document guarantee about evaluation of associated consts

Author: RalfJung

src/expressions/block-expr.md

@@ -138,7 +138,7 @@ fn foo<T>() -> usize {
 }
 ```
 
-is equivalent to:
+is roughly equivalent to:
 
 ```rust
 fn foo<T>() -> usize {
@@ -152,8 +152,13 @@ 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