updated code

CHANGELOG.md

@@ -2295,6 +2295,7 @@ Released 2018-09-13
 <!-- begin autogenerated links to lint list -->
 [`absurd_extreme_comparisons`]: https://rust-lang.github.io/rust-clippy/master/index.html#absurd_extreme_comparisons
 [`almost_swapped`]: https://rust-lang.github.io/rust-clippy/master/index.html#almost_swapped
+[`append_instead_of_extend`]: https://rust-lang.github.io/rust-clippy/master/index.html#append_instead_of_extend
 [`approx_constant`]: https://rust-lang.github.io/rust-clippy/master/index.html#approx_constant
 [`as_conversions`]: https://rust-lang.github.io/rust-clippy/master/index.html#as_conversions
 [`assertions_on_constants`]: https://rust-lang.github.io/rust-clippy/master/index.html#assertions_on_constants

clippy_lints/src/append_instead_of_extend.rs

@@ -0,0 +1,61 @@
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+// use rustc_ast::ast::*;
+use clippy_utils::diagnostics::span_lint_and_help;
+use clippy_utils::ty::is_type_diagnostic_item;
+use if_chain::if_chain;
+use rustc_hir::{Expr, ExprKind};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_span::symbol::sym;
+
+declare_clippy_lint! {
+    /// **What it does:**
+    ///
+    /// **Why is this bad?**
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    ///
+    /// ```rust
+    /// // example code where clippy issues a warning
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// // example code which does not raise clippy warning
+    /// ```
+    pub APPEND_INSTEAD_OF_EXTEND,
+    perf,
+    "default lint description"
+}
+
+declare_lint_pass!(AppendInsteadOfExtend => [APPEND_INSTEAD_OF_EXTEND]);
+
+impl<'tcx> LateLintPass<'tcx> for AppendInsteadOfExtend {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+        if is_vec_an_append(cx, expr) {
+            span_lint_and_help(
+                cx,
+                APPEND_INSTEAD_OF_EXTEND,
+                expr.span,
+                "use of `vec.extend(other_vec.drain(..))`",
+                None,
+                "consider using `vec![].append(&mut vec![]);` instead",
+            );
+        }
+    }
+}
+
+fn is_vec_an_append<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) -> bool {
+    if_chain! {
+        if let ExprKind::MethodCall(method_name, _, exprs, _) = expr.kind;
+        if method_name.ident.as_str() == "extend";
+        let ty = cx.typeck_results().expr_ty(&exprs[0]).peel_refs();
+        if is_type_diagnostic_item(cx, ty, sym::vec_type);
+        if let ExprKind::MethodCall(method_name2, _, _exprs2, _) = &exprs[1].kind;
+        if method_name2.ident.as_str() == "drain";
+        then {
+            return true
+        }
+    }
+    false
+}

clippy_lints/src/lib.rs

@@ -167,6 +167,7 @@ mod deprecated_lints;
 
 // begin lints modules, do not remove this comment, it’s used in `update_lints`
 mod absurd_extreme_comparisons;
+mod append_instead_of_extend;
 mod approx_const;
 mod arithmetic;
 mod as_conversions;
@@ -534,6 +535,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         #[cfg(feature = "internal-lints")]
         utils::internal_lints::UNNECESSARY_SYMBOL_STR,
         absurd_extreme_comparisons::ABSURD_EXTREME_COMPARISONS,
+        append_instead_of_extend::APPEND_INSTEAD_OF_EXTEND,
         approx_const::APPROX_CONSTANT,
         arithmetic::FLOAT_ARITHMETIC,
         arithmetic::INTEGER_ARITHMETIC,
@@ -1166,6 +1168,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
 
     store.register_group(true, "clippy::all", Some("clippy"), vec![
         LintId::of(absurd_extreme_comparisons::ABSURD_EXTREME_COMPARISONS),
+        LintId::of(append_instead_of_extend::APPEND_INSTEAD_OF_EXTEND),
         LintId::of(approx_const::APPROX_CONSTANT),
         LintId::of(assertions_on_constants::ASSERTIONS_ON_CONSTANTS),
         LintId::of(assign_ops::ASSIGN_OP_PATTERN),
@@ -1744,6 +1747,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     ]);
 
     store.register_group(true, "clippy::perf", Some("clippy_perf"), vec![
+        LintId::of(append_instead_of_extend::APPEND_INSTEAD_OF_EXTEND),
         LintId::of(entry::MAP_ENTRY),
         LintId::of(escape::BOXED_LOCAL),
         LintId::of(large_const_arrays::LARGE_CONST_ARRAYS),
@@ -2078,7 +2082,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(move || box if_then_some_else_none::IfThenSomeElseNone::new(msrv));
     store.register_early_pass(|| box bool_assert_comparison::BoolAssertComparison);
     store.register_late_pass(|| box unused_async::UnusedAsync);
-
+    store.register_late_pass(|| box append_instead_of_extend::AppendInsteadOfExtend);
 }
 
 #[rustfmt::skip]

tests/ui/append_instead_of_extend.rs

@@ -0,0 +1,13 @@
+#![warn(clippy::append_instead_of_extend)]
+
+fn main() {
+    let mut a = vec![0u8; 1024];
+    let mut b = Vec::new();
+
+    b.extend(a.drain(..));
+
+    let mut c = vec![0u8, 10];
+    let mut d: std::vec::Vec<u8> = Vec::new();
+
+    c.extend(d.drain(1..3))
+}

tests/ui/append_instead_of_extend.stderr

@@ -0,0 +1,11 @@
+error: use of `vec.extend(other_vec.drain(..))`
+  --> $DIR/append_instead_of_extend.rs:7:5
+   |
+LL |     b.extend(a.drain(..));
+   |     ^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::append-instead-of-extend` implied by `-D warnings`
+   = help: consider using `vec![].append(&mut vec![]);` instead
+
+error: aborting due to previous error
+