Add definition of niche to the glossary

Author: LeSeulArtichaut

src/appendix/glossary.md

@@ -47,6 +47,7 @@ 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, like the `NonZero*` integers or the reference `&T` are invalid for certain bit patterns, in these cases 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 ellision 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.
 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))