move const/static section to other promotion contexts

Author: RalfJung

promotion.md

@@ -44,41 +44,7 @@ attribute, introduced in
 specify these parameters and (aggressively, see below) try to promote the
 corresponding arguments.
 
-## Implicit and explicit promotion
-
-On top of what applies to [consts](const.md), promoteds suffer from the additional issue that *the user did not ask for them to be evaluated at compile-time*.
-Thus, if CTFE fails but the code would have worked fine at run-time, we broke the user's code for no good reason.
-Even if we are sure we found an error in the user's code, we are only allowed to [emit a warning, not a hard error][warn-rfc].
-We call this *implicit* promotion, and we have to be very conservative with what can and cannot be implicitly promoted.
-
-CTFE of implicitly promoted code must never fail to evaluate except if the
-run-time code also would have failed. This means we cannot permit calling
-arbitrary `const fn`, as discussed in detail in
-[rust-lang/const-eval#19](https://github.com/rust-lang/const-eval/issues/19).
-Thus, only functions marked `#[rustc_promotable]` are implicitly promotable (see
-below).
-
-On the other hand, when a user passes an expression to a function with
-`#[rustc_args_required_const]`, the only way for this code to compile is to
-promote it.  In that sense, the user is explicitly asking for that expression to
-be evaluated at compile-time even though they have not written it in a `const`
-declaration. We can thus be less conservative. This is called *explicit*
-promotion.
-
-Currently, the following are considered explicit promotion contexts:
-* `#[rustc_args_required_const]` arguments
-* the bodies of `const` and `static` items and array lengths
-
-Everything else is an implicit promotion context, including `const fn` bodies and non-`Copy` array initializers.
-
-In an explicit promotion context, we promote every closed expression (i.e.,
-expressions that do not depend on other variables) of reference type, subject to
-the usual restrictions of [consts](const.md). This means that interior
-mutability and values that need dropping are not promoted.
-
-[warn-rfc]: https://github.com/rust-lang/rfcs/blob/master/text/1229-compile-time-asserts.md
-
-## Promotion inside `const` and `static`
+### Promotion inside `const` and `static`
 
 Lifetime extension is also responsible for making code like this work:
 
@@ -108,6 +74,39 @@ the "enclosing scope", similar to how `let x = &mut x;` creates a reference
 whose lifetime lasts for the enclosing scope. This is decided during MIR
 building already, and does not involve lifetime extension.
 
+## Implicit and explicit promotion
+
+On top of what applies to [consts](const.md), promoteds suffer from the additional issue that *the user did not ask for them to be evaluated at compile-time*.
+Thus, if CTFE fails but the code would have worked fine at run-time, we broke the user's code for no good reason.
+Even if we are sure we found an error in the user's code, we are only allowed to [emit a warning, not a hard error][warn-rfc].
+We call this *implicit* promotion, and we have to be very conservative with what can and cannot be implicitly promoted.
+
+CTFE of implicitly promoted code must never fail to evaluate except if the
+run-time code also would have failed. This means we cannot permit calling
+arbitrary `const fn`, as discussed in detail in
+[rust-lang/const-eval#19](https://github.com/rust-lang/const-eval/issues/19).
+Thus, only functions marked `#[rustc_promotable]` are implicitly promotable (see
+below).
+
+On the other hand, when a user passes an expression to a function with
+`#[rustc_args_required_const]`, the only way for this code to compile is to
+promote it.  In that sense, the user is explicitly asking for that expression to
+be evaluated at compile-time even though they have not written it in a `const`
+declaration. We can thus be less conservative. This is called *explicit*
+promotion.
+
+Currently, the following are considered explicit promotion contexts:
+* `#[rustc_args_required_const]` arguments
+* lifetime extension in the bodies of `const` and `static` items and array lengths
+
+Everything else is an implicit promotion context, including `const fn` bodies and non-`Copy` array initializers.
+
+In an explicit promotion context, we consider almost everything promotable,
+subject to the usual restrictions of [consts](const.md). This means that
+interior mutability and values that need dropping are not promoted.
+
+[warn-rfc]: https://github.com/rust-lang/rfcs/blob/master/text/1229-compile-time-asserts.md
+
 ## Promotability
 
 We have described the circumstances where promotion is desirable, but what