Adds union_must_use_added lint (#845)

* adds union_must_use_added lint, with the correct changes in the documentation

* fixes windows double slashes

* Apply suggestions from code review

---------

Co-authored-by: Predrag Gruevski <[email protected]>

README.md

@@ -416,6 +416,7 @@ inherent_method_must_use_added = "warn"
 struct_must_use_added = "warn"
 enum_must_use_added = "warn"
 trait_must_use_added = "warn"
+union_must_use_added = "warn"
 ```
 
 #### Common configurations: Disable `#[must_use]` lints entirely
@@ -428,6 +429,7 @@ inherent_method_must_use_added = "allow"
 struct_must_use_added = "allow"
 enum_must_use_added = "allow"
 trait_must_use_added = "allow"
+union_must_use_added = "allow"
 ```
 
 #### Implementation details & limitations

src/lints/union_must_use_added.ron

@@ -0,0 +1,64 @@
+SemverQuery(
+    id: "union_must_use_added",
+    human_readable_name: "union #[must_use] added",
+    description: "A union has been marked with #[must_use].",
+    required_update: Minor,
+    lint_level: Deny,
+    reference_link: Some("https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-must_use-attribute"),
+    query: r#"
+    {
+        CrateDiff {
+            current {
+                item {
+                    ... on Union {
+                        visibility_limit @filter(op: "=", value: ["$public"])
+                        name @output
+
+                        importable_path {
+                            path @tag @output
+                            public_api @filter(op: "=", value: ["$true"])
+                        }
+
+                        attribute {
+                            new_attr: raw_attribute @output
+                            content {
+                                base @filter(op: "=", value: ["$must_use"])
+                            }
+                        }
+
+                        span_: span @optional {
+                            filename @output
+                            begin_line @output
+                        }
+                    }
+                }
+            }
+            baseline {
+                item {
+                    ... on Union {
+                        visibility_limit @filter(op: "=", value: ["$public"]) @output
+
+                        importable_path {
+                            path @filter(op: "=", value: ["%path"])
+                            public_api @filter(op: "=", value: ["$true"])
+                        }
+
+                        attribute @fold @transform(op: "count") @filter(op: "=", value: ["$zero"]) {
+                            content {
+                                base @filter(op: "=", value: ["$must_use"])
+                            }
+                        }
+                    }
+                }
+            }
+        }
+    }"#,
+    arguments: {
+        "public": "public",
+        "must_use": "must_use",
+        "true": true,
+        "zero": 0,
+    },
+    error_message: "A union is now #[must_use]. Downstream crates that did not use its value will get a compiler lint.",
+    per_result_error_template: Some("union {{name}} in {{span_filename}}:{{span_begin_line}}"),
+)

src/query.rs

@@ -801,6 +801,7 @@ add_lints!(
     type_marked_deprecated,
     union_field_missing,
     union_missing,
+    union_must_use_added,
     union_now_doc_hidden,
     union_pub_field_now_doc_hidden,
     unit_struct_changed_kind,

test_crates/union_must_use_added/new/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+publish = false
+name = "union_must_use_added"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

test_crates/union_must_use_added/new/src/lib.rs

@@ -0,0 +1,62 @@
+// These unions did not have the #[must_use] attribute in the old version.
+// Addition of the attribute should be reported by this rule.
+
+#[must_use]
+pub union UnionToMustUseUnion {
+    bar: u64,
+}
+
+#[must_use = "Foo"]
+pub union UnionToMustUseMessageUnion {
+    bar: u64,
+}
+
+
+// These unions had the #[must_use] attribute in the old version. Changes of
+// the attribute, including deletion, should NOT be reported by this rule.
+
+pub union MustUseUnionToUnion {
+    bar: u64,
+}
+
+#[must_use = "Foo"]
+pub union MustUseUnionToMustUseMessageUnion {
+    bar: u64,
+}
+
+
+// These unions 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 union MustUseMessageUnionToUnion {
+    bar: u64,
+}
+
+#[must_use]
+pub union MustUseMessageUnionToMustUseUnion {
+    bar: u64,
+}
+
+#[must_use = "Baz"]
+pub union MustUseMessageUnionToMustUseMessageUnion {
+    bar: u64,
+}
+
+
+// This union is private and should NOT be reported by this rule.
+
+#[must_use]
+union MustUsePrivateUnion {
+    bar: u64,
+}
+
+
+// This union was added in the new version of the crate with its attribute.
+// It should NOT be reported by this rule to avoid duplicate lints.
+// It should be reported as a new pub type that is part of the crate's API.
+
+#[must_use]
+pub union MustUseNewUnion {
+    bar: u64,
+}

test_crates/union_must_use_added/old/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+publish = false
+name = "union_must_use_added"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

test_crates/union_must_use_added/old/src/lib.rs

@@ -0,0 +1,51 @@
+// These unions did not have the #[must_use] attribute in the old version.
+// Addition of the attribute should be reported by this rule.
+
+pub union UnionToMustUseUnion {
+    bar: u64,
+}
+
+pub union UnionToMustUseMessageUnion {
+    bar: u64,
+}
+
+
+// These unions 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 union MustUseUnionToUnion {
+    bar: u64,
+}
+
+#[must_use]
+pub union MustUseUnionToMustUseMessageUnion {
+    bar: u64,
+}
+
+
+// These unions 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 union MustUseMessageUnionToUnion {
+    bar: u64,
+}
+
+#[must_use = "Foo"]
+pub union MustUseMessageUnionToMustUseUnion {
+    bar: u64,
+}
+
+#[must_use = "Foo"]
+pub union MustUseMessageUnionToMustUseMessageUnion {
+    bar: u64,
+}
+
+
+// This union is private and should NOT be reported by this rule.
+
+union MustUsePrivateUnion {
+    bar: u64,
+}

test_outputs/union_must_use_added.output.ron

@@ -0,0 +1,26 @@
+{
+    "./test_crates/union_must_use_added/": [
+        {
+            "name": String("UnionToMustUseUnion"),
+            "new_attr": String("#[must_use]"),
+            "path": List([
+                String("union_must_use_added"),
+                String("UnionToMustUseUnion"),
+            ]),
+            "span_begin_line": Uint64(5),
+            "span_filename": String("src/lib.rs"),
+            "visibility_limit": String("public"),
+        },
+        {
+            "name": String("UnionToMustUseMessageUnion"),
+            "new_attr": String("#[must_use = \"Foo\"]"),
+            "path": List([
+                String("union_must_use_added"),
+                String("UnionToMustUseMessageUnion"),
+            ]),
+            "span_begin_line": Uint64(10),
+            "span_filename": String("src/lib.rs"),
+            "visibility_limit": String("public"),
+        },
+    ],
+}