Add trait_method_requires_different_const_generic_params lint. (#1060)

Catches cases where trait methods require a different number of const
generic parameters than they used to.

src/lints/trait_method_requires_different_const_generic_params.ron

@@ -0,0 +1,93 @@
+SemverQuery(
+    id: "trait_method_requires_different_const_generic_params",
+    human_readable_name: "trait method now requires a different number of const generic parameters",
+    // Currently, const generics in functions and methods cannot have defaults set.
+    // This is why we have only one lint ("requires different number") instead of
+    // two separate lints ("requires" / "allows") like for structs/traits etc.
+    description: "A trait method now requires a different number of const generic parameters than before.",
+    required_update: Major,
+    lint_level: Deny,
+    reference_link: Some("https://doc.rust-lang.org/reference/items/generics.html#const-generics"),
+    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"])
+                        }
+
+                        method {
+                            public_api_eligible @filter(op: "=", value: ["$true"])
+                            method: name @output @tag
+
+                            generic_parameter @fold
+                                              @transform(op: "count")
+                                              @tag(name: "old_required_const_count")
+                                              @output(name: "old_required_const_count") {
+                                ... on GenericConstParameter {
+                                    old_required_consts: name @output
+                                }
+                            }
+                        }
+                    }
+                }
+            }
+            current {
+                item {
+                    ... on Trait {
+                        visibility_limit @filter(op: "=", value: ["$public"]) @output
+
+                        importable_path {
+                            path @filter(op: "=", value: ["%path"])
+                            public_api @filter(op: "=", value: ["$true"])
+                        }
+
+                        method {
+                            public_api_eligible @filter(op: "=", value: ["$true"])
+                            name @filter(op: "=", value: ["%method"])
+
+                            generic_parameter @fold
+                                              @transform(op: "count")
+                                              @filter(op: "!=", value: ["%old_required_const_count"])
+                                              @output(name: "new_required_const_count") {
+                                ... on GenericConstParameter {
+                                    new_required_consts: name @output
+                                }
+                            }
+
+                            span_: span @optional {
+                                filename @output
+                                begin_line @output
+                            }
+                        }
+
+                    }
+                }
+            }
+        }
+    }"#,
+    arguments: {
+        "public": "public",
+        "true": true,
+    },
+    error_message: "A trait method now requires a different number of const generic parameters than it used to. Calls or implementations of this trait method using the previous number of const generics will be broken.",
+    per_result_error_template: Some("{{name}}::{{method}} ({{old_required_const_count}} -> {{new_required_const_count}} const generics) in {{span_filename}}:{{span_begin_line}}"),
+    // TODO: see https://github.com/obi1kenobi/cargo-semver-checks/blob/main/CONTRIBUTING.md#adding-a-witness
+    // for information about this field.
+    //
+    // The witness here depends on whether the trait method is implementable or sealed,
+    // and whether the method is callable.
+    // - If the method is callable, the witness would be a trait method invocation
+    //   with the old number of const generics, which is insufficient for the new definition.
+    // - If the trait is not sealed, and the method is implementable, then the witness would be
+    //   an implementation of that trait that supplies an implementation of the method with the old
+    //   number of const generics. Refinement by making a method implementation more generic isn't
+    //   supported, so this is always breaking in current Rust.
+    witness: None,
+)

src/query.rs

@@ -1207,6 +1207,7 @@ add_lints!(
     trait_method_added,
     trait_method_missing,
     trait_method_now_doc_hidden,
+    trait_method_requires_different_const_generic_params,
     trait_method_unsafe_added,
     trait_method_unsafe_removed,
     trait_mismatched_generic_lifetimes,

test_crates/trait_method_requires_different_const_generic_params/new/Cargo.toml

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

test_crates/trait_method_requires_different_const_generic_params/new/src/lib.rs

@@ -0,0 +1,7 @@
+pub trait Example {
+    fn becomes_generic<const N: usize>(data: [i64; N]) -> [i64; 4];
+
+    fn gains_const_generics<const N: usize, const M: usize>(data: [i64; N]) -> [i64; M];
+
+    fn loses_const_generics<const N: usize>(data: [i64; N]) -> [i64; 4];
+}

test_crates/trait_method_requires_different_const_generic_params/old/Cargo.toml

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

test_crates/trait_method_requires_different_const_generic_params/old/src/lib.rs

@@ -0,0 +1,7 @@
+pub trait Example {
+    fn becomes_generic(data: [i64; 4]) -> [i64; 4];
+
+    fn gains_const_generics<const N: usize>(data: [i64; N]) -> [i64; 4];
+
+    fn loses_const_generics<const N: usize, const M: usize>(data: [i64; N]) -> [i64; M];
+}

test_outputs/query_execution/trait_method_requires_different_const_generic_params.snap

@@ -0,0 +1,66 @@
+---
+source: src/query.rs
+expression: "&query_execution_results"
+snapshot_kind: text
+---
+{
+  "./test_crates/trait_method_requires_different_const_generic_params/": [
+    {
+      "method": String("becomes_generic"),
+      "name": String("Example"),
+      "new_required_const_count": Uint64(1),
+      "new_required_consts": List([
+        String("N"),
+      ]),
+      "old_required_const_count": Uint64(0),
+      "old_required_consts": List([]),
+      "path": List([
+        String("trait_method_requires_different_const_generic_params"),
+        String("Example"),
+      ]),
+      "span_begin_line": Uint64(2),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+    {
+      "method": String("gains_const_generics"),
+      "name": String("Example"),
+      "new_required_const_count": Uint64(2),
+      "new_required_consts": List([
+        String("N"),
+        String("M"),
+      ]),
+      "old_required_const_count": Uint64(1),
+      "old_required_consts": List([
+        String("N"),
+      ]),
+      "path": List([
+        String("trait_method_requires_different_const_generic_params"),
+        String("Example"),
+      ]),
+      "span_begin_line": Uint64(4),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+    {
+      "method": String("loses_const_generics"),
+      "name": String("Example"),
+      "new_required_const_count": Uint64(1),
+      "new_required_consts": List([
+        String("N"),
+      ]),
+      "old_required_const_count": Uint64(2),
+      "old_required_consts": List([
+        String("N"),
+        String("M"),
+      ]),
+      "path": List([
+        String("trait_method_requires_different_const_generic_params"),
+        String("Example"),
+      ]),
+      "span_begin_line": Uint64(6),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+  ],
+}