Add semver check for unit structs becoming plain structs.

scripts/regenerate_test_rustdocs.sh

@@ -24,6 +24,7 @@ features=(
     'struct_pub_field_missing'
     'enum_missing'
     'enum_variant_missing'
+    'unit_struct_changed_kind'
 )
 for feat in "${features[@]}"
 do

semver_tests/Cargo.toml

@@ -13,3 +13,4 @@ struct_missing = []
 struct_pub_field_missing = []
 enum_missing = []
 enum_variant_missing = []
+unit_struct_changed_kind = []

semver_tests/src/test_cases/mod.rs

@@ -23,3 +23,17 @@ pub enum VariantWillBeRemoved {
     #[cfg(not(feature = "enum_variant_missing"))]
     Bar,
 }
+
+#[cfg(not(feature = "unit_struct_changed_kind"))]
+pub struct UnitStructToPlain;
+
+#[cfg(feature = "unit_struct_changed_kind")]
+pub struct UnitStructToPlain {}
+
+#[cfg(not(feature = "unit_struct_changed_kind"))]
+#[non_exhaustive]
+pub struct NonExhaustiveUnitStructToPlain;
+
+#[cfg(feature = "unit_struct_changed_kind")]
+#[non_exhaustive]
+pub struct NonExhaustiveUnitStructToPlain {}

src/adapter.rs

@@ -748,6 +748,7 @@ mod tests {
         enum_variant_missing,
         struct_missing,
         struct_pub_field_missing,
+        unit_struct_changed_kind,
         struct_marked_non_exhaustive,
     );
 }

src/queries/unit_struct_changed_kind.ron

@@ -0,0 +1,59 @@
+SemverQuery(
+    id: "unit_struct_changed_kind",
+    human_readable_name: "unit struct changed kind",
+    description: "A public struct that was previously a unit struct is now a plain struct. The unit struct was not marked #[non_exhaustive], so it could be constructed outside of the defining crate. Plain structs cannot be constructed using the syntax allowed for unit structs, so this is a major breaking change for code that depends on it.",
+    required_update: Major,
+
+    // TODO: Change the reference link once this cargo docs PR merges:
+    // https://github.com/rust-lang/cargo/pull/10871
+    //
+    // Change to this link:
+    // https://doc.rust-lang.org/cargo/reference/semver.html#struct-unit-to-normal
+    reference_link: Some("https://github.com/rust-lang/cargo/pull/10871"),
+    query: r#"
+    {
+        CrateDiff {
+            baseline {
+                item {
+                    ... on Struct {
+                        visibility_limit @filter(op: "=", value: ["$public"]) @output
+                        name @output @tag
+                        struct_type @filter(op: "=", value: ["$unit"])
+                        attrs @filter(op: "not_contains", value: ["$non_exhaustive"])
+
+                        path {
+                            path @output @tag
+                        }
+
+                        span_: span @optional {
+                            filename @output
+                            begin_line @output
+                        }
+                    }
+                }
+            }
+            current @fold @transform(op: "count") @filter(op: ">=", value: ["$one"]) {
+                item {
+                    ... on Struct {
+                        visibility_limit @filter(op: "=", value: ["$public"])
+                        name @filter(op: "=", value: ["%name"])
+                        struct_type @filter(op: "=", value: ["$plain"])
+
+                        path {
+                            path @filter(op: "=", value: ["%path"])
+                        }
+                    }
+                }
+            }
+        }
+    }"#,
+    arguments: {
+        "public": "public",
+        "unit": "unit",
+        "plain": "plain",
+        "one": 1,
+        "non_exhaustive": "#[non_exhaustive]",
+    },
+    error_message: "A public unit struct has been changed to a normal (curly-braces) struct, which cannot be constructed using the same struct literal syntax.",
+    per_result_error_template: Some("struct {{name}}, previously in file {{span_filename}}:{{span_begin_line}}"),
+)

src/test_data/unit_struct_changed_kind.output.ron

@@ -0,0 +1,9 @@
+[
+    {
+        "name": String("UnitStructToPlain"),
+        "path": List([String("semver_tests"), String("test_cases"), String("UnitStructToPlain")]),
+        "visibility_limit": String("public"),
+        "span_filename": String("src/test_cases/mod.rs"),
+        "span_begin_line": Uint64(28),
+    }
+]