Add single_option_map lint

tests/ui/single_option_map.rs

@@ -0,0 +1,69 @@
+#![warn(clippy::single_option_map)]
+
+use std::sync::atomic::{AtomicUsize, Ordering};
+
+static ATOM: AtomicUsize = AtomicUsize::new(42);
+static MAYBE_ATOMIC: Option<&AtomicUsize> = Some(&ATOM);
+
+fn h(arg: Option<u32>) -> Option<u32> {
+    //~^ ERROR: `fn` that only maps over argument
+    arg.map(|x| x * 2)
+}
+
+fn j(arg: Option<u64>) -> Option<u64> {
+    //~^ ERROR: `fn` that only maps over argument
+    arg.map(|x| x * 2)
+}
+
+fn mul_args(a: String, b: u64) -> String {
+    a
+}
+
+fn mul_args_opt(a: Option<String>, b: u64) -> Option<String> {
+    //~^ ERROR: `fn` that only maps over argument
+    a.map(|val| mul_args(val, b + 1))
+}
+
+// No lint: no `Option` argument argument
+fn maps_static_option() -> Option<usize> {
+    MAYBE_ATOMIC.map(|a| a.load(Ordering::Relaxed))
+}
+
+// No lint: wrapped by another function
+fn manipulate(i: i32) -> i32 {
+    i + 1
+}
+// No lint: wraps another function to do the optional thing
+fn manipulate_opt(opt_i: Option<i32>) -> Option<i32> {
+    opt_i.map(manipulate)
+}
+
+// No lint: maps other than the receiver
+fn map_not_arg(arg: Option<u32>) -> Option<u32> {
+    maps_static_option().map(|_| arg.unwrap())
+}
+
+// No lint: wrapper function with η-expanded form
+#[allow(clippy::redundant_closure)]
+fn manipulate_opt_explicit(opt_i: Option<i32>) -> Option<i32> {
+    opt_i.map(|x| manipulate(x))
+}
+
+// No lint
+fn multi_args(a: String, b: bool, c: u64) -> String {
+    a
+}
+
+// No lint: contains only map of a closure that binds other arguments
+fn multi_args_opt(a: Option<String>, b: bool, c: u64) -> Option<String> {
+    a.map(|a| multi_args(a, b, c))
+}
+
+fn main() {
+    let answer = Some(42u32);
+    let h_result = h(answer);
+
+    let answer = Some(42u64);
+    let j_result = j(answer);
+    maps_static_option();
+}

tests/ui/single_option_map.stderr

@@ -0,0 +1,37 @@
+error: `fn` that only maps over argument
+  --> tests/ui/single_option_map.rs:8:1
+   |
+LL | / fn h(arg: Option<u32>) -> Option<u32> {
+LL | |
+LL | |     arg.map(|x| x * 2)
+LL | | }
+   | |_^
+   |
+   = help: move the `.map` to the caller or to an `_opt` function
+   = note: `-D clippy::single-option-map` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::single_option_map)]`
+
+error: `fn` that only maps over argument
+  --> tests/ui/single_option_map.rs:13:1
+   |
+LL | / fn j(arg: Option<u64>) -> Option<u64> {
+LL | |
+LL | |     arg.map(|x| x * 2)
+LL | | }
+   | |_^
+   |
+   = help: move the `.map` to the caller or to an `_opt` function
+
+error: `fn` that only maps over argument
+  --> tests/ui/single_option_map.rs:22:1
+   |
+LL | / fn mul_args_opt(a: Option<String>, b: u64) -> Option<String> {
+LL | |
+LL | |     a.map(|val| mul_args(val, b + 1))
+LL | | }
+   | |_^
+   |
+   = help: move the `.map` to the caller or to an `_opt` function
+
+error: aborting due to 3 previous errors
+