<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="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).