Add lint for opt.map_or(None, f)

clippy_lints/src/methods.rs

@@ -193,6 +193,24 @@ declare_lint! {
      `map_or_else(g, f)`"
 }
 
+/// **What it does:** Checks for usage of `_.map_or(None, _)`.
+///
+/// **Why is this bad?** Readability, this can be written more concisely as
+/// `_.and_then(_)`.
+///
+/// **Known problems:** None.
+///
+/// **Example:**
+/// ```rust
+/// opt.map_or(None, |a| a + 1)
+/// ```
+declare_lint! {
+    pub OPTION_MAP_OR_NONE,
+    Warn,
+    "using `Option.map_or(None, f)`, which is more succinctly expressed as \
+     `map_or_else(g, f)`"
+}
+
 /// **What it does:** Checks for usage of `_.filter(_).next()`.
 ///
 /// **Why is this bad?** Readability, this can be written more concisely as
@@ -574,6 +592,7 @@ impl LintPass for Pass {
             OK_EXPECT,
             OPTION_MAP_UNWRAP_OR,
             OPTION_MAP_UNWRAP_OR_ELSE,
+            OPTION_MAP_OR_NONE,
             OR_FUN_CALL,
             CHARS_NEXT_CMP,
             CHARS_LAST_CMP,
@@ -620,6 +639,8 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
                     lint_map_unwrap_or(cx, expr, arglists[0], arglists[1]);
                 } else if let Some(arglists) = method_chain_args(expr, &["map", "unwrap_or_else"]) {
                     lint_map_unwrap_or_else(cx, expr, arglists[0], arglists[1]);
+                } else if let Some(arglists) = method_chain_args(expr, &["map_or"]) {
+                    lint_map_or_none(cx, expr, arglists[0]);
                 } else if let Some(arglists) = method_chain_args(expr, &["filter", "next"]) {
                     lint_filter_next(cx, expr, arglists[0]);
                 } else if let Some(arglists) = method_chain_args(expr, &["filter", "map"]) {
@@ -1220,6 +1241,37 @@ fn lint_map_unwrap_or_else<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, expr: &'tcx hir
     }
 }
 
+/// lint use of `_.map_or(None, _)` for `Option`s
+fn lint_map_or_none<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, expr: &'tcx hir::Expr, map_or_args: &'tcx [hir::Expr]) {
+    // check if the first non-self argument to map_or() is None
+    let map_or_arg_is_none = if let hir::Expr_::ExprPath(ref qpath) = map_or_args[1].node {
+        match_qpath(qpath, &paths::OPTION_NONE)
+    } else {
+        false
+    };
+
+    if match_type(cx, cx.tables.expr_ty(&map_or_args[0]), &paths::OPTION) && map_or_arg_is_none {
+        // lint message
+        let msg = "called `map_or(None, f)` on an Option value. This can be done more directly by calling \
+                   `and_then(f)` instead";
+        let map_or_none_snippet = snippet(cx, map_or_args[1].span, "..");
+        let map_or_func_snippet = snippet(cx, map_or_args[2].span, "..");
+        let multiline = map_or_func_snippet.lines().count() > 1 || map_or_none_snippet.lines().count() > 1;
+        if multiline {
+            span_lint(cx, OPTION_MAP_OR_NONE, expr.span, msg);
+        } else {
+            span_note_and_lint(
+                cx,
+                OPTION_MAP_OR_NONE,
+                expr.span,
+                msg,
+                expr.span,
+                &format!("replace `map_or({0}, {1})` with `and_then({1})`", map_or_none_snippet, map_or_func_snippet)
+            );
+        }
+    }
+}
+
 /// lint use of `filter().next()` for `Iterators`
 fn lint_filter_next<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, expr: &'tcx hir::Expr, filter_args: &'tcx [hir::Expr]) {
     // lint if caller of `.filter().next()` is an Iterator

tests/ui/methods.rs

@@ -91,6 +91,7 @@ macro_rules! opt_map {
 /// Checks implementation of the following lints:
 /// * `OPTION_MAP_UNWRAP_OR`
 /// * `OPTION_MAP_UNWRAP_OR_ELSE`
+/// * `OPTION_MAP_OR_NONE`
 fn option_methods() {
     let opt = Some(1);
 
@@ -137,6 +138,15 @@ fn option_methods() {
                 );
     // macro case
     let _ = opt_map!(opt, |x| x + 1).unwrap_or_else(|| 0); // should not lint
+
+    // Check OPTION_MAP_OR_NONE
+    // single line case
+    let _ = opt.map_or(None, |x| Some(x + 1));
+    // multi line case
+    let _ = opt.map_or(None, |x| {
+                        Some(x + 1)
+                       }
+                );
 }
 
 /// Struct to generate false positives for things with .iter()