link to 'two kinds of invariants'

[web.git] / personal / _posts / 2020-07-15-unused-data.md

diff --git a/personal/_posts/2020-07-15-unused-data.md b/personal/_posts/2020-07-15-unused-data.md
index 16068e87b781dd52f2d0ec44eac186c47f14c101..ccde20408a75ca3b29f95bb6d08e8cbadaed2ecc 100644 (file)
--- a/personal/_posts/2020-07-15-unused-data.md
+++ b/personal/_posts/2020-07-15-unused-data.md
@@ -1,6 +1,7 @@
 ---
 title: "Why even unused data needs to be valid"
 categories: rust
+forum: https://internals.rust-lang.org/t/why-even-unused-data-needs-to-be-valid/12734
 ---
 
 The Rust compiler has a few assumptions that it makes about the behavior of all code.
@@ -9,7 +10,7 @@ Since Rust is a safe-by-default language, programmers usually do not have to wor
 but authors of `unsafe` code are themselves responsible for upholding these requirements.
 
 Those assumptions are [listed in the Rust reference](https://doc.rust-lang.org/reference/behavior-considered-undefined.html).
-The one that seems to be most surprising to many people is the clause which says that unsafe code may not *produce* "[...] an invalid value, even in private fields and locals".
+The one that seems to be most surprising to many people is the clause which says that Rust code may not *produce* "[...] an invalid value, even in private fields and locals".
 The reference goes on to explain that "*producing* a value happens any time a value is assigned to or read from a place, passed to a function/primitive operation or returned from a function/primitive operation".
 In other words, even just *constructing*, for example, an invalid `bool`, is Undefined Behavior---no matter whether that `bool` is ever actually "used" by the program.
 The purpose of this post is to explain why that rule is so strict.
@@ -29,6 +30,8 @@ fn example(b: bool) -> i32 {
 
 I hope it is not very surprising that calling `example` on, e.g., `3` transmuted to `bool` is Undefined Behavior (UB).
 When compiling `if`, the compiler assumes that `0` and `1` are the only possible values; there is no saying what could go wrong when that assumption is violated.
+(This is a compiler-understood *validity invariant* that is fixed in the language specification, which is very different from a user-defined *safety invariant*.
+See [this earlier post]({% post_url 2018-08-22-two-kinds-of-invariants.md %}) for more details on that distinction.)
 
 What is less obvious is why calling `example` on `3` is UB even when there is no such `if` being executed.
 To understand why that is important, let us consider the following example:
@@ -85,4 +88,4 @@ But this means our hands are pretty much tied: since we cannot take into account
 To support inlining and outlining, we also do not want the function boundary to be relevant, which ultimately leads us to the rule that Rust requires today: whenever data of a given type is *produced* anywhere, the data needs to be valid for that type.
 
 I hope this post was helpful in explaining why Undefined Behavior in Rust is defined the way it is.
-As usual, if you have any comments or questions, let me know in the forums.
+As usual, if you have any comments or questions, let me know in the [forums](https://internals.rust-lang.org/t/why-even-unused-data-needs-to-be-valid/12734).