Add trait_mismatched_generic_lifetimes lint. (#1040)

Analogous to #1039, but this time for traits instead of
structs/enums/unions. For traits, the proof of breakage is in trait
bounds where all lifetimes must be explicitly specified.

src/lints/trait_mismatched_generic_lifetimes.ron

@@ -0,0 +1,69 @@
+SemverQuery(
+    id: "trait_mismatched_generic_lifetimes",
+    human_readable_name: "trait now takes a different number of generic lifetimes",
+    description: "A trait now takes a different number of generic lifetime parameters, breaking uses of that trait.",
+    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 Trait {
+                        visibility_limit @filter(op: "=", value: ["$public"])
+                        name @output
+
+                        importable_path {
+                            path @tag @output
+                            public_api @filter(op: "=", value: ["$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 Trait {
+                        visibility_limit @filter(op: "=", value: ["$public"]) @output
+
+                        importable_path {
+                            path @filter(op: "=", value: ["%path"])
+                            public_api @filter(op: "=", value: ["$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 trait now takes a different number of generic lifetime parameters. Uses of this trait that name the previous number of parameters, such as in trait bounds, will be broken.",
+    per_result_error_template: Some("trait {{name}} ({{old_lifetimes_count}} -> {{new_lifetimes_count}} lifetime params) in {{span_filename}}:{{span_begin_line}}"),
+)

src/query.rs

@@ -1135,6 +1135,7 @@ add_lints!(
     trait_method_now_doc_hidden,
     trait_method_unsafe_added,
     trait_method_unsafe_removed,
+    trait_mismatched_generic_lifetimes,
     trait_missing,
     trait_must_use_added,
     trait_newly_sealed,

test_crates/trait_mismatched_generic_lifetimes/new/Cargo.toml

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

test_crates/trait_mismatched_generic_lifetimes/new/src/lib.rs

@@ -0,0 +1,23 @@
+// All these changes are breaking.
+//
+// The best witness is using the trait in a bound:
+// ```
+// fn witness<T: Example>() {}
+// ```
+// where supplying any number other than the exact number of lifetime parameters is an error:
+// ```
+//   |
+// 6 | fn witness<T: Example>() {}
+//   |               ^^^^^^^ expected 2 lifetime parameters
+//   |
+//   = note: for more information on higher-ranked polymorphism, visit https://doc.rust-lang.org/nomicon/hrtb.html
+// help: consider making the bound lifetime-generic with a new `'a` lifetime
+// ```
+
+pub trait NotGeneric<'a> {}
+
+pub trait GainsLifetimes<'a, 'b> {}
+
+pub trait LosesLifetimes<'a> {}
+
+pub trait StopsBeingGeneric {}

test_crates/trait_mismatched_generic_lifetimes/old/Cargo.toml

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

test_crates/trait_mismatched_generic_lifetimes/old/src/lib.rs

@@ -0,0 +1,23 @@
+// All these changes are breaking.
+//
+// The best witness is using the trait in a bound:
+// ```
+// fn witness<T: Example>() {}
+// ```
+// where supplying any number other than the exact number of lifetime parameters is an error:
+// ```
+//   |
+// 6 | fn witness<T: Example>() {}
+//   |               ^^^^^^^ expected 2 lifetime parameters
+//   |
+//   = note: for more information on higher-ranked polymorphism, visit https://doc.rust-lang.org/nomicon/hrtb.html
+// help: consider making the bound lifetime-generic with a new `'a` lifetime
+// ```
+
+pub trait NotGeneric {}
+
+pub trait GainsLifetimes<'a> {}
+
+pub trait LosesLifetimes<'a, 'b> {}
+
+pub trait StopsBeingGeneric<'a, 'b> {}

test_outputs/query_execution/trait_mismatched_generic_lifetimes.snap

@@ -0,0 +1,80 @@
+---
+source: src/query.rs
+expression: "&query_execution_results"
+snapshot_kind: text
+---
+{
+  "./test_crates/trait_mismatched_generic_lifetimes/": [
+    {
+      "name": String("NotGeneric"),
+      "new_lifetimes": List([
+        String("\'a"),
+      ]),
+      "new_lifetimes_count": Uint64(1),
+      "old_lifetimes": List([]),
+      "old_lifetimes_count": Uint64(0),
+      "path": List([
+        String("trait_mismatched_generic_lifetimes"),
+        String("NotGeneric"),
+      ]),
+      "span_begin_line": Uint64(17),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+    {
+      "name": String("GainsLifetimes"),
+      "new_lifetimes": List([
+        String("\'a"),
+        String("\'b"),
+      ]),
+      "new_lifetimes_count": Uint64(2),
+      "old_lifetimes": List([
+        String("\'a"),
+      ]),
+      "old_lifetimes_count": Uint64(1),
+      "path": List([
+        String("trait_mismatched_generic_lifetimes"),
+        String("GainsLifetimes"),
+      ]),
+      "span_begin_line": Uint64(19),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+    {
+      "name": String("LosesLifetimes"),
+      "new_lifetimes": List([
+        String("\'a"),
+      ]),
+      "new_lifetimes_count": Uint64(1),
+      "old_lifetimes": List([
+        String("\'a"),
+        String("\'b"),
+      ]),
+      "old_lifetimes_count": Uint64(2),
+      "path": List([
+        String("trait_mismatched_generic_lifetimes"),
+        String("LosesLifetimes"),
+      ]),
+      "span_begin_line": Uint64(21),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+    {
+      "name": String("StopsBeingGeneric"),
+      "new_lifetimes": List([]),
+      "new_lifetimes_count": Uint64(0),
+      "old_lifetimes": List([
+        String("\'a"),
+        String("\'b"),
+      ]),
+      "old_lifetimes_count": Uint64(2),
+      "path": List([
+        String("trait_mismatched_generic_lifetimes"),
+        String("StopsBeingGeneric"),
+      ]),
+      "span_begin_line": Uint64(23),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+  ],
+}