Implement "suspicious_map" lint

CHANGELOG.md

@@ -1152,6 +1152,7 @@ Released 2018-09-13
 [`suspicious_arithmetic_impl`]: https://rust-lang.github.io/rust-clippy/master/index.html#suspicious_arithmetic_impl
 [`suspicious_assignment_formatting`]: https://rust-lang.github.io/rust-clippy/master/index.html#suspicious_assignment_formatting
 [`suspicious_else_formatting`]: https://rust-lang.github.io/rust-clippy/master/index.html#suspicious_else_formatting
+[`suspicious_map`]: https://rust-lang.github.io/rust-clippy/master/index.html#suspicious_map
 [`suspicious_op_assign_impl`]: https://rust-lang.github.io/rust-clippy/master/index.html#suspicious_op_assign_impl
 [`temporary_assignment`]: https://rust-lang.github.io/rust-clippy/master/index.html#temporary_assignment
 [`temporary_cstring_as_ptr`]: https://rust-lang.github.io/rust-clippy/master/index.html#temporary_cstring_as_ptr

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 309 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
+[There are 310 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

@@ -800,6 +800,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry<'_>, conf: &Conf) {
         methods::SHOULD_IMPLEMENT_TRAIT,
         methods::SINGLE_CHAR_PATTERN,
         methods::STRING_EXTEND_CHARS,
+        methods::SUSPICIOUS_MAP,
         methods::TEMPORARY_CSTRING_AS_PTR,
         methods::UNNECESSARY_FILTER_MAP,
         methods::UNNECESSARY_FOLD,
@@ -1033,6 +1034,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry<'_>, conf: &Conf) {
         methods::FILTER_NEXT,
         methods::FLAT_MAP_IDENTITY,
         methods::SEARCH_IS_SOME,
+        methods::SUSPICIOUS_MAP,
         methods::UNNECESSARY_FILTER_MAP,
         methods::USELESS_ASREF,
         misc::SHORT_CIRCUIT_STATEMENT,

clippy_lints/src/methods/mod.rs

@@ -18,7 +18,6 @@ use syntax::ast;
 use syntax::source_map::Span;
 use syntax::symbol::LocalInternedString;
 
-use crate::utils::paths;
 use crate::utils::sugg;
 use crate::utils::usage::mutated_variables;
 use crate::utils::{
@@ -28,6 +27,7 @@ use crate::utils::{
     snippet, snippet_with_applicability, snippet_with_macro_callsite, span_lint, span_lint_and_sugg,
     span_lint_and_then, span_note_and_lint, walk_ptrs_ty, walk_ptrs_ty_depth, SpanlessEq,
 };
+use crate::utils::{paths, span_help_and_lint};
 
 declare_clippy_lint! {
     /// **What it does:** Checks for `.unwrap()` calls on `Option`s.
@@ -889,6 +889,24 @@ declare_clippy_lint! {
     "using `.into_iter()` on a reference"
 }
 
+declare_clippy_lint! {
+    /// **What it does:** Checks for calls to `map` followed by a `count`.
+    ///
+    /// **Why is this bad?** It looks suspicious. Maybe `map` was confused with `filter`.
+    /// If the `map` call is intentional, this should be rewritten.
+    ///
+    /// **Known problems:** None
+    ///
+    /// **Example:**
+    ///
+    /// ```rust
+    /// let _ = (0..3).map(|x| x + 2).count();
+    /// ```
+    pub SUSPICIOUS_MAP,
+    complexity,
+    "suspicious usage of map"
+}
+
 declare_lint_pass!(Methods => [
     OPTION_UNWRAP_USED,
     RESULT_UNWRAP_USED,
@@ -927,6 +945,7 @@ declare_lint_pass!(Methods => [
     UNNECESSARY_FILTER_MAP,
     INTO_ITER_ON_ARRAY,
     INTO_ITER_ON_REF,
+    SUSPICIOUS_MAP,
 ]);
 
 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Methods {
@@ -972,6 +991,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Methods {
             ["as_mut"] => lint_asref(cx, expr, "as_mut", arg_lists[0]),
             ["fold", ..] => lint_unnecessary_fold(cx, expr, arg_lists[0]),
             ["filter_map", ..] => unnecessary_filter_map::lint(cx, expr, arg_lists[0]),
+            ["count", "map"] => lint_suspicious_map(cx, expr),
             _ => {},
         }
 
@@ -2519,6 +2539,16 @@ fn lint_into_iter(cx: &LateContext<'_, '_>, expr: &hir::Expr, self_ref_ty: Ty<'_
     }
 }
 
+fn lint_suspicious_map(cx: &LateContext<'_, '_>, expr: &hir::Expr) {
+    span_help_and_lint(
+        cx,
+        SUSPICIOUS_MAP,
+        expr.span,
+        "this call to `map()` won't have an effect on the call to `count()`",
+        "make sure you did not confuse `map` with `filter`",
+    );
+}
+
 /// Given a `Result<T, E>` type, return its error type (`E`).
 fn get_error_type<'a>(cx: &LateContext<'_, '_>, ty: Ty<'a>) -> Option<Ty<'a>> {
     if let ty::Adt(_, substs) = ty.sty {

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; 309] = [
+pub const ALL_LINTS: [Lint; 310] = [
     Lint {
         name: "absurd_extreme_comparisons",
         group: "correctness",
@@ -1736,6 +1736,13 @@ pub const ALL_LINTS: [Lint; 309] = [
         deprecation: None,
         module: "formatting",
     },
+    Lint {
+        name: "suspicious_map",
+        group: "complexity",
+        desc: "suspicious usage of map",
+        deprecation: None,
+        module: "methods",
+    },
     Lint {
         name: "suspicious_op_assign_impl",
         group: "correctness",

tests/ui/suspicious_map.rs

@@ -0,0 +1,5 @@
+#![warn(clippy::suspicious_map)]
+
+fn main() {
+    let _ = (0..3).map(|x| x + 2).count();
+}

tests/ui/suspicious_map.stderr

@@ -0,0 +1,11 @@
+error: this call to `map()` won't have an effect on the call to `count()`
+  --> $DIR/suspicious_map.rs:4:13
+   |
+LL |     let _ = (0..3).map(|x| x + 2).count();
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::suspicious-map` implied by `-D warnings`
+   = help: make sure you did not confuse `map` with `filter`
+
+error: aborting due to previous error
+