Constant Evaluation

Constant evaluation is the process of computing values at compile time. For a specific item (constant/static/array length) this happens after the MIR for the item is borrow-checked and optimized. In many cases trying to const evaluate an item will trigger the computation of its MIR for the first time.

Prominent examples are

  • The initializer of a static
  • Array length
    • needs to be known to reserve stack or heap space
  • Enum variant discriminants
    • needs to be known to prevent two variants from having the same discriminant
  • Patterns
    • need to be known to check for overlapping patterns

Additionally constant evaluation can be used to reduce the workload or binary size at runtime by precomputing complex operations at compiletime and only storing the result.

Constant evaluation can be done by calling the const_eval query of TyCtxt.

The const_eval query takes a ParamEnv of environment in which the constant is evaluated (e.g. the function within which the constant is used) and a GlobalId. The GlobalId is made up of an Instance referring to a constant or static or of an Instance of a function and an index into the function's Promoted table.

Constant evaluation returns a Result with either the error, or the simplest representation of the constant. "simplest" meaning if it is representable as an integer or fat pointer, it will directly yield the value (via Value::ByVal or Value::ByValPair), instead of referring to the miri virtual memory allocation (via Value::ByRef). This means that the const_eval function cannot be used to create miri-pointers to the evaluated constant or static. If you need that, you need to directly work with the functions in src/librustc_mir/const_eval.rs.