Suggest into_iter() over drain(..)

Add doc

Add description

iter_with_drain dogfood

Disable emiting on struct field.

Fix clippy

Add eq_path for SpanlessEq

Fix tests

Better error message

Fix doc test

Fix version

Apply suggestions

clippy_lints/src/lib.register_all.rs

@@ -164,6 +164,7 @@ store.register_group(true, "clippy::all", Some("clippy_all"), vec![
     LintId::of(methods::ITER_NTH_ZERO),
     LintId::of(methods::ITER_OVEREAGER_CLONED),
     LintId::of(methods::ITER_SKIP_NEXT),
+    LintId::of(methods::ITER_WITH_DRAIN),
     LintId::of(methods::MANUAL_FILTER_MAP),
     LintId::of(methods::MANUAL_FIND_MAP),
     LintId::of(methods::MANUAL_SATURATING_ARITHMETIC),

clippy_lints/src/lib.register_lints.rs

@@ -299,6 +299,7 @@ store.register_lints(&[
     methods::ITER_NTH_ZERO,
     methods::ITER_OVEREAGER_CLONED,
     methods::ITER_SKIP_NEXT,
+    methods::ITER_WITH_DRAIN,
     methods::MANUAL_FILTER_MAP,
     methods::MANUAL_FIND_MAP,
     methods::MANUAL_SATURATING_ARITHMETIC,

clippy_lints/src/lib.register_perf.rs

@@ -16,6 +16,7 @@ store.register_group(true, "clippy::perf", Some("clippy_perf"), vec![
     LintId::of(methods::EXTEND_WITH_DRAIN),
     LintId::of(methods::ITER_NTH),
     LintId::of(methods::ITER_OVEREAGER_CLONED),
+    LintId::of(methods::ITER_WITH_DRAIN),
     LintId::of(methods::MANUAL_STR_REPEAT),
     LintId::of(methods::OR_FUN_CALL),
     LintId::of(methods::SINGLE_CHAR_PATTERN),

clippy_lints/src/methods/iter_with_drain.rs

@@ -0,0 +1,72 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::is_integer_const;
+use clippy_utils::ty::is_type_diagnostic_item;
+use clippy_utils::{
+    higher::{self, Range},
+    SpanlessEq,
+};
+use rustc_ast::ast::RangeLimits;
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, ExprKind, QPath};
+use rustc_lint::LateContext;
+use rustc_span::symbol::{sym, Symbol};
+use rustc_span::Span;
+
+use super::ITER_WITH_DRAIN;
+
+const DRAIN_TYPES: &[Symbol] = &[sym::Vec, sym::VecDeque];
+
+pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, recv: &Expr<'_>, span: Span, arg: &Expr<'_>) {
+    let ty = cx.typeck_results().expr_ty(recv).peel_refs();
+    if let Some(drained_type) = DRAIN_TYPES.iter().find(|&&sym| is_type_diagnostic_item(cx, ty, sym)) {
+        // Refuse to emit `into_iter` suggestion on draining struct fields due
+        // to the strong possibility of processing unmovable field.
+        if let ExprKind::Field(..) = recv.kind {
+            return;
+        }
+
+        if let Some(range) = higher::Range::hir(arg) {
+            let left_full = match range {
+                Range { start: Some(start), .. } if is_integer_const(cx, start, 0) => true,
+                Range { start: None, .. } => true,
+                _ => false,
+            };
+            let full = left_full
+                && match range {
+                    Range {
+                        end: Some(end),
+                        limits: RangeLimits::HalfOpen,
+                        ..
+                    } => {
+                        // `x.drain(..x.len())` call
+                        if_chain! {
+                            if let ExprKind::MethodCall(len_path, len_args, _) = end.kind;
+                            if len_path.ident.name == sym::len && len_args.len() == 1;
+                            if let ExprKind::Path(QPath::Resolved(_, drain_path)) = recv.kind;
+                            if let ExprKind::Path(QPath::Resolved(_, len_path)) = len_args[0].kind;
+                            if SpanlessEq::new(cx).eq_path(drain_path, len_path);
+                            then { true }
+                            else { false }
+                        }
+                    },
+                    Range {
+                        end: None,
+                        limits: RangeLimits::HalfOpen,
+                        ..
+                    } => true,
+                    _ => false,
+                };
+            if full {
+                span_lint_and_sugg(
+                    cx,
+                    ITER_WITH_DRAIN,
+                    span.with_hi(expr.span.hi()),
+                    &format!("`drain(..)` used on a `{}`", drained_type),
+                    "try this",
+                    "into_iter()".to_string(),
+                    Applicability::MaybeIncorrect,
+                );
+            }
+        }
+    }
+}

clippy_lints/src/methods/mod.rs

@@ -32,6 +32,7 @@ mod iter_nth;
 mod iter_nth_zero;
 mod iter_overeager_cloned;
 mod iter_skip_next;
+mod iter_with_drain;
 mod iterator_step_by_zero;
 mod manual_saturating_arithmetic;
 mod manual_str_repeat;
@@ -1118,6 +1119,31 @@ declare_clippy_lint! {
     "using `.skip(x).next()` on an iterator"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for use of `.drain(..)` on `Vec` and `VecDeque` for iteration.
+    ///
+    /// ### Why is this bad?
+    /// `.into_iter()` is simpler with better performance.
+    ///
+    /// ### Example
+    /// ```rust
+    /// # use std::collections::HashSet;
+    /// let mut foo = vec![0, 1, 2, 3];
+    /// let bar: HashSet<usize> = foo.drain(..).collect();
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// # use std::collections::HashSet;
+    /// let foo = vec![0, 1, 2, 3];
+    /// let bar: HashSet<usize> = foo.into_iter().collect();
+    /// ```
+    #[clippy::version = "1.61.0"]
+    pub ITER_WITH_DRAIN,
+    perf,
+    "replace `.drain(..)` with `.into_iter()`"
+}
+
 declare_clippy_lint! {
     /// ### What it does
     /// Checks for use of `.get().unwrap()` (or
@@ -2047,6 +2073,7 @@ impl_lint_pass!(Methods => [
     GET_UNWRAP,
     STRING_EXTEND_CHARS,
     ITER_CLONED_COLLECT,
+    ITER_WITH_DRAIN,
     USELESS_ASREF,
     UNNECESSARY_FOLD,
     UNNECESSARY_FILTER_MAP,
@@ -2327,6 +2354,9 @@ fn check_methods<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, msrv: Optio
                 Some(("map", [_, arg], _)) => suspicious_map::check(cx, expr, recv, arg),
                 _ => {},
             },
+            ("drain", [arg]) => {
+                iter_with_drain::check(cx, expr, recv, span, arg);
+            },
             ("expect", [_]) => match method_call(recv) {
                 Some(("ok", [recv], _)) => ok_expect::check(cx, expr, recv),
                 _ => expect_used::check(cx, expr, recv),

clippy_lints/src/suspicious_operation_groupings.rs

@@ -399,9 +399,9 @@ fn if_statment_binops(kind: &ExprKind) -> Option<Vec<BinaryOp<'_>>> {
 
 fn append_opt_vecs<A>(target_opt: Option<Vec<A>>, source_opt: Option<Vec<A>>) -> Option<Vec<A>> {
     match (target_opt, source_opt) {
-        (Some(mut target), Some(mut source)) => {
+        (Some(mut target), Some(source)) => {
             target.reserve(source.len());
-            for op in source.drain(..) {
+            for op in source {
                 target.push(op);
             }
             Some(target)
@@ -436,9 +436,9 @@ fn chained_binops_helper<'expr>(left_outer: &'expr Expr, right_outer: &'expr Exp
             chained_binops_helper(left_left, left_right),
             chained_binops_helper(right_left, right_right),
         ) {
-            (Some(mut left_ops), Some(mut right_ops)) => {
+            (Some(mut left_ops), Some(right_ops)) => {
                 left_ops.reserve(right_ops.len());
-                for op in right_ops.drain(..) {
+                for op in right_ops {
                     left_ops.push(op);
                 }
                 Some(left_ops)

clippy_lints/src/utils/internal_lints/metadata_collector.rs

@@ -473,7 +473,7 @@ impl<'hir> LateLintPass<'hir> for MetadataCollector {
     /// ```
     fn check_expr(&mut self, cx: &LateContext<'hir>, expr: &'hir hir::Expr<'_>) {
         if let Some(args) = match_lint_emission(cx, expr) {
-            let mut emission_info = extract_emission_info(cx, args);
+            let emission_info = extract_emission_info(cx, args);
             if emission_info.is_empty() {
                 // See:
                 // - src/misc.rs:734:9
@@ -483,7 +483,7 @@ impl<'hir> LateLintPass<'hir> for MetadataCollector {
                 return;
             }
 
-            for (lint_name, applicability, is_multi_part) in emission_info.drain(..) {
+            for (lint_name, applicability, is_multi_part) in emission_info {
                 let app_info = self.applicability_info.entry(lint_name).or_default();
                 app_info.applicability = applicability;
                 app_info.is_multi_part_suggestion = is_multi_part;
@@ -693,7 +693,7 @@ fn extract_emission_info<'hir>(
     }
 
     lints
-        .drain(..)
+        .into_iter()
         .map(|lint_name| (lint_name, applicability, multi_part))
         .collect()
 }