clarify conflusion

[web.git] / ralf / _posts / 2018-07-19-const.md

diff --git a/ralf/_posts/2018-07-19-const.md b/ralf/_posts/2018-07-19-const.md
index d523935c8030bcb6ea4e55a1a6f6e795be0a76d2..f511b9f9da80797dd55855523cf005f1af616520 100644 (file)
--- a/ralf/_posts/2018-07-19-const.md
+++ b/ralf/_posts/2018-07-19-const.md
@@ -13,7 +13,7 @@ Expect something like a structured brain dump, so there are some unanswered ques
 
 ## Some Background
 
-CTFE is the mechanism used by the compiler, primarily, to evaluate items like `const x : T = ...;`.
+CTFE is the mechanism used by the compiler, primarily, to evaluate items like `const x: T = ...;`.
 The `...` here is going to be Rust code that must be "run" at compile-time, because it can be used as a constant in the code -- for example, it can be used for array lengths.
 
 Notice that CTFE is *not* the same as constant propagation: Constant propagation is an optimization pass done by LLVM that will opportunistically change code like `3 + 4` into `7` to avoid run-time work.
@@ -23,8 +23,8 @@ You can statically see, just from the syntax of the code, whether CTFE applies t
 CTFE is only used in places like the value of a `const` or the length of an array.
 {% highlight rust %}
 fn demo() {
-  const X : u32 = 3 + 4; // CTFE
-  let x : u32 = 4 + 3; // no CTFE (but maybe constant propagation)
+  const X: u32 = 3 + 4; // CTFE
+  let x: u32 = 4 + 3; // no CTFE (but maybe constant propagation)
 }
 {% endhighlight %}
 We say that the `3 + 4` above is in *const context* and hence subject to CTFE, but the `4 + 3` is not.
@@ -228,6 +228,8 @@ I am not sure which effect that should or will have for promotion.
 ## Conclusion
 
 I have discussed the notions of CTFE determinism and CTFE correctness (which are properties of a CTFE engine like miri), as well as const safety (property of a piece of code) and const soundness (property of a type system).
+In particular, I propose that *when type-checking safe code in const context, we guarantee that this code is const-safe*, i.e., that it will not hit a CTFE error (though panics are allowed, just like they are in "run-time" Rust code).
+
 There are still plenty of open questions, in particular around the interaction of [`const fn` and traits](https://github.com/rust-lang/rust/issues/24111#issuecomment-311029471), but I hope this terminology is useful when having those discussions.
 Let the type systems guide us :)