1`. We call these regions "placeholder
 regions" – they represent, basically, "some unknown region".
@@ -198,7 +197,7 @@ fn bar<'a, T>(t: &'a T) {
 ```
 
 Here, the name `'b` is not part of the root universe. Instead, when we
-"enter" into this `for<'b>` (e.g., by placeholder it), we will create
+"enter" into this `for<'b>` (e.g., by replace it with a placeholder), we will create
 a child universe of the root, let's call it U1:
 
 ```text
@@ -411,7 +410,7 @@ for<'a> fn(&'a u32, &'a u32)
 for<'b, 'c> fn(&'b u32, &'c u32)
 ```
 
-Here we would placeholer the supertype, as before, yielding:
+Here we would replace the bound region in the supertype with a placeholder, as before, yielding:
 
 ```text
 for<'a> fn(&'a u32, &'a u32)
@@ -476,7 +475,7 @@ an error. That's good: the problem is that we've gone from a fn that promises
 to return one of its two arguments, to a fn that is promising to return the
 first one. That is unsound. Let's see how it plays out.
 
-First, we placeholder the supertype:
+First, we replace the bound region in the supertype with a placeholder:
 
 ```text
 for<'a> fn(&'a u32, &'a u32) -> &'a u32

src/traits/hrtb.md

@@ -36,20 +36,20 @@ to the subtyping for higher-ranked types (which is described [here][hrsubtype]
 and also in a [paper by SPJ]. If you wish to understand higher-ranked
 subtyping, we recommend you read the paper). There are a few parts:
 
-**TODO**: We should define _placeholder_.
-
-1. _Skolemize_ the obligation.
-2. Match the impl against the placeholder obligation.
+1. replace bound regions in the obligation with placeholders.
+2. Match the impl against the [placeholder] obligation.
 3. Check for _placeholder leaks_.
 
+[placeholder]: ../appendix/glossary.html#appendix-c-glossary
 [hrsubtype]: https://github.com/rust-lang/rust/tree/master/src/librustc/infer/higher_ranked/README.md
 [paper by SPJ]: http://research.microsoft.com/en-us/um/people/simonpj/papers/higher-rank/
 
 So let's work through our example.
 
 1. The first thing we would do is to
-placeholder the obligation, yielding `AnyInt : Foo<&'0 isize>` (here `'0`
-represents placeholder region #0). Note that we now have no quantifiers;
+replace the bound region in the obligation with a placeholder, yielding 
+`AnyInt : Foo<&'0 isize>` (here `'0` represents placeholder region #0). 
+Note that we now have no quantifiers;
 in terms of the compiler type, this changes from a `ty::PolyTraitRef`
 to a `TraitRef`. We would then create the `TraitRef` from the impl,
 using fresh variables for it's bound regions (and thus getting
@@ -78,7 +78,7 @@ impl Foo<&'static isize> for StaticInt;
 
 We want the obligation `StaticInt : for<'a> Foo<&'a isize>` to be
 considered unsatisfied. The check begins just as before. `'a` is
-placeholder to `'0` and the impl trait reference is instantiated to
+replaced with a placeholder `'0` and the impl trait reference is instantiated to
 `Foo<&'static isize>`. When we relate those two, we get a constraint
 like `'static == '0`. This means that the taint set for `'0` is `{'0,
 'static}`, which fails the leak check.