Add lint for exhaustive struct variants gaining new fields.

src/lints/enum_struct_variant_field_added.ron

@@ -0,0 +1,75 @@
+SemverQuery(
+    id: "enum_struct_variant_field_added",
+    human_readable_name: "pub enum struct variant field added",
+    description: "An enum's exhaustive struct variant has a new field.",
+    required_update: Major,
+    reference_link: Some("https://doc.rust-lang.org/reference/attributes/type_system.html#the-non_exhaustive-attribute"),
+    query: r#"
+    {
+        CrateDiff {
+            current {
+                item {
+                    ... on Enum {
+                        visibility_limit @filter(op: "=", value: ["$public"])
+                        enum_name: name @output @tag
+
+                        importable_path {
+                            path @output @tag
+                        }
+
+                        variant {
+                            ... on StructVariant {
+                                # If the variant is now marked `#[non_exhaustive]`,
+                                # that's already a breaking change that has its own rule.
+                                # Don't also report new field additions, since the programmer has
+                                # clearly stated that they don't consider it exhaustive anymore.
+                                attrs @filter(op: "not_contains", value: ["$non_exhaustive"])
+
+                                variant_name: name @output @tag
+
+                                field {
+                                    field_name: name @output @tag
+
+                                    span_: span @optional {
+                                        filename @output
+                                        begin_line @output
+                                    }
+                                }
+                            }
+                        }
+                    }
+                }
+            }
+            baseline {
+                item {
+                    ... on Enum {
+                        visibility_limit @filter(op: "=", value: ["$public"])
+                        name @filter(op: "=", value: ["%enum_name"])
+
+                        importable_path {
+                            path @filter(op: "=", value: ["%path"])
+                        }
+
+                        variant {
+                            ... on StructVariant {
+                                name @filter(op: "=", value: ["%variant_name"])
+                                attrs @filter(op: "not_contains", value: ["$non_exhaustive"])
+
+                                field @fold @transform(op: "count") @filter(op: "=", value: ["$zero"]) {
+                                    name @filter(op: "=", value: ["%field_name"])
+                                }
+                            }
+                        }
+                    }
+                }
+            }
+        }
+    }"#,
+    arguments: {
+        "public": "public",
+        "zero": 0,
+        "non_exhaustive": "#[non_exhaustive]",
+    },
+    error_message: "An enum's exhaustive struct variant has a new field, which has to be included when constructing or matching on this variant.",
+    per_result_error_template: Some("field {{field_name}} of variant {{enum_name}}::{{variant_name}} in {{span_filename}}:{{span_begin_line}}"),
+)

src/query.rs

@@ -342,9 +342,10 @@ add_lints!(
     enum_repr_c_removed,
     enum_repr_int_changed,
     enum_repr_int_removed,
+    enum_struct_variant_field_added,
+    enum_struct_variant_field_missing,
     enum_variant_added,
     enum_variant_missing,
-    enum_struct_variant_field_missing,
     function_const_removed,
     function_missing,
     function_parameter_count_changed,

test_crates/enum_struct_variant_field_added/new/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "enum_struct_variant_field_added"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

test_crates/enum_struct_variant_field_added/new/src/lib.rs

@@ -0,0 +1,36 @@
+pub enum PubEnum {
+    Foo {
+        x: i64,
+        y: usize,
+    }
+}
+
+// This enum is not public, so it should not be reported by the lint,
+// regardless of the new field in the variant.
+pub(crate) enum PrivateEnum {
+    Foo {
+        x: i64,
+        y: usize,
+    }
+}
+
+// This enum's variant is non-exhaustive, so it can't be externally constructed
+// and can't be matched on without a `..`, and therefore should not be reported by the lint,
+// regardless of the new field in the variant.
+pub enum EnumWithNonExhaustiveVariant {
+    #[non_exhaustive]
+    Variant {
+        x: i64,
+        y: usize,
+    }
+}
+
+// This enum's variant will become non-exhaustive while also gaining a field.
+// That should trigger a separate lint, and not this one.
+pub enum EnumWithExhaustiveVariant {
+    #[non_exhaustive]
+    ExhaustiveVariant {
+        x: i64,
+        y: usize,
+    }
+}

test_crates/enum_struct_variant_field_added/old/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "enum_struct_variant_field_added"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

test_crates/enum_struct_variant_field_added/old/src/lib.rs

@@ -0,0 +1,31 @@
+pub enum PubEnum {
+    Foo {
+        x: i64,
+    }
+}
+
+// This enum is not public, so it should not be reported by the lint,
+// regardless of the new field in the variant.
+pub(crate) enum PrivateEnum {
+    Foo {
+        x: i64,
+    }
+}
+
+// This enum's variant is non-exhaustive, so it can't be externally constructed
+// and can't be matched on without a `..`, and therefore should not be reported by the lint,
+// regardless of the new field in the variant.
+pub enum EnumWithNonExhaustiveVariant {
+    #[non_exhaustive]
+    Variant {
+        x: i64,
+    }
+}
+
+// This enum's variant will become non-exhaustive while also gaining a field.
+// That should trigger a separate lint, and not this one.
+pub enum EnumWithExhaustiveVariant {
+    ExhaustiveVariant {
+        x: i64,
+    }
+}

test_outputs/enum_struct_variant_field_added.output.ron

@@ -0,0 +1,15 @@
+{
+    "./test_crates/enum_struct_variant_field_added/": [
+        {
+            "enum_name": String("PubEnum"),
+            "field_name": String("y"),
+            "path": List([
+                String("enum_struct_variant_field_added"),
+                String("PubEnum"),
+            ]),
+            "span_begin_line": Uint64(4),
+            "span_filename": String("src/lib.rs"),
+            "variant_name": String("Foo"),
+        },
+    ],
+}

test_outputs/variant_marked_non_exhaustive.output.ron

@@ -1,4 +1,17 @@
 {
+    "./test_crates/enum_struct_variant_field_added/": [
+        {
+            "name": String("EnumWithExhaustiveVariant"),
+            "path": List([
+                String("enum_struct_variant_field_added"),
+                String("EnumWithExhaustiveVariant"),
+            ]),
+            "span_begin_line": Uint64(32),
+            "span_filename": String("src/lib.rs"),
+            "variant_name": String("ExhaustiveVariant"),
+            "visibility_limit": String("public"),
+        },
+    ],
     "./test_crates/non_exhaustive/": [
         {
             "name": String("MyEnum"),