Trait Qualif

pub trait Qualif {
    const ANALYSIS_NAME: &'static str;
    const IS_CLEARED_ON_MOVE: bool = false;
    const ALLOW_PROMOTED: bool = false;

    // Required methods
    fn in_qualifs(qualifs: &ConstQualifs) -> bool;
    fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool;
    fn in_adt_inherently<'tcx>(
        cx: &ConstCx<'_, 'tcx>,
        adt: AdtDef<'tcx>,
        args: GenericArgsRef<'tcx>,
    ) -> bool;
    fn deref_structural<'tcx>(cx: &ConstCx<'_, 'tcx>) -> bool;
}

A “qualif”(-ication) is a way to look for something “bad” in the MIR that would disqualify some code for promotion or prevent it from evaluating at compile time.

Normally, we would determine what qualifications apply to each type and error when an illegal operation is performed on such a type. However, this was found to be too imprecise, especially in the presence of enums. If only a single variant of an enum has a certain qualification, we needn’t reject code unless it actually constructs and operates on the qualified variant.

To accomplish this, const-checking and promotion use a value-based analysis (as opposed to a type-based one). Qualifications propagate structurally across variables: If a local (or a projection of a local) is assigned a qualified value, that local itself becomes qualified.

Required Associated Constants

const ANALYSIS_NAME: &'static str

The name of the file used to debug the dataflow analysis that computes this qualif.

Provided Associated Constants

const IS_CLEARED_ON_MOVE: bool = false

Whether this Qualif is cleared when a local is moved from.

const ALLOW_PROMOTED: bool = false

Whether this Qualif might be evaluated after the promotion and can encounter a promoted.

Required Methods

fn in_qualifs(qualifs: &ConstQualifs) -> bool

Extracts the field of ConstQualifs that corresponds to this Qualif.

fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool

Returns true if any value of the given type could possibly have this Qualif.

This function determines Qualifs when we cannot do a value-based analysis. Since qualif propagation is context-insensitive, this includes function arguments and values returned from a call to another function.

It also determines the Qualifs for primitive types.

fn in_adt_inherently<'tcx>( cx: &ConstCx<'_, 'tcx>, adt: AdtDef<'tcx>, args: GenericArgsRef<'tcx>, ) -> bool

Returns true if this Qualif is inherent to the given struct or enum.

By default, Qualifs propagate into ADTs in a structural way: An ADT only becomes qualified if part of it is assigned a value with that Qualif. However, some ADTs always have a certain Qualif, regardless of whether their fields have it. For example, a type with a custom Drop impl is inherently NeedsDrop.

Returning true for in_adt_inherently but false for in_any_value_of_ty is unsound.

fn deref_structural<'tcx>(cx: &ConstCx<'_, 'tcx>) -> bool

Returns true if this Qualif behaves sructurally for pointers and references: the pointer/reference qualifies if and only if the pointee qualifies.

(This is currently false for all our instances, but that may change in the future. Also, by keeping it abstract, the handling of Deref in in_place becomes more clear.)

Dyn Compatibility

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.

Implementors

impl Qualif for HasMutInterior

const ANALYSIS_NAME: &'static str = "flow_has_mut_interior"

impl Qualif for NeedsDrop

const ANALYSIS_NAME: &'static str = "flow_needs_drop"

const IS_CLEARED_ON_MOVE: bool = true

impl Qualif for NeedsNonConstDrop

const ANALYSIS_NAME: &'static str = "flow_needs_nonconst_drop"

const IS_CLEARED_ON_MOVE: bool = true

const ALLOW_PROMOTED: bool = true