Glossary

TermMeaning
arena/arena allocationAn arena is a large memory buffer from which other memory allocations are made. This style of allocation is called arena allocation. See this chapter for more info.
ASTThe abstract syntax tree produced by the rustc_ast crate; reflects user syntax very closely.
binderA "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.
BodyIdAn identifier that refers to a specific body (definition of a function or constant) in the crate. See the HIR chapter for more.
bound variableA "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
codegenThe code to translate MIR into LLVM IR.
codegen unitWhen 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)
completenessA 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").
control-flow graphA representation of the control-flow of a program; see the background chapter for more
CTFEShort for Compile-Time Function Evaluation, this is the ability of the compiler to evaluate const fns at compile time. This is part of the compiler's constant evaluation system. (see more)
cxWe tend to use "cx" as an abbreviation for context. See also tcx, infcx, etc.
ctxtWe also use "ctxt" as an abbreviation for context, e.g. TyCtxt. See also cx or tcx.
DAGA directed acyclic graph is used during compilation to keep track of dependencies between queries. (see more)
data-flow analysisA 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.
DeBruijn IndexA 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)
DefIdAn index identifying a definition (see rustc_middle/src/hir/def_id.rs). Uniquely identifies a DefPath. See the HIR chapter for more.
DiscriminantThe underlying value associated with an enum variant or generator state to indicate it as "active" (but not to be confused with its "variant index"). At runtime, the discriminant of the active variant is encoded in the tag.
Double pointerA pointer with additional metadata. See "fat pointer" for more.
drop glue(internal) compiler-generated instructions that handle calling the destructors (Drop) for data types.
DSTShort 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]).
early-bound lifetimeA lifetime region that is substituted at its definition site. Bound in an item's Generics and substituted using a Substs. Contrast with late-bound lifetime. (see more)
empty typesee "uninhabited type".
Fat pointerA 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".
free variableA "free variable" is one that is not bound within an expression or term; see the background chapter for more
genericsThe set of generic type parameters defined on a type or item.
HIRThe High-level IR, created by lowering and desugaring the AST. (see more)
HirIdIdentifies a particular node in the HIR by combining a def-id with an "intra-definition offset". See the HIR chapter for more.
HIR MapThe HIR map, accessible via tcx.hir, allows you to quickly navigate the HIR and convert between various forms of identifiers.
ICEShort for internal compiler error, this is when the compiler crashes.
ICHShort 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.
infcxThe inference context (see rustc_middle::infer)
inference variableWhen doing type or region 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.
internInterning 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 for more info.
intrinsicIntrinsics 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)
IRShort 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.
IRLOIRLO or irlo is sometimes used as an abbreviation for internals.rust-lang.org.
itemA kind of "definition" in the language, such as a static, const, use statement, module, struct, etc. Concretely, this corresponds to the Item type.
lang itemItems 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)
late-bound lifetimeA 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)
local crateThe crate currently being compiled.
LTOShort 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 and here.
LLVM(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.
memoizationThe 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.
MIRThe Mid-level IR that is created after type-checking for use by borrowck and codegen. (see more)
miriAn interpreter for MIR used for constant evaluation. (see more)
monomorphizationThe 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).
normalizeA general term for converting to a more canonical form, but in the case of rustc typically refers to associated type normalization.
newtypeA 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.
NicheInvalid 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, which allows using a type's niche as the "tag" for an enum without requiring a separate field.
NLLShort for non-lexical lifetimes, this is an extension to Rust's borrowing system to make it be based on the control-flow graph.
node-id or NodeIdAn index identifying a particular node in the AST or HIR; gradually being phased out and replaced with HirId. See the HIR chapter for more.
obligationSomething that must be proven by the trait system. (see more)
placeholderNOTE: 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 for more details.
pointUsed 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.
polymorphizeAn optimization that avoids unnecessary monomorphisation. (see more)
projectionA general term for a "relative path", e.g. x.f is a "field projection", and T::Item is an "associated type projection".
promoted constantsConstants extracted from a function and lifted to static scope; see this section for more details.
providerThe function that executes a query. (see more)
quantifiedIn 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.
queryPerhaps some sub-computation during compilation. (see more)
recoveryRecovery 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).
regionAnother term for "lifetime" often used in the literature and in the borrow checker.
ribA data structure in the name resolver that keeps track of a single scope for names. (see more)
scrutineeA 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.
sessThe compiler session, which stores global data used throughout compilation
side tablesBecause the 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.
sigilLike a keyword but composed entirely of non-alphanumeric tokens. For example, & is a sigil for references.
soundnessA 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").
spanA 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.
substsThe substitutions for a given generic type or item (e.g. the i32, u32 in HashMap<i32, u32>).
sysrootThe directory for build artifacts that are loaded by the compiler at runtime. (see more)
TagThe "tag" of an enum/generator encodes the discriminant of the active variant/state. Tags can either be "direct" (simply storing the discriminant in a field) or use a "niche".
tcxThe "typing context", main data structure of the compiler. (see more)
'tcxThe lifetime of the allocation arena. (see more)
tokenThe smallest unit of parsing. Tokens are produced after lexing (see more).
TLSThread-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.
trait referenceThe name of a trait along with a suitable set of input type/lifetimes. (see more)
transThe code to translate MIR into LLVM IR. Renamed to codegen.
tyThe internal representation of a type. (see more)
TyCtxtThe data structure often referred to as tcx in code
UFCSShort for Universal Function Call Syntax, this is an unambiguous syntax for calling a method. (see more)
uninhabited typeA 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".
upvarA variable captured by a closure from outside the closure.
varianceDetermines 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 for a more general explanation. See the variance chapter for an explanation of how type checking handles variance.
Variant indexIn an enum, identifies a variant by assigning them indices starting at 0. This is purely internal and not to be confused with the "discrimiant" which can be overwritten by the user (e.g. enum Bool { True = 42, False = 0 }).
Wide pointerA pointer with additional metadata. See "fat pointer" for more.
ZSTZero-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.