Add enum variant discriminant changed lint

src/lints/enum_variant_discriminant_changed.ron

@@ -0,0 +1,65 @@
+SemverQuery(
+    id: "enum_variant_discriminant_changed",
+    human_readable_name: "Public enum's variant had its discriminant changed from its previous value",
+    description: "A public enum's variant had its discriminant changed from its previous value.",
+    reference: Some("The public enum's variant had its discriminant changed from its previous value. This can cause compatibility issues, which may break FFI use cases."),
+    required_update: Major,
+    lint_level: Deny,
+    reference_link: Some("https://doc.rust-lang.org/reference/items/enumerations.html#discriminants"),
+    query: r#"
+    {
+        CrateDiff {
+            baseline {
+                item {
+                    ... on Enum {
+                        visibility_limit @filter(op: "=", value: ["$public"]) @output
+                        enum_name: name @output @tag
+
+                        importable_path {
+                            path @output @tag
+                            public_api @filter(op: "=", value: ["$true"])
+                        }
+
+                        variant {
+                            variant_name: name @output @tag
+                            discriminant @optional {
+                                old_value: value @output @tag
+                            }
+                        }
+                    }
+                }
+            }
+            current {
+                item {
+                    ... on Enum {
+                        visibility_limit @filter(op: "=", value: ["$public"])
+                        name @filter(op: "=", value: ["%enum_name"])
+
+                        importable_path {
+                            path @filter(op: "=", value: ["%path"])
+                            public_api @filter(op: "=", value: ["$true"])
+                        }
+
+                        variant {
+                            name @filter(op: "=", value: ["%variant_name"])
+                            discriminant {
+                                new_value: value @output @filter (op: "!=", value: ["%old_value"])
+                            }
+                        }
+
+                        span_: span @optional {
+                            filename @output
+                            begin_line @output
+                        }
+                    }
+                }
+            }
+        }
+    }"#,
+    arguments: {
+        "public": "public",
+        "true": true,
+    },
+    error_message: "The public enum's variant had its discriminant changed from its previous value. This can cause compatibility issues, which may break FFI use cases.",
+    per_result_error_template: Some("variant {{enum_name}}::{{variant_name}} {{old_value}} -> {{new_value}} in {{span_filename}}:{{span_begin_line}}"),
+)

src/query.rs

@@ -805,6 +805,7 @@ add_lints!(
     enum_tuple_variant_field_now_doc_hidden,
     enum_unit_variant_changed_kind,
     enum_variant_added,
+    enum_variant_discriminant_changed,
     enum_variant_marked_non_exhaustive,
     enum_variant_missing,
     exported_function_changed_abi,

test_crates/enum_variant_discriminant_changed/new/Cargo.toml

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

test_crates/enum_variant_discriminant_changed/new/src/lib.rs

@@ -0,0 +1,31 @@
+// Explicit discriminant changed values. By doing so, it changed the implicit
+// discriminant's value as well, should be reported.
+#[repr(u8, C)]
+pub enum ExplicitAndImplicitDiscriminantsAreChanged {
+    First = 2,
+    Second,
+    Third = 5,
+}
+
+// Implicit discriminant changed values when becoming explicit, should be reported.
+#[repr(u8)]
+pub enum ImplicitDiscriminantBecomesExplicit {
+    Tuple(),
+    Struct,
+    Unit = 5,
+}
+
+// Discriminant changed to be doc hidden and explicit. Being doc hidden is not relevant
+// since it's still part of the public API, should be reported.
+pub enum DiscriminantBecomesDocHiddenAndExplicit {
+    First,
+    #[doc(hidden)]
+    Second = 2,
+}
+
+// Explicit discriminants changed values, but being private dominates, should not be
+// reported.
+enum PrivateEnum {
+    First = 10,
+    Second = 11,
+}

test_crates/enum_variant_discriminant_changed/old/Cargo.toml

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

test_crates/enum_variant_discriminant_changed/old/src/lib.rs

@@ -0,0 +1,30 @@
+// Explicit discriminant changed values. By doing so, it changed the implicit
+// discriminant's value as well, should be reported.
+#[repr(u8, C)]
+pub enum ExplicitAndImplicitDiscriminantsAreChanged {
+    First = 1,
+    Second,
+    Third = 5,
+}
+
+// Implicit discriminant changed values when becoming explicit, should be reported.
+#[repr(u8)]
+pub enum ImplicitDiscriminantBecomesExplicit {
+    Tuple(),
+    Struct {},
+    Unit,
+}
+
+// Discriminant changed to be doc hidden and explicit. Being doc hidden is not relevant
+// since it's still part of the public API, should be reported.
+pub enum DiscriminantBecomesDocHiddenAndExplicit {
+    First,
+    Second,
+}
+
+// Explicit discriminants changed values, but being private dominates, should not be
+// reported.
+enum PrivateEnum {
+    First = 1,
+    Second = 2,
+}

test_outputs/enum_variant_discriminant_changed.output.ron

@@ -0,0 +1,56 @@
+{
+    "./test_crates/enum_variant_discriminant_changed/": [
+        {
+            "enum_name": String("ExplicitAndImplicitDiscriminantsAreChanged"),
+            "new_value": String("2"),
+            "old_value": String("1"),
+            "path": List([
+                String("enum_variant_discriminant_changed"),
+                String("ExplicitAndImplicitDiscriminantsAreChanged"),
+            ]),
+            "span_begin_line": Uint64(4),
+            "span_filename": String("src/lib.rs"),
+            "variant_name": String("First"),
+            "visibility_limit": String("public"),
+        },
+        {
+            "enum_name": String("ExplicitAndImplicitDiscriminantsAreChanged"),
+            "new_value": String("3"),
+            "old_value": String("2"),
+            "path": List([
+                String("enum_variant_discriminant_changed"),
+                String("ExplicitAndImplicitDiscriminantsAreChanged"),
+            ]),
+            "span_begin_line": Uint64(4),
+            "span_filename": String("src/lib.rs"),
+            "variant_name": String("Second"),
+            "visibility_limit": String("public"),
+        },
+        {
+            "enum_name": String("ImplicitDiscriminantBecomesExplicit"),
+            "new_value": String("5"),
+            "old_value": String("2"),
+            "path": List([
+                String("enum_variant_discriminant_changed"),
+                String("ImplicitDiscriminantBecomesExplicit"),
+            ]),
+            "span_begin_line": Uint64(12),
+            "span_filename": String("src/lib.rs"),
+            "variant_name": String("Unit"),
+            "visibility_limit": String("public"),
+        },
+        {
+            "enum_name": String("DiscriminantBecomesDocHiddenAndExplicit"),
+            "new_value": String("2"),
+            "old_value": String("1"),
+            "path": List([
+                String("enum_variant_discriminant_changed"),
+                String("DiscriminantBecomesDocHiddenAndExplicit"),
+            ]),
+            "span_begin_line": Uint64(20),
+            "span_filename": String("src/lib.rs"),
+            "variant_name": String("Second"),
+            "visibility_limit": String("public"),
+        },
+    ]
+}