Add a 'Reserved prefixes' section to the Tokens page

Author: mattheww

src/tokens.md

@@ -659,3 +659,34 @@ them are referred to as "token trees" in [macros].  The three types of brackets
 [use declarations]: items/use-declarations.md
 [use wildcards]: items/use-declarations.md
 [while let]: expressions/loop-expr.md#predicate-pattern-loops
+
+## Reserved prefixes
+
+Some lexical forms known as _reserved prefixes_ are reserved for future use.
+
+Source input which would otherwise be lexically interpreted as a non-raw identifier (or a keyword) which is immediately followed by a `#`, `'`, or `"` character (without intervening whitespace) is identified as a reserved prefix.
+
+Note that raw identifiers, raw string literals, and raw byte string literals may contain a `#` character but are not interpreted as containing a reserved prefix.
+
+Similarly the `r`, `b`, and `br` prefixes used in raw string literals, byte literals, byte string literals, and raw byte string literals are not interpreted as reserved prefixes.
+
+> **Edition Differences**: Starting with the 2021 edition, reserved prefixes are reported as an error by the lexer (in particular, they cannot be passed to macros).
+>
+> Before the 2021 edition, a reserved prefixes are accepted by the lexer and interpreted as multiple tokens (for example, one token for the identifier or keyword, followed by a `#` token).
+>
+> Examples accepted in all editions:
+> ```rust
+> macro_rules! lexes {($($_:tt)*) => {}}
+> lexes!{a #foo}
+> lexes!{continue 'foo}
+> lexes!{match "..." {}}
+> lexes!{r#let#foo}         // three tokens: r#let # foo
+> ```
+>
+> Examples accepted before the 2021 edition but rejected later:
+> ```rust,edition2018
+> macro_rules! lexes {($($_:tt)*) => {}}
+> lexes!{a#foo}
+> lexes!{continue'foo}
+> lexes!{match"..." {}}
+> ```