<span id="dag">DAG</span>                      |  A _directed acyclic graph_ is used during compilation to keep track of dependencies between queries. ([see more](../queries/incremental-compilation.md))
<span id="data-flow">data-flow analysis</span> |  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.md#dataflow).
<span id="debruijn">de Bruijn index</span>     |  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#what-is-a-debruijn-index))
<span id="def-id">`DefId`</span>               |  An index identifying a definition (see `rustc_middle/src/hir/def_id.rs`). Uniquely identifies a `DefPath`. See [the HIR chapter for more](../hir.md#identifiers-in-the-hir).
<span id="discriminant">discriminant</span>    |  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).
<span id="double-ptr">double pointer</span>    |  A pointer with additional metadata. See [fat pointer](#fat-ptr) for more.
<span id="drop-glue">drop glue</span>          |  (Internal) compiler-generated instructions that handle calling the destructors (`Drop`) for data types.
<span id="dst">DST</span>                      |  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]`).
<span id="ebl">early-bound lifetime</span>     |  A lifetime region that is substituted at its definition site. Bound in an item's `Generics` and substituted/instantiated using a `GenericArgs`. Contrast with **late-bound lifetime**. ([see more](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_type_ir/region_kind/enum.RegionKind.html#bound-regions))
<span id="effect">effects</span>               |  Right now only means const traits and `~const` bounds. ([see more](../effects.md))
<span id="empty-type">empty type</span>        |  See [uninhabited type](#ut).
<span id="fat-ptr">fat pointer</span>          |  A two word value carrying the address of some value, along with some further information necessary to put the value to use. Rust includes two kinds of _fat pointers_: references to slices, and trait objects. A reference to a slice carries the starting address of the slice and its length. A trait object carries a value's address and a pointer to the trait's implementation appropriate to that value. "Fat pointers" are also known as "wide pointers", and "double pointers".
<span id="free-var">free variable</span>       |  A _free variable_ is one that is not bound within an expression or term; see [the background chapter for more](./background.md#free-vs-bound)
<span id="generics">generics</span>            |  The list of generic parameters defined on an item. There are three kinds of generic parameters: Type, lifetime and const parameters.
<span id="hir">HIR</span>                      |  The _high-level [IR](#ir)_, created by lowering and desugaring the AST. ([see more](../hir.md))
<span id="hir-id">`HirId`</span>               |  Identifies a particular node in the HIR by combining a def-id with an "intra-definition offset". See [the HIR chapter for more](../hir.md#identifiers-in-the-hir).
<span id="ice">ICE</span>                      |  Short for _internal compiler error_, this is when the compiler crashes.
<span id="ich">ICH</span>                      |  Short for _incremental compilation hash_, these 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.