New "module_missing" lint to detect missing, renamed, or non-visible modules

src/lints/module_missing.ron

@@ -0,0 +1,46 @@
+SemverQuery(
+    id: "module_missing",
+    human_readable_name: "pub module removed or renamed",
+    description: "A module can no longer be imported by its prior path",
+    required_update: Major,
+    reference_link:  Some("https://doc.rust-lang.org/cargo/reference/semver.html#item-remove"),
+    query: r#"
+    {
+        CrateDiff {
+            baseline {
+                item {
+                    ... on Module {
+                        visibility_limit @filter(op: "=", value: ["$public"]) @output
+                        name @output
+
+                        importable_path {
+                            path @output @tag
+                        }
+
+                        span_: span @optional {
+                            filename @output
+                            begin_line @output
+                        }
+                    }
+                }
+            }
+            current @fold @transform(op: "count") @filter(op: "=", value: ["$zero"]) {
+                item {
+                    ... on Module {
+                        visibility_limit @filter(op: "=", value: ["$public"])
+
+                        importable_path {
+                            path @filter(op: "=", value: ["%path"])
+                        }
+                    }
+                }
+            }
+        }
+    }"#,
+    arguments: {
+        "public": "public",
+        "zero": 0,
+    },
+    error_message: "A publicly-visible module cannot be imported by its prior path. A `pub use` may have been removed, or the module may have been renamed, removed, or made non-visible.",
+    per_result_error_template: Some("{mod {{join \"::\" path}}, previously in file {{span_filename}}:{{span_begin_line}}"),
+)

src/query.rs

@@ -514,4 +514,5 @@ add_lints!(
     pub_module_level_const_missing,
     pub_static_missing,
     trait_removed_associated_type,
+    module_missing,
 );

test_crates/module_missing/new/Cargo.toml

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

test_crates/module_missing/new/src/lib.rs

@@ -0,0 +1,28 @@
+// Removing this, but no warning should happen:
+// it isn't public.
+//
+//mod a {
+//}
+
+mod b {
+    // Removing this, but no warning should happen:
+    // it isn't visible.
+    // pub mod b {
+    // }
+}
+
+pub mod bb {
+    // Removing this should cause a warning.
+    //pub mod will_remove {
+    //}
+}
+
+// Making this private should cause a warning
+pub(crate) mod will_make_private {
+    mod e {
+   }
+}
+
+// Adding a module shouldn't cause problems.
+pub mod new_module {
+}

test_crates/module_missing/old/Cargo.toml

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

test_crates/module_missing/old/src/lib.rs

@@ -0,0 +1,17 @@
+mod a {
+}
+
+mod b {
+    pub mod b {
+    }
+}
+
+pub mod bb {
+    pub mod will_remove {
+    }
+}
+
+pub mod will_make_private {
+    mod e {
+    }
+}

test_outputs/module_missing.output.ron

@@ -0,0 +1,49 @@
+{
+    "./test_crates/module_missing/": [
+        {
+            "name": String("will_remove"),
+            "path": List([
+                String("module_missing"),
+                String("bb"),
+                String("will_remove"),
+            ]),
+            "span_begin_line": Uint64(10),
+            "span_filename": String("src/lib.rs"),
+            "visibility_limit": String("public"),
+	},
+	{
+            "name": String("will_make_private"),
+            "path": List([
+                String("module_missing"),
+                String("will_make_private"),
+            ]),
+            "span_begin_line": Uint64(14),
+            "span_filename": String("src/lib.rs"),
+            "visibility_limit": String("public"),
+        },
+    ],
+    "./test_crates/trait_missing/": [
+        {
+            "name": String("my_pub_mod"),
+           "path": List([
+                String("trait_missing"),
+                String("my_pub_mod"),
+            ]),
+            "span_begin_line": Uint64(5),
+            "span_filename": String("src/lib.rs"),
+            "visibility_limit": String("public"),
+        },
+    ],
+    "./test_crates/trait_missing_with_major_bump/": [
+        {
+            "name": String("my_pub_mod"),
+            "path": List([
+                String("trait_missing_with_major_bump"),
+                String("my_pub_mod"),
+            ]),
+            "span_begin_line": Uint64(5),
+            "span_filename": String("src/lib.rs"),
+            "visibility_limit": String("public"),
+        },
+    ],
+}