Add single_option_map lint

Author: Yusuf Raji

CHANGELOG.md

@@ -6061,6 +6061,7 @@ Released 2018-09-13
 [`single_element_loop`]: https://rust-lang.github.io/rust-clippy/master/index.html#single_element_loop
 [`single_match`]: https://rust-lang.github.io/rust-clippy/master/index.html#single_match
 [`single_match_else`]: https://rust-lang.github.io/rust-clippy/master/index.html#single_match_else
+[`single_option_map`]: https://rust-lang.github.io/rust-clippy/master/index.html#single_option_map
 [`single_range_in_vec_init`]: https://rust-lang.github.io/rust-clippy/master/index.html#single_range_in_vec_init
 [`size_of_in_element_count`]: https://rust-lang.github.io/rust-clippy/master/index.html#size_of_in_element_count
 [`size_of_ref`]: https://rust-lang.github.io/rust-clippy/master/index.html#size_of_ref

clippy_lints/src/declared_lints.rs

@@ -678,6 +678,7 @@ pub static LINTS: &[&crate::LintInfo] = &[
     crate::single_call_fn::SINGLE_CALL_FN_INFO,
     crate::single_char_lifetime_names::SINGLE_CHAR_LIFETIME_NAMES_INFO,
     crate::single_component_path_imports::SINGLE_COMPONENT_PATH_IMPORTS_INFO,
+    crate::single_option_map::SINGLE_OPTION_MAP_INFO,
     crate::single_range_in_vec_init::SINGLE_RANGE_IN_VEC_INIT_INFO,
     crate::size_of_in_element_count::SIZE_OF_IN_ELEMENT_COUNT_INFO,
     crate::size_of_ref::SIZE_OF_REF_INFO,

clippy_lints/src/lib.rs

@@ -337,6 +337,7 @@ mod significant_drop_tightening;
 mod single_call_fn;
 mod single_char_lifetime_names;
 mod single_component_path_imports;
+mod single_option_map;
 mod single_range_in_vec_init;
 mod size_of_in_element_count;
 mod size_of_ref;
@@ -974,5 +975,6 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
     store.register_late_pass(move |_| Box::new(arbitrary_source_item_ordering::ArbitrarySourceItemOrdering::new(conf)));
     store.register_late_pass(|_| Box::new(unneeded_struct_pattern::UnneededStructPattern));
     store.register_late_pass(|_| Box::new(unnecessary_semicolon::UnnecessarySemicolon));
+    store.register_late_pass(|_| Box::new(single_option_map::SingleOptionMap));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }

clippy_lints/src/single_option_map.rs

@@ -0,0 +1,68 @@
+use clippy_utils::diagnostics::span_lint;
+use clippy_utils::peel_blocks;
+use clippy_utils::ty::is_type_diagnostic_item;
+use rustc_hir::def_id::LocalDefId;
+use rustc_hir::intravisit::FnKind;
+use rustc_hir::{Body, ExprKind, FnDecl, FnRetTy};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::declare_lint_pass;
+use rustc_span::{Span, sym};
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for functions with method calls to `.map(_)` on an arg
+    /// of type `Option` as the outermost expression.
+    ///
+    /// ### Why is this bad?
+    /// Taking and returning an `Option<T>` may require additional
+    /// `Some(_)` and `unwrap` if all you have is a `T`
+    ///
+    /// ### Example
+    /// ```no_run
+    /// fn double(param: Option<u32>) -> Option<u32> {
+    ///     param.map(|x| x * 2)
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```no_run
+    /// fn double(param: u32) -> u32 {
+    ///     param * 2
+    /// }
+    /// ```
+    #[clippy::version = "1.86.0"]
+    pub SINGLE_OPTION_MAP,
+    nursery,
+    "Checks for functions with method calls to `.map(_)` on an arg of type `Option` as the outermost expression."
+}
+
+declare_lint_pass!(SingleOptionMap => [SINGLE_OPTION_MAP]);
+
+impl<'tcx> LateLintPass<'tcx> for SingleOptionMap {
+    fn check_fn(
+        &mut self,
+        cx: &LateContext<'tcx>,
+        kind: FnKind<'tcx>,
+        decl: &'tcx FnDecl<'tcx>,
+        body: &'tcx Body<'tcx>,
+        span: Span,
+        _fn_def: LocalDefId,
+    ) {
+        if let FnRetTy::Return(_ret) = decl.output
+            && matches!(kind, FnKind::ItemFn(_, _, _) | FnKind::Method(_, _))
+        {
+            let func_body = peel_blocks(body.value);
+            if let ExprKind::MethodCall(method_name, callee, _args, _span) = func_body.kind
+                && method_name.ident.name == sym::map
+                && let callee_type = cx.typeck_results().expr_ty(callee)
+                && is_type_diagnostic_item(cx, callee_type, sym::Option)
+            {
+                span_lint(
+                    cx,
+                    SINGLE_OPTION_MAP,
+                    span,
+                    "mapping over single function argument of `Option<T>`, to return `Option<T>`, can be best expressed by applying the map outside of the function.",
+                );
+            }
+        }
+    }
+}

tests/ui/single_option_map.rs

@@ -0,0 +1,17 @@
+#![warn(clippy::single_option_map)]
+
+fn h(arg: Option<u32>) -> Option<u32> {
+    arg.map(|x| x * 2)
+}
+
+fn j(arg: Option<u64>) -> Option<u64> {
+    arg.map(|x| x * 2)
+}
+
+fn main() {
+    let answer = Some(42u32);
+    let h_result = h(answer);
+
+    let answer = Some(42u64);
+    let j_result = j(answer);
+}

tests/ui/single_option_map.stderr

@@ -0,0 +1,21 @@
+error: mapping over single function argument of `Option<T>`, to return `Option<T>`, can be best expressed by applying the map outside of the function.
+  --> tests/ui/single_option_map.rs:3:1
+   |
+LL | / fn h(arg: Option<u32>) -> Option<u32> {
+LL | |     arg.map(|x| x * 2)
+LL | | }
+   | |_^
+   |
+   = note: `-D clippy::single-option-map` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::single_option_map)]`
+
+error: mapping over single function argument of `Option<T>`, to return `Option<T>`, can be best expressed by applying the map outside of the function.
+  --> tests/ui/single_option_map.rs:7:1
+   |
+LL | / fn j(arg: Option<u64>) -> Option<u64> {
+LL | |     arg.map(|x| x * 2)
+LL | | }
+   | |_^
+
+error: aborting due to 2 previous errors
+