tag/niche terminology (#747)

Author: RalfJung

src/appendix/glossary.md

@@ -17,6 +17,7 @@ DAG <div id="dag"/>                      |  A directed acyclic graph is used dur
 data-flow analysis <div id="data-flow"/> |  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).
 DeBruijn Index <div id="debruijn">       |  A technique for describing which binder a variable is bound by using only integers. It has the benefit that it is invariant under variable renaming. ([see more](./background.md#debruijn))
 DefId <div id="def-id"/>                 |  An index identifying a definition (see `librustc_middle/hir/def_id.rs`). Uniquely identifies a `DefPath`. See [the HIR chapter for more](../hir.html#identifiers-in-the-hir).
+Discriminant <div id="discriminant"/>    |  The underlying value associated with an enum variant or generator state to indicate it as "active" (but not to be confused with its ["variant index"](#variant-idx)). At runtime, the discriminant of the active variant is encoded in the [tag](#tag).
 Double pointer <div id="double-ptr"/>    |  A pointer with additional metadata. See "fat pointer" for more.
 drop glue <div id="drop-glue"/>          |  (internal) compiler-generated instructions that handle calling the destructors (`Drop`) for data types.
 DST <div id="dst"/>                      |  Short for Dynamically-Sized Type, this is a type for which the compiler cannot statically know the size in memory (e.g. `str` or `[u8]`). Such types don't implement `Sized` and cannot be allocated on the stack. They can only occur as the last field in a struct. They can only be used behind a pointer (e.g. `&str` or `&[u8]`).
@@ -48,7 +49,7 @@ miri <div id="miri"/>                    |  An interpreter for MIR used for cons
 monomorphization <div id="mono"/>        |  The process of taking generic implementations of types and functions and instantiating them with concrete types. For example, in the code we might have `Vec<T>`, but in the final executable, we will have a copy of the `Vec` code for every concrete type used in the program (e.g. a copy for `Vec<usize>`, a copy for `Vec<MyStruct>`, etc).
 normalize <div id="normalize"/>          |  A general term for converting to a more canonical form, but in the case of rustc typically refers to [associated type normalization](../traits/goals-and-clauses.html#normalizeprojection---type).
 newtype <div id="newtype"/>              |  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.
-Niche                                    |  Invalid bit patterns for a type *that can be used* for layout optimizations. Some types cannot have certain bit patterns. For example, the `NonZero*` integers or the reference `&T` cannot be represented by a 0 bitstring. This means the compiler can perform layout optimizations by taking advantage of the invalid "niche value". An example application for this is the [*Discriminant elision on `Option`-like enums*](https://rust-lang.github.io/unsafe-code-guidelines/layout/enums.html#discriminant-elision-on-option-like-enums), which allows using a type's niche to determine the `enum` variant without storing a discriminant.
+Niche <div id="niche"/>                  |  Invalid bit patterns for a type *that can be used* for layout optimizations. Some types cannot have certain bit patterns. For example, the `NonZero*` integers or the reference `&T` cannot be represented by a 0 bitstring. This means the compiler can perform layout optimizations by taking advantage of the invalid "niche value". An example application for this is the [*Discriminant elision on `Option`-like enums*](https://rust-lang.github.io/unsafe-code-guidelines/layout/enums.html#discriminant-elision-on-option-like-enums), which allows using a type's niche as the ["tag"](#tag) for an `enum` without requiring a separate field.
 NLL <div id="nll"/>                      |  Short for [non-lexical lifetimes](../borrow_check/region_inference.html), this is an extension to Rust's borrowing system to make it be based on the control-flow graph.
 node-id or NodeId <div id="node-id"/>    |  An index identifying a particular node in the AST or HIR; gradually being phased out and replaced with `HirId`. See [the HIR chapter for more](../hir.html#identifiers-in-the-hir).
 obligation <div id="obligation"/>        |  Something that must be proven by the trait system. ([see more](../traits/resolution.html))
@@ -68,6 +69,7 @@ sigil <div id="sigil"/>                  |  Like a keyword but composed entirely
 soundness <div id="soundness"/>          |  A technical term in type theory. Roughly, if a type system is sound, then a program that type-checks is type-safe. That is, one can never (in safe rust) force a value into a variable of the wrong type. (see "completeness").
 span <div id="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 <div id="substs"/>                |  The substitutions for a given generic type or item (e.g. the `i32`, `u32` in `HashMap<i32, u32>`).
+Tag <div id="tag"/>                      |  The "tag" of an enum/generator encodes the [discriminant](#discriminant) of the active variant/state.  Tags can either be "direct" (simply storing the discriminant in a field) or use a ["niche"](#niche).
 tcx <div id="tcx"/>                      |  The "typing context", main data structure of the compiler. ([see more](../ty.html))
 'tcx <div id="lifetime-tcx"/>            |  The lifetime of the allocation arena. ([see more](../ty.html))
 token <div id="token"/>                  |  The smallest unit of parsing. Tokens are produced after lexing ([see more](../the-parser.html)).
@@ -79,6 +81,7 @@ UFCS <div id="ufcs"/>                    |  Short for Universal Function Call Sy
 uninhabited type <div id="ut"/>          |  A type which has _no_ values. This is not the same as a ZST, which has exactly 1 value. An example of an uninhabited type is `enum Foo {}`, which has no variants, and so, can never be created. The compiler can treat code that deals with uninhabited types as dead code, since there is no such value to be manipulated. `!` (the never type) is an uninhabited type. Uninhabited types are also called "empty types".
 upvar <div id="upvar"/>                  |  A variable captured by a closure from outside the closure.
 variance <div id="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](./background.html#variance) for a more general explanation. See the [variance chapter](../variance.html) for an explanation of how type checking handles variance.
+Variant index <div id="variant-idx"/>    |  In an enum, identifies a variant by assigning them indices starting at 0.  This is purely internal and not to be confused with the ["discrimiant"](#discriminant) which can be overwritten by the user (e.g. `enum Bool { True = 42, False = 0 }`).
 Wide pointer <div id="wide-ptr"/>        |  A pointer with additional metadata. See "fat pointer" for more.
 ZST <div id="zst"/>                      |  Zero-Sized Type. A type whose values have size 0 bytes. Since `2^0 = 1`, such types can have exactly one value. For example, `()` (unit) is a ZST. `struct Foo;` is also a ZST. The compiler can do some nice optimizations around ZSTs.