Adding #[must_use] on an item

src/lints/enum_must_use_added.ron

@@ -0,0 +1,64 @@
+SemverQuery(
+    id: "enum_must_use_added",
+    human_readable_name: "enum #[must_use] added",
+    description: "An enum has been marked with #[must_use].",
+    required_update: Minor,
+
+    // TODO: Change the reference link to point to the cargo semver reference
+    //       once it has a section on attribute [must_use].
+    reference_link: Some("https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-must_use-attribute"),
+    query: r#"
+    {
+        CrateDiff {
+            baseline {
+                item {
+                    ... on Enum {
+                        visibility_limit @filter(op: "=", value: ["$public"]) @output
+                        name @tag @output
+
+                        importable_path {
+                            path @tag @output
+                        }
+
+                        attribute @fold @transform(op: "count") @filter(op: "=", value: ["$zero"]) {
+                            content {
+                                base @filter(op: "=", value: ["$must_use"])
+                            }
+                        }
+                    }
+                }
+            }
+            current {
+                item {
+                    ... on Enum {
+                        visibility_limit @filter(op: "=", value: ["$public"])
+                        name @filter(op: "=", value: ["%name"])
+
+                        importable_path {
+                            path @filter(op: "=", value: ["%path"])
+                        }
+
+                        attribute {
+                            new_attr: raw_attribute @output
+                            content {
+                                base @filter(op: "=", value: ["$must_use"])
+                            }
+                        }
+
+                        span_: span @optional {
+                            filename @output
+                            begin_line @output
+                        }
+                    }
+                }
+            }
+        }
+    }"#,
+    arguments: {
+        "public": "public",
+        "must_use": "must_use",
+        "zero": 0,
+    },
+    error_message: "An enum has been marked with #[must_use]. This can cause downstream crates that did not use this enum's value to get a compiler lint.",
+    per_result_error_template: Some("enum {{name}} in {{span_filename}}:{{span_begin_line}}"),
+)

src/query.rs

@@ -436,4 +436,5 @@ add_lints!(
     tuple_struct_to_plain_struct,
     unit_struct_changed_kind,
     variant_marked_non_exhaustive,
+    enum_must_use_added,
 );

test_crates/enum_must_use_added/new/Cargo.toml

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

test_crates/enum_must_use_added/new/src/lib.rs

@@ -0,0 +1,43 @@
+// These enums did not have the #[must_use] attribute in the old version.
+// Addition of the attribute should be reported by this rule.
+
+#[must_use]
+pub enum EnumToMustUseEnum {
+    Bar,
+}
+#[must_use = "Foo"]
+pub enum EnumToMustUseMessageEnum {
+    Bar,
+}
+
+
+// These enums had the #[must_use] attribute in the old version. Changes of
+// the attribute, including deletion, should not be reported by this rule.
+
+pub enum MustUseEnumToEnum {
+    Bar,
+}
+
+#[must_use = "Foo"]
+pub enum MustUseEnumToMustUseMessageEnum {
+    Bar,
+}
+
+
+// These enums had the #[must_use] attribute in the old version.
+// They also included the user-defined warning message. Changes of
+// the attribute, including deletion, should not be reported by this rule.
+
+pub enum MustUseMessageEnumToEnum {
+    Bar,
+}
+
+#[must_use]
+pub enum MustUseMessageEnumToMustUseEnum {
+    Bar,
+}
+
+#[must_use = "Baz"]
+pub enum MustUseMessageEnumToMustUseMessageEnum {
+    Bar,
+}

test_crates/enum_must_use_added/old/Cargo.toml

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

test_crates/enum_must_use_added/old/src/lib.rs

@@ -0,0 +1,44 @@
+// These enums did not have the #[must_use] attribute in the old version.
+// Addition of the attribute should be reported by this rule.
+
+pub enum EnumToMustUseEnum {
+    Bar,
+}
+
+pub enum EnumToMustUseMessageEnum {
+    Bar,
+}
+
+
+// These enums had the #[must_use] attribute in the old version. Changes of
+// the attribute, including deletion, should not be reported by this rule.
+
+#[must_use]
+pub enum MustUseEnumToEnum {
+    Bar,
+}
+
+#[must_use]
+pub enum MustUseEnumToMustUseMessageEnum {
+    Bar,
+}
+
+
+// These enums had the #[must_use] attribute in the old version.
+// They also included the user-defined warning message. Changes of
+// the attribute, including deletion, should not be reported by this rule.
+
+#[must_use = "Foo"]
+pub enum MustUseMessageEnumToEnum {
+    Bar,
+}
+
+#[must_use = "Foo"]
+pub enum MustUseMessageEnumToMustUseEnum {
+    Bar,
+}
+
+#[must_use = "Foo"]
+pub enum MustUseMessageEnumToMustUseMessageEnum {
+    Bar,
+}

test_outputs/enum_must_use_added.output.ron

@@ -0,0 +1,26 @@
+{
+    "./test_crates/enum_must_use_added/": [
+        {
+            "name": String("EnumToMustUseEnum"),
+            "new_attr": String("#[must_use]"),
+            "path": List([
+                String("enum_must_use_added"),
+                String("EnumToMustUseEnum"),
+            ]),
+            "span_begin_line": Uint64(5),
+            "span_filename": String("src/lib.rs"),
+            "visibility_limit": String("public"),
+        },
+        {
+            "name": String("EnumToMustUseMessageEnum"),
+            "new_attr": String("#[must_use = \"Foo\"]"),
+            "path": List([
+                String("enum_must_use_added"),
+                String("EnumToMustUseMessageEnum"),
+            ]),
+            "span_begin_line": Uint64(9),
+            "span_filename": String("src/lib.rs"),
+            "visibility_limit": String("public"),
+        },
+    ],
+}