Add two lints for cases where enum discriminants are no longer defined.

- `enum_discriminants_undefined_non_exhaustive_variant` checks for enums that gain a new non-exhaustive variant, which then causes the discriminant to become undefined.
- `enum_discriminants_undefined_non_unit_variant` checks for enums that gain a new non-unit variant, which then causes the discriminant to become undefined.

Resolves #898.

src/lints/enum_discriminants_undefined_non_exhaustive_variant.ron

@@ -0,0 +1,83 @@
+SemverQuery(
+    id: "enum_discriminants_undefined_non_exhaustive_variant",
+    human_readable_name: "enum's variants no longer have defined discriminants due to a non-exhaustive variant",
+    description: "A public enum's variants no longer have well-defined discriminants value due to a non-exhaustive variant.",
+    reference: Some("A public enum's variants no longer have well-defined discriminants due to a non-exhaustive variant. This breaks downstream code that accessed the discriminant via a numeric cast like `as isize`."),
+    required_update: Major,
+    lint_level: Deny,
+    reference_link: Some("https://doc.rust-lang.org/reference/items/enumerations.html#assigning-discriminant-values"),
+    query: r#"
+    {
+        CrateDiff {
+            baseline {
+                item {
+                    ... on Enum {
+                        visibility_limit @filter(op: "=", value: ["$public"]) @output
+                        enum_name: name @output @tag
+
+                        attribute @fold @transform(op: "count") @filter(op: "=", value: ["$zero"]) {
+                            content {
+                                base @filter(op: "=", value: ["$repr"])
+                            }
+                        }
+
+                        importable_path {
+                            path @output @tag
+                            public_api @filter(op: "=", value: ["$true"])
+                        }
+
+                        variant @fold @transform(op: "count") @filter(op: ">", value: ["$zero"]) {
+                            discriminant {
+                                value
+                            }
+                        }
+
+                        variant @fold @transform(op: "count") @filter(op: "=", value: ["$zero"]) {
+                            __typename @filter(op: "!=", value: ["$plain_variant"])
+                        }
+
+                        variant @fold @transform(op: "count") @filter(op: "=", value: ["$zero"]) {
+                            attrs @filter(op: "contains", value: ["$non_exhaustive"])
+                        }
+                    }
+                }
+            }
+            current {
+                item {
+                    ... on Enum {
+                        visibility_limit @filter(op: "=", value: ["$public"])
+                        name @filter(op: "=", value: ["%enum_name"])
+
+                        importable_path {
+                            path @filter(op: "=", value: ["%path"])
+                            public_api @filter(op: "=", value: ["$true"])
+                        }
+
+                        variant @fold @transform(op: "count") @filter(op: "=", value: ["$zero"]) {
+                            __typename @filter(op: "!=", value: ["$plain_variant"])
+                        }
+
+                        variant @fold @transform(op: "count") @filter(op: "=", value: ["$zero"]) {
+                            attrs @filter(op: "contains", value: ["$non_exhaustive"])
+                        }
+
+                        span_: span @optional {
+                            filename @output
+                            begin_line @output
+                        }
+                    }
+                }
+            }
+        }
+    }"#,
+    arguments: {
+        "public": "public",
+        "repr": "repr",
+        "non_exhaustive": "#[non_exhaustive]",
+        "plain_variant": "PlainVariant",
+        "zero": 0,
+        "true": true,
+    },
+    error_message: "An enum's variants no longer have well-defined discriminant values due to a non-exhaustive variant in the enum. This breaks downstream code that accesses discriminants via a numeric cast like `as isize`.",
+    per_result_error_template: Some("enum {{enum_name}} in {{span_filename}}:{{span_begin_line}}"),
+)

src/lints/enum_discriminants_undefined_non_unit_variant.ron

@@ -0,0 +1,79 @@
+SemverQuery(
+    id: "enum_discriminants_undefined_non_unit_variant",
+    human_readable_name: "enum's variants no longer have defined discriminants due to non-unit variant",
+    description: "A public enum's variants no longer have well-defined discriminants value due to a non-unit variant.",
+    reference: Some("A public enum's variants no longer have well-defined discriminants due to a non-unit variant. This breaks downstream code that accessed the discriminant via a numeric cast like `as isize`."),
+    required_update: Major,
+    lint_level: Deny,
+    reference_link: Some("https://doc.rust-lang.org/reference/items/enumerations.html#assigning-discriminant-values"),
+    query: r#"
+    {
+        CrateDiff {
+            baseline {
+                item {
+                    ... on Enum {
+                        visibility_limit @filter(op: "=", value: ["$public"]) @output
+                        enum_name: name @output @tag
+
+                        attribute @fold @transform(op: "count") @filter(op: "=", value: ["$zero"]) {
+                            content {
+                                base @filter(op: "=", value: ["$repr"])
+                            }
+                        }
+
+                        importable_path {
+                            path @output @tag
+                            public_api @filter(op: "=", value: ["$true"])
+                        }
+
+                        variant @fold @transform(op: "count") @filter(op: ">", value: ["$zero"]) {
+                            discriminant {
+                                value
+                            }
+                        }
+
+                        variant @fold @transform(op: "count") @filter(op: "=", value: ["$zero"]) {
+                            __typename @filter(op: "!=", value: ["$plain_variant"])
+                        }
+
+                        variant @fold @transform(op: "count") @filter(op: "=", value: ["$zero"]) {
+                            attrs @filter(op: "contains", value: ["$non_exhaustive"])
+                        }
+                    }
+                }
+            }
+            current {
+                item {
+                    ... on Enum {
+                        visibility_limit @filter(op: "=", value: ["$public"])
+                        name @filter(op: "=", value: ["%enum_name"])
+
+                        importable_path {
+                            path @filter(op: "=", value: ["%path"])
+                            public_api @filter(op: "=", value: ["$true"])
+                        }
+
+                        variant @fold @transform(op: "count") @filter(op: ">", value: ["$zero"]) {
+                            __typename @filter(op: "!=", value: ["$plain_variant"])
+                        }
+
+                        span_: span @optional {
+                            filename @output
+                            begin_line @output
+                        }
+                    }
+                }
+            }
+        }
+    }"#,
+    arguments: {
+        "public": "public",
+        "repr": "repr",
+        "non_exhaustive": "#[non_exhaustive]",
+        "plain_variant": "PlainVariant",
+        "zero": 0,
+        "true": true,
+    },
+    error_message: "An enum's variants no longer have well-defined discriminant values due to a tuple or struct variant in the enum. This breaks downstream code that accesses discriminants via a numeric cast like `as isize`.",
+    per_result_error_template: Some("enum {{enum_name}} in {{span_filename}}:{{span_begin_line}}"),
+)

src/query.rs

@@ -1057,6 +1057,8 @@ add_lints!(
     derive_helper_attr_removed,
     derive_proc_macro_missing,
     derive_trait_impl_removed,
+    enum_discriminants_undefined_non_exhaustive_variant,
+    enum_discriminants_undefined_non_unit_variant,
     enum_marked_non_exhaustive,
     enum_missing,
     enum_must_use_added,

test_crates/enum_discriminant_no_longer_defined/new/Cargo.toml

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

test_crates/enum_discriminant_no_longer_defined/new/src/lib.rs

@@ -0,0 +1,68 @@
+// It's not allowed to use `as isize` to cast another crate's enum when
+// the enum contains a non-exhaustive variant:
+//
+// ---- src/lib.rs - GainsNonExhaustiveVariant (line 1) stdout ----
+// error[E0606]: casting `GainsNonExhaustiveVariant` as `isize` is invalid
+//  --> src/lib.rs:4:5
+//   |
+// 6 |     value as isize;
+//   |     ^^^^^^^^^^^^^^
+//   |
+//   = note: cannot cast an enum with a non-exhaustive variant when it's defined in another crate
+//
+// error: aborting due to 1 previous error
+//
+// To see this, run the following doctest:
+/// ```rust
+/// fn example(value: enum_discriminant_no_longer_defined::GainsNonExhaustiveVariant) {
+///     value as isize;
+/// }
+/// ```
+#[non_exhaustive]
+pub enum GainsNonExhaustiveVariant {
+    First,
+    Second,
+    #[non_exhaustive]  // TODO: this needs to be flagged but currently isn't.
+    Third,
+}
+
+/// This shouldn't be reported: this enum's discriminants were not well-defined to begin with
+/// because of the non-unit variant.
+#[non_exhaustive]
+pub enum NonUnitVariantButGainsNonExhaustiveVariant {
+    First,
+    Second(u16),
+    #[non_exhaustive]
+    Third,
+}
+
+// It's not allowed to use `as isize` to cast another crate's enum when
+// the enum contains a non-unit variant:
+//
+// ---- src/lib.rs - GainsTupleVariant (line 1) stdout ----
+// error[E0605]: non-primitive cast: `GainsTupleVariant` as `isize`
+//  --> src/lib.rs:4:5
+//   |
+// 6 |     value as isize;
+//   |     ^^^^^^^^^^^^^^ an `as` expression can be used to convert enum types to numeric types only if the enum type is unit-only or field-less
+//   |
+//   = note: see https://doc.rust-lang.org/reference/items/enumerations.html#casting for more information
+//
+// To see this, run the following doctest:
+/// ```rust
+/// fn example(value: enum_discriminant_no_longer_defined::GainsTupleVariant) {
+///     value as isize;
+/// }
+/// ```
+#[non_exhaustive]
+pub enum GainsTupleVariant {
+    None,
+    Never,
+    Some(core::num::NonZeroUsize),
+}
+
+// Same as above, just with a struct variant instead of a tuple variant.
+pub enum GainsStructVariant {
+    None,
+    Some { value: core::num::NonZeroUsize },
+}

test_crates/enum_discriminant_no_longer_defined/old/Cargo.toml

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

test_crates/enum_discriminant_no_longer_defined/old/src/lib.rs

@@ -0,0 +1,62 @@
+// It's not allowed to use `as isize` to cast another crate's enum when
+// the enum contains a non-exhaustive variant:
+//
+// ---- src/lib.rs - GainsNonExhaustiveVariant (line 1) stdout ----
+// error[E0606]: casting `GainsNonExhaustiveVariant` as `isize` is invalid
+//  --> src/lib.rs:4:5
+//   |
+// 6 |     value as isize;
+//   |     ^^^^^^^^^^^^^^
+//   |
+//   = note: cannot cast an enum with a non-exhaustive variant when it's defined in another crate
+//
+// error: aborting due to 1 previous error
+//
+// To see this, run the following doctest:
+/// ```rust
+/// fn example(value: enum_discriminant_no_longer_defined::GainsNonExhaustiveVariant) {
+///     value as isize;
+/// }
+/// ```
+#[non_exhaustive]
+pub enum GainsNonExhaustiveVariant {
+    First,
+    Second,
+}
+
+/// This shouldn't be reported: this enum's discriminants were not well-defined to begin with
+/// because of the non-unit variant.
+#[non_exhaustive]
+pub enum NonUnitVariantButGainsNonExhaustiveVariant {
+    First,
+    Second(u16),
+}
+
+// It's not allowed to use `as isize` to cast another crate's enum when
+// the enum contains a non-unit variant:
+//
+// ---- src/lib.rs - GainsTupleVariant (line 1) stdout ----
+// error[E0605]: non-primitive cast: `GainsTupleVariant` as `isize`
+//  --> src/lib.rs:4:5
+//   |
+// 6 |     value as isize;
+//   |     ^^^^^^^^^^^^^^ an `as` expression can be used to convert enum types to numeric types only if the enum type is unit-only or field-less
+//   |
+//   = note: see https://doc.rust-lang.org/reference/items/enumerations.html#casting for more information
+//
+// To see this, run the following doctest:
+/// ```rust
+/// fn example(value: enum_discriminant_no_longer_defined::GainsTupleVariant) {
+///     value as isize;
+/// }
+/// ```
+#[non_exhaustive]
+pub enum GainsTupleVariant {
+    None,
+    Never,
+}
+
+// Same as above, just with a struct variant instead of a tuple variant.
+pub enum GainsStructVariant {
+    None,
+}