Handle calls with 'std::convert::identity'

clippy_lints/src/methods/mod.rs

@@ -2186,6 +2186,24 @@ fn lint_flat_map<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, expr: &'tcx hir::Expr, fl
             span_lint(cx, FLAT_MAP, expr.span, msg);
         }
     }
+
+    if_chain! {
+        if match_trait_method(cx, expr, &paths::ITERATOR);
+
+        if flat_map_args.len() == 2;
+
+        let expr = &flat_map_args[1];
+
+        if let hir::ExprKind::Path(ref qpath) = expr.node;
+
+        if match_qpath(qpath, &paths::STD_CONVERT_IDENTITY);
+
+        then {
+            let msg = "called `flat_map(std::convert::identity)` on an `Iterator`. \
+                       This can be simplified by calling `flatten().`";
+            span_lint(cx, FLAT_MAP, expr.span, msg);
+        }
+    }
 }
 
 /// lint searching an Iterator followed by `is_some()`

clippy_lints/src/utils/paths.rs

@@ -95,6 +95,7 @@
 pub const SLICE_ITER: [&str; 3] = ["core", "slice", "Iter"];
 pub const STDERR: [&str; 4] = ["std", "io", "stdio", "stderr"];
 pub const STDOUT: [&str; 4] = ["std", "io", "stdio", "stdout"];
+pub const STD_CONVERT_IDENTITY: [&str; 3] = ["std", "convert", "identity"];
 pub const STD_MEM_TRANSMUTE: [&str; 3] = ["std", "mem", "transmute"];
 pub const STD_PTR_NULL: [&str; 3] = ["std", "ptr", "null"];
 pub const STRING: [&str; 3] = ["alloc", "string", "String"];

src/lintlist/mod.rs

@@ -6,7 +6,7 @@
 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",

tests/ui/unnecessary_flat_map.rs

@@ -1,6 +1,11 @@
 #![warn(clippy::flat_map)]
 
+use std::convert;
+
 fn main() {
     let iterator = [[0, 1], [2, 3], [4, 5]].iter();
     iterator.flat_map(|x| x);
+
+    let iterator = [[0, 1], [2, 3], [4, 5]].iter();
+    iterator.flat_map(convert::identity);
 }

tests/ui/unnecessary_flat_map.stderr

@@ -1,10 +1,16 @@
 error: called `flat_map(|x| x)` on an `Iterator`. This can be simplified by calling `flatten().`
-  --> $DIR/unnecessary_flat_map.rs:5:5
+  --> $DIR/unnecessary_flat_map.rs:7:5
    |
 LL |     iterator.flat_map(|x| x);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `-D clippy::flat-map` implied by `-D warnings`
 
-error: aborting due to previous error
+error: called `flat_map(std::convert::identity)` on an `Iterator`. This can be simplified by calling `flatten().`
+  --> $DIR/unnecessary_flat_map.rs:10:23
+   |
+LL |     iterator.flat_map(convert::identity);
+   |                       ^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors