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.

All uses of constant evaluation can either be categorized as "influencing the type system" (array lengths, enum variant discriminants, const generic parameters), or as solely being done to precompute expressions to be used at runtime.

Constant evaluation can be done by calling the const_eval_* functions of TyCtxt. They're the wrappers of the const_eval query.

  • const_eval_global_id_for_typeck evaluates a constant to a valtree, so the result value can be further inspected by the compiler.
  • const_eval_global_id evaluate a constant to an "opaque blob" containing its final value; this is only useful for codegen backends and the CTFE evaluator engine itself.
  • eval_static_initializer specifically computes the initial values of a static. Statics are special; all other functions do not represent statics correctly and have thus assertions preventing their use on statics.

The const_eval_* functions use 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 an EvalToValTreeResult for type system constants or EvalToConstValueResult with either the error, or a representation of the constant.

Constants for the type system are encoded in "valtree representation". The ValTree datastructure allows us to represent

  • arrays,
  • many structs,
  • tuples,
  • enums and,
  • most primitives.

The basic rule for being permitted in the type system is that every value must be uniquely represented. In other words: a specific value must only be representable in one specific way. For example: there is only one way to represent an array of two integers as a ValTree: ValTree::Branch(&[ValTree::Leaf(first_int), ValTree;:Leaf(second_int)]). Even though theoretically a [u32; 2] could be encoded in a u64 and thus just be a ValTree::Leaf(bits_of_two_u32), that is not a legal construction of ValTree (and is very complex to do, so it is unlikely anyone is tempted to do so).

These rules also mean that some values are not representable. There can be no unions in type level constants, as it is not clear how they should be represented, because their active variant is unknown. Similarly there is no way to represent raw pointers, as addresses are unknown at compile-time and thus we cannot make any assumptions about them. References on the other hand can be represented, as equality for references is defined as equality on their value, so we ignore their address and just look at the backing value. We must make sure that the pointer values of the references are not observable at compile time. We thus encode &42 exactly like 42. Any conversion from valtree back to codegen constants must reintroduce an actual indirection. At codegen time the addresses may be deduplicated between multiple uses or not, entirely depending on arbitrary optimization choices.

As a consequence, all decoding of ValTree must happen by matching on the type first and making decisions depending on that. The value itself gives no useful information without the type that belongs to it.

Other constants get represented as ConstValue::Scalar or ConstValue::Slice if possible. These values are only useful outside the compile-time interpreter. If you need the value of a constant during interpretation, you need to directly work with const_to_op.