Document rvalue promotion to 'static.

Author: eddyb

src/expressions.md

@@ -84,9 +84,15 @@ The following expressions can create mutable lvalues:
 ### Temporary lifetimes
 
 When using an rvalue in most lvalue contexts, a temporary unnamed lvalue is
-created and used instead. The lifetime of temporary values is typically the
-innermost enclosing statement; the tail expression of a block is considered
-part of the statement that encloses the block.
+created and used instead, if not promoted to `'static`. Promotion of an
+rvalue expression to a `'static` slot occurs when the expression could be
+written in a constant, borrowed, and dereferencing that borrow where the
+expression was the originally written, without changing the runtime behavior.
+That is, the promoted expression can be evaluated at compile-time and the
+resulting value does not contain interior mutability or destructors.
+Otherwise, lifetime of temporary values is typically the innermost enclosing
+statement; the tail expression of a block is considered part of the statement
+that encloses the block
 
 When a temporary rvalue is being created that is assigned into a `let`
 declaration, however, the temporary is created with the lifetime of the