Add a lint for slice.iter().cloned().collect()

If one uses `slice.iter().cloned().collect()` to create a new `Vec`,
it should be `slice.to_owned()`.

Fix #1292

Author: F001

clippy_lints/src/methods.rs

@@ -494,13 +494,33 @@ declare_lint! {
 /// s.push_str(abc);
 /// s.push_str(&def));
 /// ```
-
 declare_lint! {
     pub STRING_EXTEND_CHARS,
     Warn,
     "using `x.extend(s.chars())` where s is a `&str` or `String`"
 }
 
+/// **What it does:** Checks for the use of `.cloned().collect()` on slice to create a Vec.
+///
+/// **Why is this bad?** `.to_owned()` is clearer
+///
+/// **Known problems:** None.
+///
+/// **Example:**
+/// ```rust
+/// let s = [1,2,3,4,5];
+/// let s2 : Vec<isize> = s.iter().cloned().collect();
+/// ```
+/// The correct use would be:
+/// ```rust
+/// let s = [1,2,3,4,5];
+/// let s2 : Vec<isize> = s.to_owned();
+/// ```
+declare_lint! {
+    pub ITER_CLONED_COLLECT,
+    Warn,
+    "using `.cloned().collect()` on slice to create a `Vec`"
+}
 
 impl LintPass for Pass {
     fn get_lints(&self) -> LintArray {
@@ -525,7 +545,8 @@ impl LintPass for Pass {
                     ITER_NTH,
                     ITER_SKIP_NEXT,
                     GET_UNWRAP,
-                    STRING_EXTEND_CHARS)
+                    STRING_EXTEND_CHARS,
+                    ITER_CLONED_COLLECT)
     }
 }
 
@@ -580,6 +601,8 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
                     lint_iter_nth(cx, expr, arglists[0], true);
                 } else if method_chain_args(expr, &["skip", "next"]).is_some() {
                     lint_iter_skip_next(cx, expr);
+                } else if let Some(arglists) = method_chain_args(expr, &["cloned", "collect"]) {
+                    lint_iter_cloned_collect(cx, expr, arglists[0]);
                 }
 
                 lint_or_fun_call(cx, expr, &name.node.as_str(), args);
@@ -879,6 +902,16 @@ fn lint_cstring_as_ptr(cx: &LateContext, expr: &hir::Expr, new: &hir::Expr, unwr
     }}
 }
 
+fn lint_iter_cloned_collect(cx: &LateContext, expr: &hir::Expr, iter_args: &[hir::Expr]) {
+    if match_type(cx, cx.tables.expr_ty(expr), &paths::VEC) &&
+        derefs_to_slice(cx, &iter_args[0], cx.tables.expr_ty(&iter_args[0])).is_some() {
+        span_lint(cx,
+                  ITER_CLONED_COLLECT,
+                  expr.span,
+                  "called `cloned().collect()` on a slice to create a `Vec`. This is more succinctly expressed by calling `to_owned(x)`");
+    }
+}
+
 fn lint_iter_nth(cx: &LateContext, expr: &hir::Expr, iter_args: &[hir::Expr], is_mut: bool) {
     let mut_str = if is_mut { "_mut" } else { "" };
     let caller_type = if derefs_to_slice(cx, &iter_args[0], cx.tables.expr_ty(&iter_args[0])).is_some() {

src/main.rs

@@ -1,7 +1,6 @@
 // error-pattern:yummy
 #![feature(box_syntax)]
 #![feature(rustc_private)]
-#![feature(static_in_const)]
 
 #![allow(unknown_lints, missing_docs_in_private_items)]
 

tests/ui/methods.rs

@@ -689,3 +689,8 @@ fn temporary_cstring() {
 
 
 }
+
+fn iter_clone_collect() {
+    let v = [1,2,3,4,5];
+    let v2 : Vec<isize> = v.iter().cloned().collect();
+}

tests/ui/methods.stderr

@@ -950,5 +950,13 @@ help: assign the `CString` to a variable to extend its lifetime
 687 |     CString::new("foo").unwrap().as_ptr();
     |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+warning: called `cloned().collect()` on a slice to create a `Vec`. This is more succinctly expressed by calling `to_owned(x)`
+   --> $DIR/methods.rs:695:27
+    |
+695 |     let v2 : Vec<isize> = v.iter().cloned().collect();
+    |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+    |
+    = note: #[warn(iter_cloned_collect)] on by default
+
 error: aborting due to 88 previous errors