Add method_requires_different_const_generic_params lint. (#1058)

It catches methods that have a different number of const generic
parameters than they used to.

src/lints/method_requires_different_const_generic_params.ron

@@ -0,0 +1,142 @@
+SemverQuery(
+    id: "method_requires_different_const_generic_params",
+    human_readable_name: "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 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 ImplOwner {
+                        visibility_limit @filter(op: "=", value: ["$public"])
+
+                        importable_path {
+                            path @tag @output
+                            public_api @filter(op: "=", value: ["$true"])
+                        }
+
+                        inherent_impl {
+                            method {
+                                method_visibility: visibility_limit @filter(op: "=", value: ["$public"]) @output
+                                method_name: name @output @tag
+                                public_api_eligible @filter(op: "=", value: ["$true"])
+
+                                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
+                                    }
+                                }
+
+                                span_: span @optional {
+                                    filename @output
+                                    begin_line @output
+                                }
+                            }
+                        }
+                    }
+                }
+            }
+            current {
+                item {
+                    ... on ImplOwner {
+                        visibility_limit @filter(op: "=", value: ["$public"]) @output
+                        name @output
+
+                        importable_path {
+                            path @filter(op: "=", value: ["%path"])
+                            public_api @filter(op: "=", value: ["$true"])
+                        }
+
+                        # We use "impl" instead of "inherent_impl" here because moving
+                        # an inherently-implemented method to a trait is not necessarily
+                        # a breaking change, but changing the number of const generics is.
+                        #
+                        # Method names are not unique on an ImplOwner! It's perfectly valid to have
+                        # both an inherent method `foo()` as well as a trait-provided method
+                        # `<Self as Bar>::foo()` at the same time. Whenever possible, rustc
+                        # attempts to "do the right thing" and dispatch to the correct one.
+                        #
+                        # Because of the above, this check has to be written as
+                        # "there is no method with the correct name and number of const generics"
+                        # rather than the (incorrect!) alternative
+                        # "the named method does not have the correct number of arguments."
+                        #
+                        # The above by itself is still not enough: say if the method was removed,
+                        # that still makes the "there is no method ..." statement true.
+                        # So we add an additional clause demanding that a method by that name
+                        # with appropriate visibility actually exists.
+                        impl @fold @transform(op: "count") @filter(op: ">", value: ["$zero"]) {
+                            method {
+                                visibility_limit @filter(op: "one_of", value: ["$public_or_default"])
+                                name @filter(op: "=", value: ["%method_name"])
+                                public_api_eligible @filter(op: "=", value: ["$true"])
+                            }
+                        }
+                        impl @fold @transform(op: "count") @filter(op: "=", value: ["$zero"]) {
+                            method {
+                                visibility_limit @filter(op: "one_of", value: ["$public_or_default"])
+                                name @filter(op: "=", value: ["%method_name"])
+                                public_api_eligible @filter(op: "=", value: ["$true"])
+
+                                generic_parameter @fold
+                                                  @transform(op: "count")
+                                                  @filter(op: "=", value: ["%old_required_const_count"]) {
+                                    ... on GenericConstParameter {
+                                        name
+                                    }
+                                }
+                            }
+                        }
+
+                        # Get the non-matching methods by that name so we can report them
+                        # in the lint error message.
+                        impl @fold {
+                            method {
+                                visibility_limit @filter(op: "one_of", value: ["$public_or_default"])
+                                name @filter(op: "=", value: ["%method_name"])
+                                public_api_eligible @filter(op: "=", value: ["$true"])
+
+                                generic_parameter @fold
+                                                  @transform(op: "count")
+                                                  @output(name: "new_required_const_count") {
+                                    ... on GenericConstParameter {
+                                        new_required_consts: name @output
+                                    }
+                                }
+
+                                non_matching_span_: span @optional {
+                                    filename @output
+                                    begin_line @output
+                                }
+                            }
+                        }
+                    }
+                }
+            }
+        }
+    }"#,
+    arguments: {
+        "public": "public",
+        "public_or_default": ["public", "default"],
+        "true": true,
+        "zero": 0,
+    },
+    error_message: "A method now requires a different number of const generic parameters than it used to. Uses of this method that supplied the previous number of const generics will be broken.",
+    per_result_error_template: Some("{{join \"::\" path}}::{{method_name}} takes {{unpack_if_singleton new_required_const_count}} const generics instead of {{old_required_const_count}}, in {{multiple_spans non_matching_span_filename non_matching_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 would be a invocation with the old number
+    // of const generics, which is insufficient for the new definition.
+    witness: None,
+)

src/query.rs

@@ -1172,6 +1172,7 @@ add_lints!(
     macro_no_longer_exported,
     macro_now_doc_hidden,
     method_parameter_count_changed,
+    method_requires_different_const_generic_params,
     module_missing,
     non_exhaustive_struct_changed_type,
     proc_macro_now_doc_hidden,

test_crates/method_requires_different_const_generic_params/new/Cargo.toml

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

test_crates/method_requires_different_const_generic_params/new/src/lib.rs

@@ -0,0 +1,15 @@
+pub struct Owner;
+
+impl Owner {
+    pub fn previously_not_generic<const N: usize>() -> [i64; N] {
+        todo!()
+    }
+
+    pub fn added_const_generic<const N: usize, const M: usize>(data: [i64; N]) -> [i64; M] {
+        todo!()
+    }
+
+    pub fn removed_const_generic<const N: usize>(data: [i64; N]) -> [i64; 4] {
+        todo!()
+    }
+}

test_crates/method_requires_different_const_generic_params/old/Cargo.toml

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

test_crates/method_requires_different_const_generic_params/old/src/lib.rs

@@ -0,0 +1,11 @@
+pub struct Owner;
+
+impl Owner {
+    pub fn previously_not_generic() {}
+
+    pub fn added_const_generic<const N: usize>(data: [i64; N]) {}
+
+    pub fn removed_const_generic<const N: usize, const M: usize>(data: [i64; N]) -> [i64; M] {
+        todo!()
+    }
+}

test_outputs/query_execution/method_requires_different_const_generic_params.snap

@@ -0,0 +1,99 @@
+---
+source: src/query.rs
+expression: "&query_execution_results"
+snapshot_kind: text
+---
+{
+  "./test_crates/method_requires_different_const_generic_params/": [
+    {
+      "method_name": String("previously_not_generic"),
+      "method_visibility": String("public"),
+      "name": String("Owner"),
+      "new_required_const_count": List([
+        Uint64(1),
+      ]),
+      "new_required_consts": List([
+        List([
+          String("N"),
+        ]),
+      ]),
+      "non_matching_span_begin_line": List([
+        Uint64(4),
+      ]),
+      "non_matching_span_filename": List([
+        String("src/lib.rs"),
+      ]),
+      "old_required_const_count": Uint64(0),
+      "old_required_consts": List([]),
+      "path": List([
+        String("method_requires_different_const_generic_params"),
+        String("Owner"),
+      ]),
+      "span_begin_line": Uint64(4),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+    {
+      "method_name": String("added_const_generic"),
+      "method_visibility": String("public"),
+      "name": String("Owner"),
+      "new_required_const_count": List([
+        Uint64(2),
+      ]),
+      "new_required_consts": List([
+        List([
+          String("N"),
+          String("M"),
+        ]),
+      ]),
+      "non_matching_span_begin_line": List([
+        Uint64(8),
+      ]),
+      "non_matching_span_filename": List([
+        String("src/lib.rs"),
+      ]),
+      "old_required_const_count": Uint64(1),
+      "old_required_consts": List([
+        String("N"),
+      ]),
+      "path": List([
+        String("method_requires_different_const_generic_params"),
+        String("Owner"),
+      ]),
+      "span_begin_line": Uint64(6),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+    {
+      "method_name": String("removed_const_generic"),
+      "method_visibility": String("public"),
+      "name": String("Owner"),
+      "new_required_const_count": List([
+        Uint64(1),
+      ]),
+      "new_required_consts": List([
+        List([
+          String("N"),
+        ]),
+      ]),
+      "non_matching_span_begin_line": List([
+        Uint64(12),
+      ]),
+      "non_matching_span_filename": List([
+        String("src/lib.rs"),
+      ]),
+      "old_required_const_count": Uint64(2),
+      "old_required_consts": List([
+        String("N"),
+        String("M"),
+      ]),
+      "path": List([
+        String("method_requires_different_const_generic_params"),
+        String("Owner"),
+      ]),
+      "span_begin_line": Uint64(8),
+      "span_filename": String("src/lib.rs"),
+      "visibility_limit": String("public"),
+    },
+  ],
+}