update docs

Author: RalfJung

promotion.md

@@ -69,22 +69,19 @@ 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 and inline assembly `const` operands everywhere
+* `#[rustc_args_required_const]` arguments and inline assembly `const` operands everywhere.
+* Everything inside the bodies of `const` and `static` items. (Note: this is handled separately from "explicit contexts" in promotion analysis, but the effect is the same.)
 
-In these contexts, we promote calls to arbitrary `const fn`. That is currently
-the only difference.
+In these contexts, we promote calls to arbitrary `const fn`.
 
-[warn-rfc]: https://github.com/rust-lang/rfcs/blob/master/text/1229-compile-time-asserts.md
-
-## Promotion in const contexts
+There is one further special case for the bodies of `const` and `static` items; here we additionally promote union field accesses.
+Both of these special cases can lead to promoting things that can fail to evaluate.
+Currently, this works out because it just leads to a warning, but longer-term it would be desirable to turn evaluation failures into hard errors, which for these promoteds means we have to guarantee that we only evaluate them on-demand.
 
-When promoting inside a const context (`const` and `static` items and `const fn`), we relax almost all promotion restrictions, except for basic constness restrictions listed below.
-We do this for all kinds of promotion.
+[See below][static access] for another special case in promotion analysis:
+accesses and references to statics are only promoted inside other statics.
 
-For `const fn`, [this is a bug](https://github.com/rust-lang/rust/issues/75586).
-
-For `const` and `static` items, this can lead to promoting things that can fail to evaluate.
-Currently, this works out because in the first case it just leads to a warning, but longer-term it would be desirable to turn evaluation failures into hard errors, which for these promoteds means we have to guarantee that we only evaluate them on-demand.
+[warn-rfc]: https://github.com/rust-lang/rfcs/blob/master/text/1229-compile-time-asserts.md
 
 ## "enclosing scope" rule
 
@@ -228,6 +225,7 @@ or `const` item and refer to that.
 the result of computing a promoted is a value that does not need dropping.
 
 ### Access to a `const` or `static`
+[access-static]: #access-to-a-const-or-static
 
 When accessing a `const` in a promotable context, its value gets computed
 at compile-time anyway, so we do not have to check the initializer.  However, the
@@ -248,8 +246,9 @@ const ANSWER: i32 = {
 let x: &'static i32 = &ANSWER;
 ```
 
-An access to a `static` is only promotable within the initializer of another
-`static`. This is for the same reason that `const` initializers
+An access to a `static`, including just taking references to a `static`, is only
+promotable within the initializer of another `static`. This is for the same
+reason that `const` initializers
 [cannot access statics](const.md#reading-statics).
 
 Crucially, however, the following is *not* legal: