Add the lint logic

clippy_lints/src/methods/clear_with_drain.rs

@@ -1,8 +1,29 @@
-use rustc_lint::{LateContext, LintContext};
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::higher::Range;
+use clippy_utils::is_range_full;
+use clippy_utils::ty::is_type_diagnostic_item;
+use rustc_errors::Applicability;
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_span::symbol::sym;
+use rustc_span::Span;
 
 use super::CLEAR_WITH_DRAIN;
 
 // TODO: Adjust the parameters as necessary
-pub(super) fn check(cx: &LateContext) {
-    todo!();
+// see clippy_lints/src/methods/mod.rs to add call to this check in `check_methods`
+pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, recv: &Expr<'_>, span: Span, arg: &Expr<'_>) {
+    let ty = cx.typeck_results().expr_ty(recv);
+    if is_type_diagnostic_item(cx, ty, sym::Vec) && let Some(range) = Range::hir(arg) && is_range_full(cx, recv, range)
+    {
+        span_lint_and_sugg(
+            cx,
+            CLEAR_WITH_DRAIN,
+            span.with_hi(expr.span.hi()),
+            "`drain` used to clear a `Vec`",
+            "try",
+            "clear()".to_string(),
+            Applicability::MachineApplicable,
+        );
+    }
 }

clippy_lints/src/methods/mod.rs

@@ -111,7 +111,7 @@ use clippy_utils::ty::{contains_ty_adt_constructor_opaque, implements_trait, is_
 use clippy_utils::{contains_return, is_bool, is_trait_method, iter_input_pats, return_ty};
 use if_chain::if_chain;
 use rustc_hir as hir;
-use rustc_hir::{Expr, ExprKind, TraitItem, TraitItemKind};
+use rustc_hir::{Expr, ExprKind, Node, Stmt, StmtKind, TraitItem, TraitItemKind};
 use rustc_hir_analysis::hir_ty_to_ty;
 use rustc_lint::{LateContext, LateLintPass, LintContext};
 use rustc_middle::lint::in_external_macro;
@@ -3590,7 +3590,13 @@ impl Methods {
                     _ => {},
                 },
                 ("drain", [arg]) => {
-                    iter_with_drain::check(cx, expr, recv, span, arg);
+                if let Node::Stmt(Stmt { hir_id: _, kind, .. }) = cx.tcx.hir().get_parent(expr.hir_id)
+                    && matches!(kind, StmtKind::Semi(_))
+                    {
+                        clear_with_drain::check(cx, expr, recv, span, arg);
+                    } else {
+                        iter_with_drain::check(cx, expr, recv, span, arg);
+                    }
                 },
                 ("ends_with", [arg]) => {
                     if let ExprKind::MethodCall(.., span) = expr.kind {