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Showing content from https://rustc-dev-guide.rust-lang.org/const-eval below:

Constant evaluation - Rust Compiler Development Guide

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 compile time 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 evaluated constant: a valtree or a MIR constant value, respectively.

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