clarify boundary

diff --git a/ralf/_posts/2018-08-22-two-kinds-of-invariants.md b/ralf/_posts/2018-08-22-two-kinds-of-invariants.md
index 785a9fd9efd311b382255f1b7a27935127f6ca85..2e4ff3102d0dbd0a51ffc77eed87b11cdf2a5b89 100644 (file)
--- a/ralf/_posts/2018-08-22-two-kinds-of-invariants.md
+++ b/ralf/_posts/2018-08-22-two-kinds-of-invariants.md
@@ -102,6 +102,10 @@ But that's okay, because this is carefully controlled unsafe code -- and by the
 
 > *Unsafe code only has to uphold safety invariants at the boundaries to safe code.*
 
+Notice that the "boundary" is not necessarily where the `unsafe` block ends.
+The boundary occurs where the unsafe code provides a public API that safe code is intended to use -- that might be at the module boundary, or it might even be at the crate level.
+That is where safe code should be able to rely on safety, so that it can interact with the unsafe code without triggering undefined behavior, and hence that is where the safety invariants come into play.
+
 This is in strong contrast to validity, which must *always* hold.
 Layout optimizations and LLVM's attributes are in effect throughout unsafe code, so it is never okay to ever have invalid data.