Add the enum_repr_variant_discriminant_changed lint.

Addresses a checkbox in #898.

src/lints/enum_repr_variant_discriminant_changed.ron

@@ -0,0 +1,89 @@
+SemverQuery(
+    id: "enum_repr_variant_discriminant_changed",
+    human_readable_name: "variant of an enum with explicit repr changed discriminant",
+    description: "The variant of an enum with explicit repr() had its discriminant change from its previous value.",
+    reference: Some("The variant of an enum with an explicit repr() had its discriminant value change. This breaks downstream code that accessed the discriminant via pointer casting."),
+    required_update: Major,
+    lint_level: Deny,
+    reference_link: Some("https://doc.rust-lang.org/reference/items/enumerations.html#pointer-casting"),
+    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"])
+                        }
+
+                        attribute {
+                            old_attr: raw_attribute @output
+                            content {
+                                base @filter(op: "=", value: ["$repr"])
+                                argument {
+                                    base @filter(op: "regex", value: ["$repr_regex"])
+                                }
+                            }
+                        }
+
+                        variant {
+                            variant_name: name @output @tag
+
+                            discriminant {
+                                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"])
+                        }
+
+                        attribute {
+                            new_attr: raw_attribute @output
+                            content {
+                                base @filter(op: "=", value: ["$repr"])
+                                argument {
+                                    base @filter(op: "regex", value: ["$repr_regex"])
+                                }
+                            }
+                        }
+
+                        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",
+        "repr": "repr",
+        "repr_regex": "[ui](\\d+|size)",
+        "true": true,
+    },
+    error_message: "An enum variant has changed its discriminant value. The enum has a defined primitive representation, so this breaks downstream code that used the discriminant value via an unsafe pointer cast.",
+    per_result_error_template: Some("variant {{enum_name}}::{{variant_name}} {{old_value}} -> {{new_value}} in {{span_filename}}:{{span_begin_line}}"),
+)

src/query.rs

@@ -1065,6 +1065,7 @@ add_lints!(
     enum_repr_int_changed,
     enum_repr_int_removed,
     enum_repr_transparent_removed,
+    enum_repr_variant_discriminant_changed,
     enum_struct_variant_field_added,
     enum_struct_variant_field_missing,
     enum_struct_variant_field_now_doc_hidden,

test_crates/enum_no_repr_variant_discriminant_changed/new/src/lib.rs

@@ -42,6 +42,14 @@ pub enum DiscriminantBecomesDocHiddenAndExplicit {
     Second = 2,
 }
 
+// This enum starts off not having repr(), then gains repr()
+// while also changing discriminant values. This should be reported here:
+// the enum originally has no ABI committments so the addition of the repr() doesn't matter.
+#[repr(u16)]
+pub enum GainsRepr {
+    First = 2,
+}
+
 // Explicit discriminants changed values, but being private dominates. Should not be
 // reported.
 enum PrivateEnum {

test_crates/enum_no_repr_variant_discriminant_changed/old/src/lib.rs

@@ -40,6 +40,13 @@ pub enum DiscriminantBecomesDocHiddenAndExplicit {
     Second,
 }
 
+// This enum starts off not having repr(), then gains repr()
+// while also changing discriminant values. This should be reported here:
+// the enum originally has no ABI committments so the addition of the repr() doesn't matter.
+pub enum GainsRepr {
+    First = 1,
+}
+
 // Explicit discriminants changed values, but being private dominates. Should not be
 // reported.
 enum PrivateEnum {

test_crates/enum_repr_variant_discriminant_changed/new/Cargo.toml

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

test_crates/enum_repr_variant_discriminant_changed/new/src/lib.rs

@@ -0,0 +1,25 @@
+// Explicit discriminant changed values. By doing so, it changed the implicit
+// discriminant's value as well. Should be reported.
+#[repr(isize)]
+pub enum ExplicitAndImplicitDiscriminantsAreChanged {
+    First = 2,
+    Second,
+    Third = 5,
+}
+
+// Discriminant changed values. Having #[non_exhaustive] on the enum should not have any effect
+// on the API or ABI breakage. Should be reported.
+#[repr(i16)]
+#[non_exhaustive]
+pub enum DiscriminantIsChanged {
+    First = 1,
+}
+
+// Discriminant changed values, while the other variant is marked `non_exhaustive`.
+// The variant's exhaustiveness is not relevant for pointer casting, so this should be reported.
+#[repr(u32)]
+pub enum DiscriminantIsChangedButAVariantIsNonExhaustive {
+    #[non_exhaustive]
+    First,
+    Second = 2,
+}

test_crates/enum_repr_variant_discriminant_changed/old/Cargo.toml

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

test_crates/enum_repr_variant_discriminant_changed/old/src/lib.rs

@@ -0,0 +1,26 @@
+// Explicit discriminant changed values. By doing so, it changed the implicit
+// discriminant's value as well. Should be reported.
+#[repr(isize)]
+pub enum ExplicitAndImplicitDiscriminantsAreChanged {
+    First = 1,
+    Second,
+    Third = 5,
+}
+
+// Discriminant changed values. Having #[non_exhaustive] on the enum should not have any effect
+// on the API or ABI breakage. Should be reported.
+#[repr(i16)]
+#[non_exhaustive]
+pub enum DiscriminantIsChanged {
+    First,
+}
+
+// Discriminant changed values, while the other variant is marked `non_exhaustive`.
+// The variant's exhaustiveness is not relevant for pointer casting, so this should be reported.
+#[non_exhaustive]
+#[repr(u32)]
+pub enum DiscriminantIsChangedButAVariantIsNonExhaustive {
+    #[non_exhaustive]
+    First,
+    Second,
+}

test_outputs/query_execution/enum_no_repr_variant_discriminant_changed.snap

@@ -1,6 +1,7 @@
 ---
 source: src/query.rs
 expression: "&query_execution_results"
+snapshot_kind: text
 ---
 {
   "./test_crates/enum_no_repr_variant_discriminant_changed/": [
@@ -56,5 +57,18 @@ expression: "&query_execution_results"
       "variant_name": String("Second"),
       "visibility_limit": String("public"),
     },
+    {
+      "enum_name": String("GainsRepr"),
+      "new_value": String("2"),
+      "old_value": String("1"),
+      "path": List([
+        String("enum_no_repr_variant_discriminant_changed"),
+        String("GainsRepr"),
+      ]),
+      "span_begin_line": Uint64(50),
+      "span_filename": String("src/lib.rs"),
+      "variant_name": String("First"),
+      "visibility_limit": String("public"),
+    },
   ],
 }

test_outputs/query_execution/enum_repr_variant_discriminant_changed.snap

@@ -0,0 +1,86 @@
+---
+source: src/query.rs
+expression: "&query_execution_results"
+snapshot_kind: text
+---
+{
+  "./test_crates/enum_no_repr_variant_discriminant_changed/": [
+    {
+      "enum_name": String("FieldlessWithDiscrimants"),
+      "new_attr": String("#[repr(u8)]"),
+      "new_value": String("21"),
+      "old_attr": String("#[repr(u8)]"),
+      "old_value": String("20"),
+      "path": List([
+        String("enum_no_repr_variant_discriminant_changed"),
+        String("FieldlessWithDiscrimants"),
+      ]),
+      "span_begin_line": Uint64(68),
+      "span_filename": String("src/lib.rs"),
+      "variant_name": String("Last"),
+      "visibility_limit": String("public"),
+    },
+  ],
+  "./test_crates/enum_repr_variant_discriminant_changed/": [
+    {
+      "enum_name": String("ExplicitAndImplicitDiscriminantsAreChanged"),
+      "new_attr": String("#[repr(isize)]"),
+      "new_value": String("2"),
+      "old_attr": String("#[repr(isize)]"),
+      "old_value": String("1"),
+      "path": List([
+        String("enum_repr_variant_discriminant_changed"),
+        String("ExplicitAndImplicitDiscriminantsAreChanged"),
+      ]),
+      "span_begin_line": Uint64(5),
+      "span_filename": String("src/lib.rs"),
+      "variant_name": String("First"),
+      "visibility_limit": String("public"),
+    },
+    {
+      "enum_name": String("ExplicitAndImplicitDiscriminantsAreChanged"),
+      "new_attr": String("#[repr(isize)]"),
+      "new_value": String("3"),
+      "old_attr": String("#[repr(isize)]"),
+      "old_value": String("2"),
+      "path": List([
+        String("enum_repr_variant_discriminant_changed"),
+        String("ExplicitAndImplicitDiscriminantsAreChanged"),
+      ]),
+      "span_begin_line": Uint64(6),
+      "span_filename": String("src/lib.rs"),
+      "variant_name": String("Second"),
+      "visibility_limit": String("public"),
+    },
+    {
+      "enum_name": String("DiscriminantIsChanged"),
+      "new_attr": String("#[repr(i16)]"),
+      "new_value": String("1"),
+      "old_attr": String("#[repr(i16)]"),
+      "old_value": String("0"),
+      "path": List([
+        String("enum_repr_variant_discriminant_changed"),
+        String("DiscriminantIsChanged"),
+      ]),
+      "span_begin_line": Uint64(15),
+      "span_filename": String("src/lib.rs"),
+      "variant_name": String("First"),
+      "visibility_limit": String("public"),
+    },
+    {
+      "enum_name": String("DiscriminantIsChangedButAVariantIsNonExhaustive"),
+      "new_attr": String("#[repr(u32)]"),
+      "new_value": String("2"),
+      "old_attr": String("#[repr(u32)]"),
+      "old_value": String("1"),
+      "path": List([
+        String("enum_repr_variant_discriminant_changed"),
+        String("DiscriminantIsChangedButAVariantIsNonExhaustive"),
+      ]),
+      "span_begin_line": Uint64(24),
+      "span_filename": String("src/lib.rs"),
+      "variant_name": String("Second"),
+      "visibility_limit": String("public"),
+    },
+  ],
+}

test_outputs/query_execution/enum_struct_variant_field_added.snap

@@ -1,6 +1,7 @@
 ---
 source: src/query.rs
 expression: "&query_execution_results"
+snapshot_kind: text
 ---
 {
   "./test_crates/enum_no_repr_variant_discriminant_changed/": [
@@ -11,7 +12,7 @@ expression: "&query_execution_results"
         String("enum_no_repr_variant_discriminant_changed"),
         String("UnitOnlyBecomesUndefined"),
       ]),
-      "span_begin_line": Uint64(77),
+      "span_begin_line": Uint64(85),
       "span_filename": String("src/lib.rs"),
       "variant_name": String("Struct"),
     },