Move const expressions to const_eval

Author: oli-obk

src/const_eval.md

@@ -1,7 +1,43 @@
 # Constant evaluation
 
 Constant evaluation is the process of computing the result of
-[expressions] during compilation.
+[expressions] during compilation. Only a subset of all expressions
+can be evaluated at compile-time.
+
+## Constant expressions
+
+Certain types of expressions can be evaluated at compile time. These are called
+_constant expressions_ and are required in const contexts. In
+other places, such as in [`let` statements](statements.html#let-statements),
+constant expressions may be evaluated at compile time. If errors, such as out
+of bounds [array indexing] or [overflow] occurs,
+then it is a compiler error if the value must be evaluated at compile time,
+otherwise it is just a warning, but the code will most likely panic when run.
+
+The following expressions are constant expressions, so long as any operands are
+also constant expressions and do not cause any [`Drop::drop`][destructors] calls
+to be ran.
+
+* [Literals].
+* [Paths] to [functions](items/functions.html) and constants.
+  Recursively defining constants is not allowed.
+* [Tuple expressions].
+* [Array expressions].
+* [Struct] expressions.
+* [Enum variant] expressions.
+* [Block expressions], including `unsafe` blocks, which only contain items and
+  possibly a constant tail expression.
+* [Field] expressions.
+* Index expressions, [array indexing] or [slice] with a `usize`.
+* [Range expressions].
+* [Closure expressions] which don't capture variables from the environment.
+* Built in [negation], [arithmetic, logical], [comparison] or [lazy boolean]
+  operators used on integer and floating point types, `bool` and `char`.
+* Shared [borrow]s, except if applied to a type with [interior mutability].
+* The [dereference operator].
+* [Grouped] expressions.
+* [Cast] expressions, except pointer to address and
+  function pointer to address casts.
 
 ## Const context
 
@@ -18,4 +54,28 @@ A _const context_ is one of the following:
 [enum discriminants]: items/enumerations.html#custom-discriminant-values-for-field-less-enumerations
 [constants]: items/constant-items.html
 [statics]: items/static-items.html
-[expressions]: expressions.html#constant-expressions
+[expressions]: expressions.html
+[array indexing]:       expressions/array-expr.html#array-and-slice-indexing-expressions
+[overflow]:             expressions/operator-expr.html#overflow
+[destructors]:          destructors.html
+[literals]:             expressions/literal-expr.html
+[paths]:                expressions/path-expr.html
+[tuple expressions]:    expressions/tuple-expr.html
+[array expressions]:    expressions/array-expr.html
+[struct]:               expressions/struct-expr.html
+[enum variant]:         expressions/enum-variant-expr.html
+[block expressions]:    expressions/block-expr.html
+[field]:                expressions/field-expr.html
+[array indexing]:       expressions/array-expr.html#array-and-slice-indexing-expressions
+[slice]:                types.html#array-and-slice-types
+[range expressions]:    expressions/range-expr.html
+[closure expressions]:  expressions/closure-expr.html
+[negation]:             expressions/operator-expr.html#negation-operators
+[arithmetic, logical]:  expressions/operator-expr.html#arithmetic-and-logical-binary-operators
+[comparison]:           expressions/operator-expr.html#comparison-operators
+[lazy boolean]:         expressions/operator-expr.html#lazy-boolean-operators
+[borrow]:               expressions/operator-expr.html#borrow-operators
+[interior mutability]:  interior-mutability.html
+[dereference operator]: expressions/operator-expr.html#the-dereference-operator
+[grouped]:              expressions/grouped-expr.html
+[cast]:                 expressions/operator-expr.html#type-cast-expressions