Fix grammar for `LiteralPattern` regarding `-`
We had documented that only numeric literals in patterns can be
prefixed by `-` (minus), but the Rust parser happily accepts a minus
ahead of all literals in patterns. E.g.:
```rust
match () {
-true | -false => (),
-'x' => (),
-b'x' => (),
-"x" => (),
-r"x" => (),
-br"x" => (),
-c"x" => (),
-cr"x" => (),
-1 => (),
-1.1 => (),
}
```
In the compiler, this happens in `Parser::parse_literal_maybe_minus`
and `Token::can_begin_literal_maybe_minus`.
Let's fix this by defining `LiteralPattern` as a `LiteralExpression`
optionally prefixed by the minus sign. This better matches how the
`rustc` AST models this.
diff --git a/src/patterns.md b/src/patterns.md
index 2d9ddb5..2818190 100644
--- a/src/patterns.md
+++ b/src/patterns.md
@@ -139,23 +139,11 @@
r[patterns.literal.syntax]
```grammar,patterns
-LiteralPattern ->
- `true` | `false`
- | CHAR_LITERAL
- | BYTE_LITERAL
- | STRING_LITERAL
- | RAW_STRING_LITERAL
- | BYTE_STRING_LITERAL
- | RAW_BYTE_STRING_LITERAL
- | C_STRING_LITERAL
- | RAW_C_STRING_LITERAL
- | `-`? INTEGER_LITERAL
- | `-`? FLOAT_LITERAL
+LiteralPattern -> `-`? LiteralExpression
```
r[patterns.literal.intro]
-_Literal patterns_ match exactly the same value as what is created by the literal.
-Since negative numbers are not [literals], literal patterns also accept an optional minus sign before the literal, which acts like the negation operator.
+_Literal patterns_ match exactly the same value as what is created by the literal. Since negative numbers are not [literals], literals in patterns may be prefixed by an optional minus sign, which acts like the negation operator.
> [!WARNING]
> C string and raw C string literals are accepted in literal patterns, but `&CStr` doesn't implement structural equality (`#[derive(Eq, PartialEq)]`) and therefore any such `match` on a `&CStr` will be rejected with a type error.
