Add validations for conflicts of flatten with other meta items

flatten doesn't work with many of the other field-level options, so emit errors when they are used together.

core/src/options/input_field.rs

@@ -45,6 +45,7 @@ impl InputField {
             skip: *self.skip.unwrap_or_default(),
             post_transform: self.post_transform.as_ref(),
             multiple: self.multiple.unwrap_or_default(),
+            flatten: self.flatten.is_present(),
         }
     }
 
@@ -132,6 +133,12 @@ impl ParseAttribute for InputField {
             }
 
             self.attr_name = FromMeta::from_meta(mi)?;
+
+            if self.flatten.is_present() {
+                return Err(
+                    Error::custom("`flatten` and `rename` cannot be used together").with_span(mi),
+                );
+            }
         } else if path.is_ident("default") {
             if self.default.is_some() {
                 return Err(Error::duplicate_field_path(path).with_span(mi));
@@ -143,12 +150,24 @@ impl ParseAttribute for InputField {
             }
 
             self.with = Some(FromMeta::from_meta(mi)?);
+
+            if self.flatten.is_present() {
+                return Err(
+                    Error::custom("`flatten` and `with` cannot be used together").with_span(mi),
+                );
+            }
         } else if path.is_ident("skip") {
             if self.skip.is_some() {
                 return Err(Error::duplicate_field_path(path).with_span(mi));
             }
 
             self.skip = FromMeta::from_meta(mi)?;
+
+            if self.skip.map(|v| *v).unwrap_or_default() && self.flatten.is_present() {
+                return Err(
+                    Error::custom("`flatten` and `skip` cannot be used together").with_span(mi),
+                );
+            }
         } else if path.is_ident("map") || path.is_ident("and_then") {
             let transformer = path.get_ident().unwrap().clone();
             if let Some(post_transform) = &self.post_transform {
@@ -173,12 +192,46 @@ impl ParseAttribute for InputField {
             }
 
             self.multiple = FromMeta::from_meta(mi)?;
+
+            if self.multiple == Some(true) && self.flatten.is_present() {
+                return Err(
+                    Error::custom("`flatten` and `multiple` cannot be used together").with_span(mi),
+                );
+            }
         } else if path.is_ident("flatten") {
             if self.flatten.is_present() {
                 return Err(Error::duplicate_field_path(path).with_span(mi));
             }
 
             self.flatten = FromMeta::from_meta(mi)?;
+
+            let mut conflicts = Error::accumulator();
+
+            if self.multiple == Some(true) {
+                conflicts.push(
+                    Error::custom("`flatten` and `multiple` cannot be used together").with_span(mi),
+                );
+            }
+
+            if self.attr_name.is_some() {
+                conflicts.push(
+                    Error::custom("`flatten` and `rename` cannot be used together").with_span(mi),
+                );
+            }
+
+            if self.with.is_some() {
+                conflicts.push(
+                    Error::custom("`flatten` and `with` cannot be used together").with_span(mi),
+                );
+            }
+
+            if self.skip.map(|v| *v).unwrap_or_default() {
+                conflicts.push(
+                    Error::custom("`flatten` and `skip` cannot be used together").with_span(mi),
+                );
+            }
+
+            conflicts.finish()?;
         } else {
             return Err(Error::unknown_field_path(path).with_span(mi));
         }

tests/compile-fail/flatten_meta_conflicts.rs

@@ -0,0 +1,21 @@
+use darling::FromMeta;
+
+#[derive(FromMeta)]
+struct Inner {
+    left: String,
+    right: String,
+}
+
+#[derive(FromMeta)]
+struct Outer {
+    #[darling(flatten, multiple, with = demo, skip = true)]
+    field: Inner,
+}
+
+#[derive(FromMeta)]
+struct ThisIsFine {
+    #[darling(flatten, multiple = false)]
+    field: Inner,
+}
+
+fn main() {}

tests/compile-fail/flatten_meta_conflicts.stderr

@@ -0,0 +1,17 @@
+error: `flatten` and `multiple` cannot be used together
+  --> tests/compile-fail/flatten_meta_conflicts.rs:11:24
+   |
+11 |     #[darling(flatten, multiple, with = demo, skip = true)]
+   |                        ^^^^^^^^
+
+error: `flatten` and `with` cannot be used together
+  --> tests/compile-fail/flatten_meta_conflicts.rs:11:34
+   |
+11 |     #[darling(flatten, multiple, with = demo, skip = true)]
+   |                                  ^^^^
+
+error: `flatten` and `skip` cannot be used together
+  --> tests/compile-fail/flatten_meta_conflicts.rs:11:47
+   |
+11 |     #[darling(flatten, multiple, with = demo, skip = true)]
+   |                                               ^^^^