Explain the new valtree system for type level constants.

Author: oli-obk

src/const-eval.md

@@ -20,17 +20,62 @@ 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`](./param_env.html) 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 [`EvalToConstValueResult`] with either the error, or a
-representation of the constant. `static` initializers are always represented as
-[`miri`](./miri.html) virtual memory allocations (via [`ConstValue::ByRef`]).
+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 `union`s 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 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 value
+of the references are not observable. 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. This means that the `const_eval_*`
 functions cannot be used to create miri-pointers to the evaluated constant.
@@ -42,4 +87,5 @@ If you need the value of a constant inside Miri, you need to directly work with
 [`ConstValue::Slice`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/interpret/value/enum.ConstValue.html#variant.Slice
 [`ConstValue::ByRef`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/interpret/value/enum.ConstValue.html#variant.ByRef
 [`EvalToConstValueResult`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/interpret/error/type.EvalToConstValueResult.html
+[`EvalToValTreeResult`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/interpret/error/type.EvalToValTreeResult.html
 [`const_to_op`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/struct.InterpCx.html#method.const_to_op