adds new trait-method-added lint (#891)

* adds new trait-method-added lint

* corrects lint to account for traits that have also become sealed, and adds tests for it

src/lints/trait_method_added.ron

@@ -0,0 +1,60 @@
+SemverQuery(
+    id: "trait_method_added",
+    human_readable_name: "pub trait method added",
+    description: "A non-sealed public trait added a new method without a default implementation",
+    required_update: Major,
+    lint_level: Deny,
+    reference_link: Some("https://doc.rust-lang.org/cargo/reference/semver.html#trait-new-item-no-default"),
+    query: r#"
+    {
+        CrateDiff {
+            current {
+                item {
+                    ... on Trait {
+                        visibility_limit @filter(op: "=", value: ["$public"]) @output
+                        name @output
+
+                        importable_path {
+                            path @output @tag
+                            public_api @filter(op: "=", value: ["$true"])
+                        }
+
+                        method {
+                            method_name: name @output @tag
+                            has_body @filter(op: "!=", value: ["$true"])
+
+                            span_: span @optional {
+                                filename @output
+                                begin_line @output
+                            }
+                        }
+                    }
+                }
+            }
+            baseline {
+                item {
+                    ... on Trait {
+                        visibility_limit @filter(op: "=", value: ["$public"])
+                        sealed @filter(op: "!=", value: ["$true"])
+
+                        importable_path {
+                            path @filter(op: "=", value: ["%path"])
+                            public_api @filter(op: "=", value: ["$true"])
+                        }
+
+                        method @fold @transform(op: "count") @filter(op: "=", value: ["$zero"]) {
+                            name @filter(op: "=", value: ["%method_name"])
+                        }
+                    }
+                }
+            }
+        }
+    }"#,
+    arguments: {
+        "true": true,
+        "public": "public",
+        "zero": 0,
+    },
+    error_message: "A non-sealed public trait added a new method without a default implementation, which breaks downstream implementations of the trait",
+    per_result_error_template: Some("trait method {{join \"::\" path}}::{{method_name}} in file {{span_filename}}:{{span_begin_line}}"),
+)

src/query.rs

@@ -840,6 +840,7 @@ add_lints!(
     trait_associated_type_default_removed,
     trait_associated_type_now_doc_hidden,
     trait_default_impl_removed,
+    trait_method_added,
     trait_method_missing,
     trait_method_now_doc_hidden,
     trait_method_unsafe_added,

test_crates/trait_method_added/new/Cargo.toml

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

test_crates/trait_method_added/new/src/lib.rs

@@ -0,0 +1,49 @@
+mod sealed {
+    pub(crate) trait Sealed {}
+}
+
+// ---- Should be reported ----
+pub trait WillGainMethodWithoutDefault {
+    fn one_method(self);
+}
+
+pub trait WillGainAnotherMethodWithoutDefault {
+    fn one_method(self);
+    fn two_method(self);
+}
+
+pub trait WillGainMultipleMethodsWithoutDefault {
+    fn one_method(self);
+    fn two_method(self);
+}
+
+pub trait WillGainMethodWithoutDefaultAndSeal: sealed::Sealed {
+    fn one_method(self);
+} 
+
+pub trait WIllGainDocHiddenMethodWithoutDefault {
+    #[doc(hidden)]
+    fn one_method(self);
+}
+
+// ---- Should not be reported ----
+pub trait WillGainMethodWithDefault {
+    fn one_method(self) {}
+}
+
+pub trait WillGainAnotherMethodWithDefault {
+    fn one_method(self);
+    fn two_method(self) {}
+}
+
+pub trait WillGainMethodWithoutDefaultSealed: sealed::Sealed {
+    fn one_method(self);
+}
+
+pub trait WillGainMethodWithoutDefaultAndLoseSeal {
+    fn one_method(self);
+}
+
+pub trait WillKeepAMethodWithoutDefault {
+    fn one_method(self);
+}

test_crates/trait_method_added/old/Cargo.toml

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

test_crates/trait_method_added/old/src/lib.rs

@@ -0,0 +1,31 @@
+mod sealed {
+    pub(crate) trait Sealed {}
+}
+
+// ---- Should be reported ----
+pub trait WillGainMethodWithoutDefault {}
+
+pub trait WillGainAnotherMethodWithoutDefault {
+    fn one_method(self);
+}
+
+pub trait WillGainMultipleMethodsWithoutDefault {}
+
+pub trait WillGainMethodWithoutDefaultAndSeal {}
+
+pub trait WIllGainDocHiddenMethodWithoutDefault {}
+
+// ---- Should not be reported ----
+pub trait WillGainMethodWithDefault {}
+
+pub trait WillGainAnotherMethodWithDefault {
+    fn one_method(self);
+}
+
+pub trait WillGainMethodWithoutDefaultSealed: sealed::Sealed {}
+
+pub trait WillGainMethodWithoutDefaultAndLoseSeal: sealed::Sealed {}
+
+pub trait WillKeepAMethodWithoutDefault {
+    fn one_method(self);
+}