disallow declarations between container fields

test/compile_errors.zig

@@ -2,6 +2,21 @@ const tests = @import("tests.zig");
 const std = @import("std");
 
 pub fn addCases(cases: *tests.CompileErrorContext) void {
+    cases.add("declaration between fields",
+        \\const S = struct {
+        \\    a: usize,
+        \\    const foo = 2;
+        \\    const bar = 2;
+        \\    const baz = 2;
+        \\    b: usize,
+        \\};
+        \\comptime {
+        \\    _ = S;
+        \\}
+    , &[_][]const u8{
+        "tmp.zig:3:5: error: declarations are not allowed between container fields",
+    });
+
     cases.add("non-extern function with var args",
         \\fn foo(args: ...) void {}
         \\export fn entry() void {