Fix unary minus rendering into -- line comments

Author: LucaCappelletti94

src/ast/mod.rs

@@ -1966,7 +1966,8 @@ impl fmt::Display for Expr {
                         | UnaryOperator::DoubleAt
                         | UnaryOperator::QuestionDash
                         | UnaryOperator::QuestionPipe
-                ) {
+                ) || (op == &UnaryOperator::Minus && starts_with_operator_char(expr))
+                {
                     write!(f, "{op} {expr}")
                 } else {
                     write!(f, "{op}{expr}")
@@ -8083,6 +8084,26 @@ impl fmt::Display for FunctionArg {
     }
 }
 
+/// Whether `expr` renders with an operator character first. A prefix `-`
+/// must not abut one, since `--` starts a line comment and operator-run
+/// dialects fuse `-@`, `-~`, `-#`, `-!!` and `-||/` into single tokens.
+fn starts_with_operator_char(expr: &Expr) -> bool {
+    use fmt::Write;
+    struct FirstChar(Option<char>);
+    impl fmt::Write for FirstChar {
+        fn write_str(&mut self, s: &str) -> fmt::Result {
+            if self.0.is_none() {
+                self.0 = s.chars().next();
+            }
+            Ok(())
+        }
+    }
+    let mut first = FirstChar(None);
+    let _ = write!(first, "{expr}");
+    const OPERATOR_CHARS: &str = "+-*/<>=~!@%#^&|";
+    first.0.is_some_and(|c| OPERATOR_CHARS.contains(c))
+}
+
 /// `FunctionArgOperator::Space` has no token of its own, so the name and the
 /// value are separated by a single space instead.
 fn fmt_named_function_arg(

tests/sqlparser_common.rs

@@ -20060,3 +20060,11 @@ fn parse_insert_by_name() {
         _ => unreachable!(),
     }
 }
+
+#[test]
+fn parse_unary_minus_never_renders_line_comment() {
+    all_dialects().verified_stmt("SELECT - -1");
+    all_dialects().verified_stmt("SELECT - - -1");
+    all_dialects().verified_stmt("SELECT -1");
+    all_dialects().verified_stmt("SELECT -x");
+}

tests/sqlparser_postgres.rs

@@ -9953,3 +9953,10 @@ fn parse_insert_by_name_keywords_as_table_and_alias() {
         statement => panic!("Expected INSERT statement, got: {statement:?}"),
     }
 }
+
+#[test]
+fn parse_unary_minus_before_pg_prefix_operators() {
+    pg().verified_stmt("SELECT - ~1");
+    pg().verified_stmt("SELECT - @2");
+    pg().one_statement_parses_to("SELECT - #x", "SELECT - # x");
+}