Revise and clarify

Rather than talking about lifetime-extended temporaries in the
top-level scope of an initializer, which is maybe a bit ambiguous,
let's speak directly to the result of the lifetime extension, which is
that these temporaries disallowed for borrows (in cases of interior
mutability) would have their lifetimes extended to the end of the
program.

Author: traviscross

src/const_eval.md

@@ -81,9 +81,7 @@ r[const-eval.const-expr.builtin-arith-logic]
   operators used on integer and floating point types, `bool`, and `char`.
 
 r[const-eval.const-expr.borrows]
-* All forms of [borrow]s, including raw borrows, with one limitation:
-  mutable borrows and shared borrows to values with interior mutability
-  are not allowed to refer to [lifetime-extended temporaries in the top-level scope of a `const` or `static` initializer expression][lifetime-extension-const].
+* All forms of [borrow]s, including raw borrows, with one limitation: mutable borrows and shared borrows of expressions whose temporary scope would be extended (see [temporary lifetime extension]) to the end of the program are not allowed when those borrows refer to values with interior mutability.
 
   ```rust,compile_fail,E0492
   # use core::sync::atomic::AtomicU8;
@@ -111,8 +109,7 @@ r[const-eval.const-expr.borrows]
   const C: &AtomicU8 = &S; // OK
   ```
 
-  In other words, they are only allowed to refer to *transient* places, to *indirect* places, or to *static* places.
-  A place is *transient* if it is based on a local variable whose lifetime is strictly contained inside the current [const context].
+  In other words, they are only allowed to refer to *transient* places, to *indirect* places, or to *static* places. A place is *transient* if it is based on a local variable whose lifetime is strictly contained inside the current [const context].
 
   ```rust
   const C: () = {
@@ -260,7 +257,6 @@ of whether you are building on a `64` bit or a `32` bit system.
 [interior mutability]:  interior-mutability.md
 [if]:                   expressions/if-expr.md#if-expressions
 [lazy boolean]:         expressions/operator-expr.md#lazy-boolean-operators
-[lifetime-extension-const]: destructors.md#r-destructors.scope.lifetime-extension.static
 [let statements]:       statements.md#let-statements
 [literals]:             expressions/literal-expr.md
 [logical]:              expressions/operator-expr.md#arithmetic-and-logical-binary-operators
@@ -275,5 +271,6 @@ of whether you are building on a `64` bit or a `32` bit system.
 [slice]:                types/slice.md
 [statics]:              items/static-items.md
 [struct]:               expressions/struct-expr.md
+[temporary lifetime extension]: destructors.scope.lifetime-extension
 [tuple expressions]:    expressions/tuple-expr.md
 [while]:                expressions/loop-expr.md#predicate-loops