document guarantee about evaluation of associated consts and const blocks

Author: RalfJung

src/expressions/block-expr.md

@@ -129,7 +129,7 @@ Const blocks allows you to define a constant value without having to define new
 It also supports type inference so there is no need to specify the type, unlike [constant items].
 
 Const blocks have the ability to reference generic parameters in scope, unlike [free][free item] constant items.
-They are desugared to associated constant items with generic parameters in scope.
+They are desugared to constant items with generic parameters in scope (similar to associated constants, but without a trait or type they are associated with).
 For example, this code:
 
 ```rust
@@ -152,8 +152,12 @@ fn foo<T>() -> usize {
 }
 ```
 
-This also means that const blocks are treated similarly to associated constants.
-For example, they are not guaranteed to be evaluated when the enclosing function is unused.
+If the function containing the const block is executed at runtime, then the const block is guaranteed to be evaluated.
+This is true even if the const block occurs in dead code.
+In particular, unsafe code inside a function can rely on the fact that all const blocks syntactically occurring in the function have successfully evaluated (and not panicked).
+Note that this is only guaranteed for all currently existing syntactic constructs;
+in the future, a new kind of block may be introduced such that constants inside that block are *not* unconditionally guaranteed to be evaluated.
+If the function is not executed at runtime, const blocks inside of it may or may not be evaluated.
 
 ## `unsafe` blocks
 

src/expressions/path-expr.md

@@ -23,10 +23,13 @@ let push_integer = Vec::<i32>::push;
 let slice_reverse = <[i32]>::reverse;
 ```
 
+Evaluation of associated constants is handled the same way as [`const` blocks].
+
 [_PathInExpression_]: ../paths.md#paths-in-expressions
 [_QualifiedPathInExpression_]: ../paths.md#qualified-paths
 [place expressions]: ../expressions.md#place-expressions-and-value-expressions
 [value expressions]: ../expressions.md#place-expressions-and-value-expressions
 [path]: ../paths.md
 [`static mut`]: ../items/static-items.md#mutable-statics
 [`unsafe` block]: block-expr.md#unsafe-blocks
+[`const` blocks]: block-expr.md#const-blocks