WIP: Filter map lint

CHANGELOG.md

@@ -1040,6 +1040,7 @@ Released 2018-09-13
 [`map_clone`]: https://rust-lang.github.io/rust-clippy/master/index.html#map_clone
 [`map_entry`]: https://rust-lang.github.io/rust-clippy/master/index.html#map_entry
 [`map_flatten`]: https://rust-lang.github.io/rust-clippy/master/index.html#map_flatten
+[`map_flatten_filtermap`]: https://rust-lang.github.io/rust-clippy/master/index.html#map_flatten_filtermap
 [`match_as_ref`]: https://rust-lang.github.io/rust-clippy/master/index.html#match_as_ref
 [`match_bool`]: https://rust-lang.github.io/rust-clippy/master/index.html#match_bool
 [`match_overlapping_arm`]: https://rust-lang.github.io/rust-clippy/master/index.html#match_overlapping_arm

README.md

@@ -6,7 +6,7 @@
 
 A collection of lints to catch common mistakes and improve your [Rust](https://github.com/rust-lang/rust) code.
 
-[There are 313 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
+[There are 314 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
 
 We have a bunch of lint categories to allow you to choose how much Clippy is supposed to ~~annoy~~ help you:
 

clippy_lints/src/lib.rs

@@ -794,6 +794,7 @@ pub fn register_plugins(reg: &mut rustc_driver::plugin::Registry<'_>, conf: &Con
         methods::ITER_NTH,
         methods::ITER_SKIP_NEXT,
         methods::MANUAL_SATURATING_ARITHMETIC,
+        methods::MAP_FLATTEN_FILTERMAP,
         methods::NEW_RET_NO_SELF,
         methods::OK_EXPECT,
         methods::OPTION_AND_THEN_SOME,
@@ -960,6 +961,7 @@ pub fn register_plugins(reg: &mut rustc_driver::plugin::Registry<'_>, conf: &Con
         methods::ITER_CLONED_COLLECT,
         methods::ITER_SKIP_NEXT,
         methods::MANUAL_SATURATING_ARITHMETIC,
+        methods::MAP_FLATTEN_FILTERMAP,
         methods::NEW_RET_NO_SELF,
         methods::OK_EXPECT,
         methods::OPTION_MAP_OR_NONE,

clippy_lints/src/methods/map_flatten_filtermap.rs

@@ -0,0 +1,19 @@
+use rustc::lint::{LateContext, LateLintPass, LintPass};
+use rustc::{declare_lint_pass, declare_tool_lint};
+use rustc::hir;
+use crate::utils::{span_lint};
+use syntax::ast::*;
+use syntax_pos::Span;
+use syntax::visit::FnKind;
+
+use super::MAP_FLATTEN_FILTERMAP;
+
+pub(super) fn lint(cx: &LateContext<'_, '_>, expr: &hir::Expr, args: &[hir::Expr]) {
+    eprintln!("LINT has been called");
+    span_lint(
+        cx,
+        MAP_FLATTEN_FILTERMAP,
+        expr.span,
+        "this `.map().flatten()` can be written more simply using `.filter_map()`",
+    );
+}

clippy_lints/src/methods/mod.rs

@@ -1,6 +1,7 @@
 mod manual_saturating_arithmetic;
 mod option_map_unwrap_or;
 mod unnecessary_filter_map;
+mod map_flatten_filtermap;
 
 use std::borrow::Cow;
 use std::fmt;
@@ -1011,6 +1012,35 @@ declare_clippy_lint! {
     "`.chcked_add/sub(x).unwrap_or(MAX/MIN)`"
 }
 
+declare_clippy_lint! {
+    /// **What it does:** Checks for `.map().flatten()`.
+    ///
+    /// **Why is this bad?** These functions can be replaced with `filter_map()`.
+    ///
+    /// *+Example:**
+    ///
+    /// ```rust
+    /// let v = [10, 20, 30];
+    ///
+    /// let filtered1 = v
+    ///     .iter()
+    ///     .map(|x| if *x > 10 { Some(x) } else { None })
+    ///     .flatten()
+    ///     .collect::<Vec<_>>();
+    /// ```
+    ///
+    /// can be written using `filter_map()` instead:
+    ///
+    /// ```rust
+    /// let filtered2 = v
+    ///     .iter()
+    ///     .filter_map(|x| if *x > 10 { Some(x) } else { None })
+    ///     .collect::<Vec<_>>();
+    pub MAP_FLATTEN_FILTERMAP,
+    style,
+    "`.map().flatten() can be replaced with `.filter_map()`"
+}
+
 declare_lint_pass!(Methods => [
     OPTION_UNWRAP_USED,
     RESULT_UNWRAP_USED,
@@ -1053,6 +1083,7 @@ declare_lint_pass!(Methods => [
     SUSPICIOUS_MAP,
     UNINIT_ASSUMED_INIT,
     MANUAL_SATURATING_ARITHMETIC,
+    MAP_FLATTEN_FILTERMAP,
 ]);
 
 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Methods {
@@ -1102,14 +1133,19 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Methods {
             ["as_ref"] => lint_asref(cx, expr, "as_ref", arg_lists[0]),
             ["as_mut"] => lint_asref(cx, expr, "as_mut", arg_lists[0]),
             ["fold", ..] => lint_unnecessary_fold(cx, expr, arg_lists[0], method_spans[0]),
-            ["filter_map", ..] => unnecessary_filter_map::lint(cx, expr, arg_lists[0]),
+            // Temporarily commented out to test the map_flatten_filtermap alone
+            //["filter_map", ..] => unnecessary_filter_map::lint(cx, expr, arg_lists[0]),
             ["count", "map"] => lint_suspicious_map(cx, expr),
             ["assume_init"] => lint_maybe_uninit(cx, &arg_lists[0][0], expr),
             ["unwrap_or", arith @ "checked_add"]
             | ["unwrap_or", arith @ "checked_sub"]
             | ["unwrap_or", arith @ "checked_mul"] => {
                 manual_saturating_arithmetic::lint(cx, expr, &arg_lists, &arith["checked_".len()..])
             },
+            ["map", "flatten"] => {
+                println!("LINT CALLED");
+                map_flatten_filtermap::lint(cx, expr, arg_lists[0])
+            }
             _ => {},
         }
 

src/lintlist/mod.rs

@@ -6,7 +6,7 @@ pub use lint::Lint;
 pub use lint::LINT_LEVELS;
 
 // begin lint list, do not remove this comment, it’s used in `update_lints`
-pub const ALL_LINTS: [Lint; 313] = [
+pub const ALL_LINTS: [Lint; 314] = [
     Lint {
         name: "absurd_extreme_comparisons",
         group: "correctness",
@@ -973,6 +973,13 @@ pub const ALL_LINTS: [Lint; 313] = [
         deprecation: None,
         module: "methods",
     },
+    Lint {
+        name: "map_flatten_filtermap",
+        group: "style",
+        desc: "`.map().flatten() can be replaced with `.filter_map()`",
+        deprecation: None,
+        module: "methods",
+    },
     Lint {
         name: "match_as_ref",
         group: "complexity",

tests/ui/map_flatten_filtermap.rs

@@ -0,0 +1,18 @@
+#![warn(clippy::map_flatten_filtermap)]
+
+fn main() {
+    let v = [10, 20, 30];
+
+    let filtered1 = v
+        .iter()
+        .map(|x| if *x > 10 { Some(x) } else { None })
+        .flatten()
+        .collect::<Vec<_>>();
+
+    let filtered2 = v
+        .iter()
+        .filter_map(|x| if *x > 10 { Some(x) } else { None })
+        .collect::<Vec<_>>();
+
+    assert_eq!(filtered1, filtered2);
+}