Merge pull request #540 from rust-lang/keywords

Fill in short definitons for keywords

second-edition/src/appendix-01-keywords.md

@@ -1,58 +1,70 @@
 ## Appendix A: Keywords
 
 The following keywords are reserved by the Rust language and may not be used as
-names of functions, variables, macros, modules, crates, constants, static
-values, attributes, struct fields, or parameters.
+identifiers such as names of functions, variables, parameters, struct fields,
+modules, crates, constants, macros, static values, attributes, types, traits,
+or lifetimes.
+
+### Keywords Currently in Use
+
+* `as` - primitive casting, disambiguating the specific trait containing an
+  item, or renaming items in `use` and `extern crate` statements
+* `break` - exit a loop immediately
+* `const` - constant items and constant raw pointers
+* `continue` - continue to the next loop iteration
+* `crate` - external crate linkage or a macro variable representing the crate
+  in which the macro is defined
+* `else` - fallback for `if` and `if let` control flow constructs
+* `enum` - defining an enumeration
+* `extern` - external crate, function, and variable linkage
+* `false` - boolean false literal
+* `fn` - function definition and function pointer type
+* `for` - iterator loop, part of trait impl syntax, and higher-ranked lifetime
+  syntax
+* `if` - conditional branching
+* `impl` - inherent and trait implementation block
+* `in` - part of `for` loop syntax
+* `let` - variable binding
+* `loop` - unconditional, infinite loop
+* `match` - pattern matching
+* `mod` - module declaration
+* `move` - makes a closure take ownership of all its captures
+* `mut` - denotes mutability in references, raw pointers, and pattern bindings
+* `pub` - denotes public visibility in struct fields, `impl` blocks, and modules
+* `ref` - by-reference binding
+* `return` - return from function
+* `Self` - type alias for the type implementing a trait
+* `self` - method subject or current module
+* `static` - global variable or lifetime lasting the entire program execution
+* `struct` - structure definition
+* `super` - parent module of the current module
+* `trait` - trait definition
+* `true` - boolean true literal
+* `type` - type alias and associated type definition
+* `unsafe` - denotes unsafe code, functions, traits, and implementations
+* `use` - import symbols into scope
+* `where` - type constraint clauses
+* `while` - conditional loop
+
+### Keywords Reserved for Future Use
+
+These keywords do not have any functionality, but are reserved by Rust for
+potential future use.
 
 * `abstract`
 * `alignof`
-* `as`
 * `become`
 * `box`
-* `break`
-* `const`
-* `continue`
-* `crate`
 * `do`
-* `else`
-* `enum`
-* `extern`
-* `false`
 * `final`
-* `fn`
-* `for`
-* `if`
-* `impl`
-* `in`
-* `let`
-* `loop`
 * `macro`
-* `match`
-* `mod`
-* `move`
-* `mut`
 * `offsetof`
 * `override`
 * `priv`
 * `proc`
-* `pub`
 * `pure`
-* `ref`
-* `return`
-* `Self`
-* `self`
 * `sizeof`
-* `static`
-* `struct`
-* `super`
-* `trait`
-* `true`
-* `type`
 * `typeof`
-* `unsafe`
 * `unsized`
-* `use`
 * `virtual`
-* `where`
-* `while`
 * `yield`