Document r#_ and 'r#_ as reserved

These are rejected by the lexer.

Author: ehuss

src/identifiers.md

@@ -14,6 +14,8 @@ r[ident.syntax]
 >
 > IDENTIFIER :\
 > NON_KEYWORD_IDENTIFIER | RAW_IDENTIFIER
+>
+> RESERVED_RAW_IDENTIFIER : `r#_`
 
 <!-- When updating the version, update the UAX links, too. -->
 r[ident.unicode]
@@ -71,6 +73,10 @@ r[ident.raw.allowed]
 Unlike a normal identifier, a raw identifier may be any strict or reserved
 keyword except the ones listed above for `RAW_IDENTIFIER`.
 
+r[ident.raw.reserved]
+It is an error to use the RESERVED_RAW_IDENTIFIER token `r#_` in order to avoid confusion with the [_WildcardPattern_].
+
+[_WildcardPattern_]: patterns.md#wildcard-pattern
 [`extern crate`]: items/extern-crates.md
 [`no_mangle`]: abi.md#the-no_mangle-attribute
 [`path` attribute]: items/modules.md#the-path-attribute

src/tokens.md

@@ -769,6 +769,9 @@ r[lex.token.life.syntax]
 > RAW_LIFETIME :\
 > &nbsp;&nbsp; `'r#` [IDENTIFIER_OR_KEYWORD][identifier] <sub>*Except `crate`, `self`, `super`, `Self`*</sub>
 >   _(not immediately followed by `'`)_
+>
+> RESERVED_RAW_LIFETIME : `'r#_`
+>   _(not immediately followed by `'`)_
 
 r[lex.token.life.intro]
 Lifetime parameters and [loop labels] use LIFETIME_OR_LABEL tokens. Any
@@ -781,6 +784,9 @@ A raw lifetime is like a normal lifetime, but its identifier is prefixed by `r#`
 r[lex.token.life.raw.allowed]
 Unlike a normal lifetime, a raw lifetime may be any strict or reserved keyword except the ones listed above for `RAW_LIFETIME`.
 
+r[lex.token.life.raw.reserved]
+It is an error to use the RESERVED_RAW_LIFETIME token `'r#_` in order to avoid confusion with the [placeholder lifetime].
+
 r[lex.token.life.raw.edition2021]
 > **Edition differences**: Raw lifetimes are accepted in the 2021
 > edition or later. In earlier additions the token `'r#lt` is lexed as `'r # lt`.
@@ -946,6 +952,7 @@ r[lex.token.reserved-prefix.edition2021]
 [numeric types]: types/numeric.md
 [paths]: paths.md
 [patterns]: patterns.md
+[placeholder lifetime]: lifetime-elision.md
 [question]: expressions/operator-expr.md#the-question-mark-operator
 [range]: expressions/range-expr.md
 [rangepat]: patterns.md#range-patterns