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Explain the new valtree system for type level constants.
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src/const-eval.md

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@@ -20,17 +20,49 @@ Additionally constant evaluation can be used to reduce the workload or binary
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size at runtime by precomputing complex operations at compiletime and only
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storing the result.
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All uses of constant evaluation can either be categorized as "influencing the type system"
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(array lengths, enum variant discriminants, const generic parameters), or as solely being
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done to precompute expressions to be used at runtime.
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Constant evaluation can be done by calling the `const_eval_*` functions of `TyCtxt`.
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They're the wrappers of the `const_eval` query.
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`static` initializers must use the `eval_static_initializer` function. All other functions
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do not represent statics correctly and have thus assertions preventing their use on statics.
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The `const_eval_*` functions use a [`ParamEnv`](./param_env.html) of environment
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in which the constant is evaluated (e.g. the function within which the constant is used)
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and a [`GlobalId`]. The `GlobalId` is made up of an `Instance` referring to a constant
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or static or of an `Instance` of a function and an index into the function's `Promoted` table.
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Constant evaluation returns a [`EvalToConstValueResult`] with either the error, or a
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representation of the constant. `static` initializers are always represented as
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[`miri`](./miri.html) virtual memory allocations (via [`ConstValue::ByRef`]).
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Constant evaluation returns a [`EvalToValTreeResult`] (for type system constants) or
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[`EvalToConstValueResult`] with either the error, or a representation of the constant.
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Constants for the type system are encoded in "valtree representation". The `ValTree` datastructure
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allows us to represent arrays, many structs, tuples, enums and most primitives. The basic rule for
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being permitted in the type system is that every value must be uniquely represented. In other
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words: a specific value must only be representable in one specific way. For example: there is only
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one way to represent an array of two integers as a `ValTree`:
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`ValTree::Branch(&[ValTree::Leaf(first_int), ValTree;:Leaf(second_int)])`.
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Even though theoretically a `[u32; 2]` could be encoded in a `u64` and thus just be a
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`ValTree::Leaf(bits_of_two_u32)`, that is not a legal construction of `ValTree`
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(and is so complex to do, so it is unlikely anyone is tempted to do so).
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These rules also mean that some values are not representable. There can be no `union`s in type
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level constants, as it is not clear how they should be represented, because their active variant
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is unknown. Similarly there is no way to represent pointers, as addresses are unknown at
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compile-time and thus we cannot make any assumptions about them. References on the other hand
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can be represented, as equality for references is defined as equality on their value, so we
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ignore their address and just look at the backing value. We must make sure that the pointer value
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of the references are not observable. We thus encode `&42` exactly like `42`. Any conversion from
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valtree back to codegen constants must reintroduce an actual indirection. At codegen time the
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addresses may be deduplicated between multiple uses or not, entirely depending on arbitrary
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optimization choices.
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As a consequence, all decoding of `ValTree` must happen by matching on the type first and making
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decisions depending on that. The value itself gives no useful information without the type that
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belongs to it.
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Other constants get represented as [`ConstValue::Scalar`]
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or [`ConstValue::Slice`] if possible. This means that the `const_eval_*`
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functions cannot be used to create miri-pointers to the evaluated constant.
@@ -42,4 +74,5 @@ If you need the value of a constant inside Miri, you need to directly work with
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[`ConstValue::Slice`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/interpret/value/enum.ConstValue.html#variant.Slice
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[`ConstValue::ByRef`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/interpret/value/enum.ConstValue.html#variant.ByRef
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[`EvalToConstValueResult`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/interpret/error/type.EvalToConstValueResult.html
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[`EvalToValTreeResult`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/interpret/error/type.EvalToValTreeResult.html
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[`const_to_op`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/struct.InterpCx.html#method.const_to_op

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