# Glossary

Term                                                  | Meaning
------------------------------------------------------|--------
<span id="arena">arena, arena allocation</span> |  An _arena_ is a large memory buffer from which other memory allocations are made. This style of allocation is called _arena allocation_. See [this chapter](../memory.md) for more info.
<span id="ast">AST</span>                      |  The _abstract syntax tree_ produced by the `rustc_ast` crate; reflects user syntax very closely.
<span id="apit">APIT</span>                    |  An argument-position `impl Trait`. Also known as an anonymous type parameter. ([see the reference](https://doc.rust-lang.org/reference/types/impl-trait.html#anonymous-type-parameters)).
<span id="binder">binder</span>                |  A _binder_ is a place where a variable or type is declared; for example, the `<T>` is a binder for the generic type parameter `T` in `fn foo<T>(..)`, and \|`a`\|` ...` is a binder for the parameter `a`. See [the background chapter for more](./background.md#free-vs-bound).
<span id="body-id">`BodyId`</span>             |  An identifier that refers to a specific body (definition of a function or constant) in the crate. See [the HIR chapter for more](../hir.md#identifiers-in-the-hir).
<span id="bound-var">bound variable</span>     |  A _bound variable_ is one that is declared within an expression/term. For example, the variable `a` is bound within the closure expression \|`a`\|` a * 2`. See [the background chapter for more](./background.md#free-vs-bound)
<span id="codegen">codegen</span>              |  Short for _code generation_. The code to translate MIR into LLVM IR.
<span id="codegen-unit">codegen unit</span>    |  When we produce LLVM IR, we group the Rust code into a number of codegen units (sometimes abbreviated as CGUs). Each of these units is processed by LLVM independently from one another, enabling parallelism. They are also the unit of incremental re-use. ([see more](../backend/codegen.md))
<span id="completeness">completeness</span>    |  A technical term in type theory, it means that every type-safe program also type-checks. Having both soundness and completeness is very hard, and usually soundness is more important. (see "soundness").
<span id="cfg">control-flow graph</span>       |  A representation of the control-flow of a program; see [the background chapter for more](./background.md#cfg)
<span id="ctfe">CTFE</span>                    |  Short for _compile-time function evaluation_, this is the ability of the compiler to evaluate `const fn`s at compile time. This is part of the compiler's constant evaluation system. ([see more](../const-eval.md))
<span id="cx">`cx`</span>                      |  We tend to use _cx_ as an abbreviation for _context_. See also `tcx`, `infcx`, etc.
<span id="ctxt">`ctxt`</span>                  |  We also use _ctxt_ as an abbreviation for _context_, e.g. [`TyCtxt`](#TyCtxt). See also [cx](#cx) or [tcx](#tcx).
<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.
<span id="infcx">`infcx`</span>                |  The type inference context (`InferCtxt`). (see `rustc_middle::infer`)
<span id="inf-var">inference variable, infer var </span> |  When doing type, region, const inference, an _inference variable_ is a kind of special type/region that represents what you are trying to infer. Think of X in algebra. For example, if we are trying to infer the type of a variable in a program, we create an inference variable to represent that unknown type.
<span id="intern">intern</span>                |  Interning refers to storing certain frequently-used constant data, such as strings, and then referring to the data by an identifier (e.g. a `Symbol`) rather than the data itself, to reduce memory usage and number of allocations. See [this chapter](../memory.md) for more info.
<span id="interpreter">interpreter</span>      |  The heart of const evaluation, running MIR code at compile time. ([see more](../const-eval/interpret.md))
<span id="intrinsic">intrinsic</span>          |  Intrinsics are special functions that are implemented in the compiler itself but exposed (often unstably) to users. They do magical and dangerous things. (See [`std::intrinsics`](https://doc.rust-lang.org/std/intrinsics/index.html))
<span id="ir">IR</span>                        |  Short for _intermediate representation_, a general term in compilers. During compilation, the code is transformed from raw source (ASCII text) to various IRs. In Rust, these are primarily HIR, MIR, and LLVM IR. Each IR is well-suited for some set of computations. For example, MIR is well-suited for the borrow checker, and LLVM IR is well-suited for codegen because LLVM accepts it.
<span id="irlo">IRLO, irlo</span>              |  Sometimes used as an abbreviation for [internals.rust-lang.org](https://internals.rust-lang.org).
<span id="item">item</span>                    |  A kind of "definition" in the language, such as a static, const, use statement, module, struct, etc. Concretely, this corresponds to the `Item` type.
<span id="lang-item">lang item</span>          |  Items that represent concepts intrinsic to the language itself, such as special built-in traits like `Sync` and `Send`; or traits representing operations such as `Add`; or functions that are called by the compiler. ([see more](https://doc.rust-lang.org/1.9.0/book/lang-items.html))
<span id="lbl">late-bound lifetime</span>      |  A lifetime region that is substituted at its call site. Bound in a HRTB and substituted by specific functions in the compiler, such as `liberate_late_bound_regions`. Contrast with **early-bound lifetime**. ([see more](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_type_ir/region_kind/enum.RegionKind.html#bound-regions))
<span id="local-crate">local crate</span>      |  The crate currently being compiled. This is in contrast to "upstream crates" which refer to dependencies of the local crate.
<span id="lto">LTO</span>                      |  Short for *link-time optimizations*, this is a set of optimizations offered by LLVM that occur just before the final binary is linked. These include optimizations like removing functions that are never used in the final program, for example. _ThinLTO_ is a variant of LTO that aims to be a bit more scalable and efficient, but possibly sacrifices some optimizations. You may also read issues in the Rust repo about "FatLTO", which is the loving nickname given to non-Thin LTO. LLVM documentation: [here][lto] and [here][thinlto].
<span id="llvm">[LLVM]</span>                  |  (actually not an acronym :P) an open-source compiler backend. It accepts LLVM IR and outputs native binaries. Various languages (e.g. Rust) can then implement a compiler front-end that outputs LLVM IR and use LLVM to compile to all the platforms LLVM supports.
<span id="memoization">memoization</span>      |  The process of storing the results of (pure) computations (such as pure function calls) to avoid having to repeat them in the future. This is typically a trade-off between execution speed and memory usage.
<span id="mir">MIR</span>                      |  The _mid-level [IR](#ir)_ that is created after type-checking for use by borrowck and codegen. ([see more](../mir/index.md))
<span id="miri">Miri</span>                    |  A tool to detect Undefined Behavior in (unsafe) Rust code. ([see more](https://github.com/rust-lang/miri))
<span id="mono">monomorphization</span>        |  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).
<span id="normalize">normalize</span>          |  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.md#normalizeprojection---type).
<span id="newtype">newtype</span>              |  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.
<span id="niche">niche</span>                  |  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.
<span id="nll">NLL</span>                      |  Short for [non-lexical lifetimes](../borrow_check/region_inference.md), this is an extension to Rust's borrowing system to make it be based on the control-flow graph.
<span id="node-id">node-id or `NodeId`</span>  |  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.md#identifiers-in-the-hir).
<span id="obligation">obligation</span>        |  Something that must be proven by the trait system. ([see more](../traits/resolution.md))
<span id="placeholder">placeholder</span>      |  **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/placeholders_and_universes.md) for more details.
<span id="point">point</span>                  |  Used in the NLL analysis to refer to some particular location in the MIR; typically used to refer to a node in the control-flow graph.
<span id="projection">projection</span>        |  A general term for a "relative path", e.g. `x.f` is a "field projection", and `T::Item` is an ["associated type projection"](../traits/goals-and-clauses.md#trait-ref).
<span id="pc">promoted constants</span>        |  Constants extracted from a function and lifted to static scope; see [this section](../mir/index.md#promoted) for more details.
<span id="provider">provider</span>            |  The function that executes a query. ([see more](../query.md))
<span id="quantified">quantified</span>        |  In math or logic, existential and universal quantification are used to ask questions like "is there any type T for which is true?" or "is this true for all types T?"; see [the background chapter for more](./background.md#quantified).
<span id="query">query</span>                  |  A sub-computation during compilation. Query results can be cached in the current session or to disk for incremental compilation. ([see more](../query.md))
<span id="recovery">recovery</span>            |  Recovery refers to handling invalid syntax during parsing (e.g. a missing comma) and continuing to parse the AST. This avoid showing spurious errors to the user (e.g. showing 'missing field' errors when the struct definition contains errors).
<span id="region">region</span>                |  Another term for "lifetime" often used in the literature and in the borrow checker.
<span id="rib">rib</span>                      |  A data structure in the name resolver that keeps track of a single scope for names. ([see more](../name-resolution.md))
<span id="rpit">RPIT</span>                    |  A return-position `impl Trait`. ([see the reference](https://doc.rust-lang.org/reference/types/impl-trait.html#abstract-return-types)).
<span id="rpitit">RPITIT</span>                |  A return-position `impl Trait` in trait. Unlike RPIT, this is desugared to a generic associated type (GAT). Introduced in [RFC 3425](https://rust-lang.github.io/rfcs/3425-return-position-impl-trait-in-traits.html). ([see more](../return-position-impl-trait-in-trait.md))
<span id="rustbuild">rustbuild</span>          |  A deprecated term for the part of bootstrap that is written in Rust
<span id="scrutinee">scrutinee</span>          |  A scrutinee is the expression that is matched on in `match` expressions and similar pattern matching constructs. For example, in `match x { A => 1, B => 2 }`, the expression `x` is the scrutinee.
<span id="sess">`sess`</span>                  |  The compiler _session_, which stores global data used throughout compilation
<span id="side-tables">side tables</span>      |  Because the [AST](#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.
<span id="sigil">sigil</span>                  |  Like a keyword but composed entirely of non-alphanumeric tokens. For example, `&` is a sigil for references.
<span id="soundness">soundness</span>          |  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 id="span">span</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.
<span id="subst">subst</span>                  |  The act of _substituting_ the generic parameters inside of a type, constant expression, etc. with concrete generic arguments by supplying [substs](#substs). Nowadays referred to as _instantiating_ in the compiler.
<span id="substs">substs</span>                |  The _substitutions_ for a given generic item (e.g. the `i32`, `u32` in `HashMap<i32, u32>`). Nowadays referred to as the list of _generic arguments_ in the compiler (but note that strictly speaking these two concepts differ, see the literature).
<span id="sysroot">sysroot</span>              |  The directory for build artifacts that are loaded by the compiler at runtime. ([see more](../building/bootstrapping/what-bootstrapping-does.html#what-is-a-sysroot))
<span id="tag">tag</span>                      |  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).
<span id="tait">TAIT</span>                    |  A type-alias `impl Trait`. Introduced in [RFC 2515](https://rust-lang.github.io/rfcs/2515-type_alias_impl_trait.html).
<span id="tcx">`tcx`</span>                    |  Standard variable name for the "typing context" (`TyCtxt`), main data structure of the compiler. ([see more](../ty.md))
<span id="lifetime-tcx">`'tcx`</span>          |  The lifetime of the allocation arenas used by `TyCtxt`. Most data interned during a compilation session will use this lifetime with the exception of HIR data which uses the `'hir` lifetime. ([see more](../ty.md))
<span id="token">token</span>                  |  The smallest unit of parsing. Tokens are produced after lexing ([see more](../the-parser.md)).
<span id="tls">[TLS]</span>                    |  *Thread-local storage*. Variables may be defined so that each thread has its own copy (rather than all threads sharing the variable). This has some interactions with LLVM. Not all platforms support TLS.
<span id="trait-ref">trait reference, trait ref </span> |  The name of a trait along with a suitable list of generic arguments. ([see more](../traits/goals-and-clauses.md#trait-ref))
<span id="trans">trans</span>                  |  Short for _translation_, the code to translate MIR into LLVM IR. Renamed to [codegen](#codegen).
<span id="ty">`Ty`</span>                      |  The internal representation of a type. ([see more](../ty.md))
<span id="tyctxt">`TyCtxt`</span>              |  The data structure often referred to as [`tcx`](#tcx) in code which provides access to session data and the query system.
<span id="ufcs">UFCS</span>                    |  Short for _universal function call syntax_, this is an unambiguous syntax for calling a method. **Term no longer in use!** Prefer _fully-qualified path/syntax_. ([see more](../type-checking.md), [see the reference](https://doc.rust-lang.org/reference/expressions/call-expr.html#disambiguating-function-calls))
<span id="ut">uninhabited type</span>          |  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_.
<span id="upvar">upvar</span>                  |  A variable captured by a closure from outside the closure.
<span id="variance">variance</span>            |  Determines how changes to a generic 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.md#variance) for a more general explanation. See the [variance chapter](../variance.md) for an explanation of how type checking handles variance.
<span id="variant-idx">variant index</span>    |  In an enum, identifies a variant by assigning them indices starting at 0. This is purely internal and not to be confused with the ["discriminant"](#discriminant) which can be overwritten by the user (e.g. `enum Bool { True = 42, False = 0 }`).
<span id="wf">well-formedness</span>           |  Semantically: An expression that evaluates to meaningful result. In type systems: A type related construct which follows rules of the type system.
<span id="wide-ptr">wide pointer</span>        |  A pointer with additional metadata. See [fat pointer](#fat-ptr) for more.
<span id="zst">ZST</span>                      |  *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.

[LLVM]: https://llvm.org/
[lto]: https://llvm.org/docs/LinkTimeOptimization.html
[`Span`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html
[thinlto]: https://clang.llvm.org/docs/ThinLTO.html
[TLS]: https://llvm.org/docs/LangRef.html#thread-local-storage-models