refactor(api): VALIDATE macros #22187

Problem:
- API validation involves too much boilerplate.
- API validation errors are not consistently worded.

Solution:
Introduce some macros. Currently these are clumsy, but they at least
help with consistency and avoid some nesting.

src/nvim/api/private/validate.h

@@ -0,0 +1,69 @@
+#ifndef NVIM_API_PRIVATE_VALIDATE_H
+#define NVIM_API_PRIVATE_VALIDATE_H
+
+#include "nvim/api/private/defs.h"
+#include "nvim/api/private/helpers.h"
+
+#define VALIDATE_INT(cond, name, val_, code) \
+  do { \
+    if (!(cond)) { \
+      api_set_error(err, kErrorTypeValidation, "Invalid " name ": %" PRId64, val_); \
+      code; \
+    } \
+  } while (0)
+
+#define VALIDATE_S(cond, name, val_, code) \
+  do { \
+    if (!(cond)) { \
+      if (strequal(val_, "")) { \
+        api_set_error(err, kErrorTypeValidation, "Invalid " name); \
+      } else { \
+        api_set_error(err, kErrorTypeValidation, "Invalid " name ": '%s'", val_); \
+      } \
+      code; \
+    } \
+  } while (0)
+
+#define VALIDATE_R(cond, name, code) \
+  do { \
+    if (!(cond)) { \
+      api_set_error(err, kErrorTypeValidation, "'" name "' is required"); \
+      code; \
+    } \
+  } while (0)
+
+#define VALIDATE_EXP(cond, name, expected, actual, code) \
+  do { \
+    if (!(cond)) { \
+      api_set_error(err, kErrorTypeValidation, "Invalid " name ": expected %s, got %s", \
+                    expected, actual); \
+      code; \
+    } \
+  } while (0)
+
+#define VALIDATE_T(name, expected_t, actual_t, code) \
+  do { \
+    if (expected_t != actual_t) { \
+      api_set_error(err, kErrorTypeValidation, "Invalid %s: expected %s, got %s", \
+                    name, api_typename(expected_t), api_typename(actual_t)); \
+      code; \
+    } \
+  } while (0)
+
+#define VALIDATE(cond, msg_, code) \
+  do { \
+    if (!(cond)) { \
+      api_set_error(err, kErrorTypeValidation, "%s", msg_); \
+      code; \
+    } \
+  } while (0)
+
+#define VALIDATE_FMT(cond, fmt_, msg_, code) \
+  do { \
+    if (!(cond)) { \
+      api_set_error(err, kErrorTypeValidation, fmt_, msg_); \
+      code; \
+    } \
+  } while (0)
+
+#endif  // NVIM_API_PRIVATE_VALIDATE_H