Parser: add warning for empty if body

But only if the semicolon is on the same line as the if

src/aro/Diagnostics.zig

@@ -223,6 +223,7 @@ pub const Options = struct {
     nonnull: Kind = .default,
     @"atomic-access": Kind = .default,
     @"gnu-designator": Kind = .default,
+    @"empty-body": Kind = .default,
 };
 
 const Diagnostics = @This();

src/aro/Diagnostics/messages.def

@@ -2633,3 +2633,13 @@ gnu_missing_eq_designator
     .msg = "use of GNU 'missing =' extension in designator"
     .kind = .warning
     .opt = W("gnu-designator")
+
+empty_if_body
+    .msg = "if statement has empty body"
+    .kind = .warning
+    .opt = W("empty-body")
+
+empty_if_body_note
+    .msg = "put the semicolon on a separate line to silence this warning"
+    .kind = .note
+    .opt = W("empty-body")

src/aro/Parser.zig

@@ -4720,6 +4720,17 @@ fn stmt(p: *Parser) Error!Node.Index {
         const then_body = try p.stmt();
         const else_body = if (p.eatToken(.keyword_else)) |_| try p.stmt() else null;
 
+        if (p.nodeIs(then_body, .null_stmt) and else_body == null) {
+            const semicolon_tok = then_body.get(&p.tree).null_stmt.semicolon_or_r_brace_tok;
+            const locs = p.pp.tokens.items(.loc);
+            const if_loc = locs[kw_if];
+            const semicolon_loc = locs[semicolon_tok];
+            if (if_loc.line == semicolon_loc.line) {
+                try p.errTok(.empty_if_body, semicolon_tok);
+                try p.errTok(.empty_if_body_note, semicolon_tok);
+            }
+        }
+
         return p.addNode(.{ .if_stmt = .{
             .if_tok = kw_if,
             .cond = cond.node,

test/cases/statements.c

@@ -35,13 +35,20 @@ void baz(int arg) {
         default:
             return;
     }
+    if (1)
+        ;
 }
 
-#define EXPECTED_ERRORS "statements.c:3:9: error: statement requires expression with scalar type ('void' invalid)" \
+#define EXPECTED_ERRORS "statements.c:2:17: warning: if statement has empty body [-Wempty-body]" \
+    "statements.c:2:17: note: put the semicolon on a separate line to silence this warning" \
+    "statements.c:3:9: error: statement requires expression with scalar type ('void' invalid)" \
+    "statements.c:3:17: warning: if statement has empty body [-Wempty-body]" \
+    "statements.c:3:17: note: put the semicolon on a separate line to silence this warning" \
     "statements.c:4:13: error: statement requires expression with integer type ('float' invalid)" \
     "statements.c:5:10: warning: expression result unused [-Wunused-value]" \
     "statements.c:5:10: error: statement requires expression with scalar type ('void' invalid)" \
     "statements.c:7:21: error: case value must be an integer constant expression" \
     "statements.c:32:16: warning: use of GNU case range extension [-Wgnu-case-range]" \
     "statements.c:33:14: error: duplicate case value '2'" \
     "statements.c:32:14: note: previous case defined here" \
+