Add type_mismatched_generic_lifetimes lint.

It catches structs, enums, and unions that have gained or lost generic lifetime parameters. Using such a type with a mismatching number of generic lifetime parameters is a compile error.

src/lints/type_mismatched_generic_lifetimes.ron

@@ -0,0 +1,71 @@
+SemverQuery(
+    id: "type_mismatched_generic_lifetimes",
+    human_readable_name: "type now takes a different number of generic lifetimes",
+    description: "A type now takes a different number of generic lifetime parameters, breaking uses of that type.",
+    required_update: Major,
+    lint_level: Deny,
+    // The cargo SemVer reference only has entries for generic *type* parameters.
+    // There's no passable place to link to when it comes to specifically lifetime parameters.
+    reference_link: None,
+    query: r#"
+    {
+        CrateDiff {
+            baseline {
+                item {
+                    ... on ImplOwner {
+                        visibility_limit @filter(op: "=", value: ["$public"])
+                        name @output
+                        owner_type: __typename @tag @output
+
+                        importable_path {
+                            path @tag @output
+                            public_api @filter(op: "=", value: ["$true"])
+                        }
+
+                        generic_parameter @fold
+                                          @transform(op: "count")
+                                          @tag(name: "old_lifetimes_count")
+                                          @output(name: "old_lifetimes_count") {
+                            ... on GenericLifetimeParameter {
+                                old_lifetimes: name @output
+                            }
+                        }
+                    }
+                }
+            }
+            current {
+                item {
+                    ... on ImplOwner {
+                        visibility_limit @filter(op: "=", value: ["$public"]) @output
+                        __typename @filter(op: "=", value: ["%owner_type"])
+
+                        importable_path {
+                            path @filter(op: "=", value: ["%path"])
+                            public_api @filter(op: "=", value: ["$true"])
+                        }
+
+                        generic_parameter @fold
+                                          @transform(op: "count")
+                                          @filter(op: "!=", value: ["%old_lifetimes_count"])
+                                          @output(name: "new_lifetimes_count") {
+                            ... on GenericLifetimeParameter {
+                                new_lifetimes: name @output
+                            }
+                        }
+
+                        span_: span @optional {
+                            filename @output
+                            begin_line @output
+                        }
+                    }
+                }
+            }
+        }
+    }"#,
+    arguments: {
+        "public": "public",
+        "true": true,
+    },
+    error_message: "A type now takes a different number of generic lifetime parameters. Uses of this type that name the previous number of parameters will be broken.",
+    per_result_error_template: Some("{{owner_type}} {{name}} ({{old_lifetimes_count}} -> {{new_lifetimes_count}} lifetime params) in {{span_filename}}:{{span_begin_line}}"),
+)

src/query.rs

@@ -1147,6 +1147,7 @@ add_lints!(
     trait_unsafe_removed,
     tuple_struct_to_plain_struct,
     type_marked_deprecated,
+    type_mismatched_generic_lifetimes,
     union_field_missing,
     union_missing,
     union_must_use_added,

test_crates/type_mismatched_generic_lifetimes/new/Cargo.toml

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

test_crates/type_mismatched_generic_lifetimes/new/src/lib.rs

@@ -0,0 +1,30 @@
+#![allow(dead_code)]
+
+pub struct Example<'a, 'b, 'c>(&'a i64, &'b i64, &'c i64);
+
+pub enum Either<'a, 'b, 'c> {
+    Left(&'a i64),
+    Right(&'b u64),
+    Mid(&'c isize),
+}
+
+pub union OneOrTheOther<'a, 'b, 'c> {
+    left: &'a i64,
+    right: &'b u64,
+    center: &'c isize,
+}
+
+// Renaming lifetimes while leaving them semantically identical is not breaking.
+// AFAIK there's no way to refer to a lifetime's defined name while using the type.
+pub struct RenamedLifetimes<'c, 'a>(&'c i64, &'a i64);
+
+// Attempting to specify lifetime parameters that don't exist is breaking.
+// This should be reported.
+pub struct NotGenericAnymore(&'static str);
+
+// This is breaking too and should be reported. The witness is just wrapping the type
+// in another type, at which point all generics must be specified.
+/// ```rust,compile_fail
+/// struct Witness(type_mismatched_generic_lifetimes::BecameGeneric);
+/// ```
+pub struct BecameGeneric<'a>(String, &'a str);

test_crates/type_mismatched_generic_lifetimes/old/Cargo.toml

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

test_crates/type_mismatched_generic_lifetimes/old/src/lib.rs

@@ -0,0 +1,29 @@
+#![allow(dead_code)]
+
+pub struct Example<'a, 'b>(&'a i64, &'b i64);
+
+#[non_exhaustive]
+pub enum Either<'a, 'b> {
+    Left(&'a i64),
+    Right(&'b u64),
+}
+
+pub union OneOrTheOther<'a, 'b> {
+    left: &'a i64,
+    right: &'b u64,
+}
+
+// Renaming lifetimes while leaving them semantically identical is not breaking.
+// AFAIK there's no way to refer to a lifetime's defined name while using the type.
+pub struct RenamedLifetimes<'a, 'b>(&'a i64, &'b i64);
+
+// Attempting to specify lifetime parameters that don't exist is breaking.
+// This should be reported.
+pub struct NotGenericAnymore<'a>(&'a str);
+
+// This is breaking too and should be reported. The witness is just wrapping the type
+// in another type, at which point all generics must be specified.
+/// ```rust
+/// struct Witness(type_mismatched_generic_lifetimes::BecameGeneric);
+/// ```
+pub struct BecameGeneric(String);

test_outputs/query_execution/type_mismatched_generic_lifetimes.snap

@@ -0,0 +1,109 @@
+---
+source: src/query.rs
+expression: "&query_execution_results"
+snapshot_kind: text
+---
+{
+  "./test_crates/type_mismatched_generic_lifetimes/": [
+    {
+      "name": String("Example"),
+      "new_lifetimes": List([
+        String("\'a"),
+        String("\'b"),
+        String("\'c"),
+      ]),
+      "new_lifetimes_count": Uint64(3),
+      "old_lifetimes": List([
+        String("\'a"),
+        String("\'b"),
+      ]),
+      "old_lifetimes_count": Uint64(2),
+      "owner_type": String("Struct"),
+      "path": List([
+        String("type_mismatched_generic_lifetimes"),
+        String("Example"),
+      ]),
+      "span_begin_line": Uint64(3),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+    {
+      "name": String("Either"),
+      "new_lifetimes": List([
+        String("\'a"),
+        String("\'b"),
+        String("\'c"),
+      ]),
+      "new_lifetimes_count": Uint64(3),
+      "old_lifetimes": List([
+        String("\'a"),
+        String("\'b"),
+      ]),
+      "old_lifetimes_count": Uint64(2),
+      "owner_type": String("Enum"),
+      "path": List([
+        String("type_mismatched_generic_lifetimes"),
+        String("Either"),
+      ]),
+      "span_begin_line": Uint64(5),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+    {
+      "name": String("OneOrTheOther"),
+      "new_lifetimes": List([
+        String("\'a"),
+        String("\'b"),
+        String("\'c"),
+      ]),
+      "new_lifetimes_count": Uint64(3),
+      "old_lifetimes": List([
+        String("\'a"),
+        String("\'b"),
+      ]),
+      "old_lifetimes_count": Uint64(2),
+      "owner_type": String("Union"),
+      "path": List([
+        String("type_mismatched_generic_lifetimes"),
+        String("OneOrTheOther"),
+      ]),
+      "span_begin_line": Uint64(11),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+    {
+      "name": String("NotGenericAnymore"),
+      "new_lifetimes": List([]),
+      "new_lifetimes_count": Uint64(0),
+      "old_lifetimes": List([
+        String("\'a"),
+      ]),
+      "old_lifetimes_count": Uint64(1),
+      "owner_type": String("Struct"),
+      "path": List([
+        String("type_mismatched_generic_lifetimes"),
+        String("NotGenericAnymore"),
+      ]),
+      "span_begin_line": Uint64(23),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+    {
+      "name": String("BecameGeneric"),
+      "new_lifetimes": List([
+        String("\'a"),
+      ]),
+      "new_lifetimes_count": Uint64(1),
+      "old_lifetimes": List([]),
+      "old_lifetimes_count": Uint64(0),
+      "owner_type": String("Struct"),
+      "path": List([
+        String("type_mismatched_generic_lifetimes"),
+        String("BecameGeneric"),
+      ]),
+      "span_begin_line": Uint64(30),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+  ],
+}