1 files changed, 82 insertions, 14 deletions

diff --git a/clippy_lints/src/box_default.rs b/clippy_lints/src/box_default.rs
index 792183ac408..bb0307e8856 100644
--- a/clippy_lints/src/box_default.rs
+++ b/clippy_lints/src/box_default.rs
@@ -1,5 +1,12 @@
-use clippy_utils::{diagnostics::span_lint_and_help, is_default_equivalent, path_def_id};
-use rustc_hir::{Expr, ExprKind, QPath};
+use clippy_utils::{
+    diagnostics::span_lint_and_sugg, get_parent_node, is_default_equivalent, macros::macro_backtrace, match_path,
+    path_def_id, paths, ty::expr_sig,
+};
+use rustc_errors::Applicability;
+use rustc_hir::{
+    intravisit::{walk_ty, Visitor},
+    Block, Expr, ExprKind, Local, Node, QPath, TyKind,
+};
 use rustc_lint::{LateContext, LateLintPass, LintContext};
 use rustc_middle::lint::in_external_macro;
 use rustc_session::{declare_lint_pass, declare_tool_lint};
@@ -15,12 +22,6 @@ declare_clippy_lint! {
     /// Second, `Box::default()` can be faster
     /// [in certain cases](https://nnethercote.github.io/perf-book/standard-library-types.html#box).
     ///
-    /// ### Known problems
-    /// The lint may miss some cases (e.g. Box::new(String::from(""))).
-    /// On the other hand, it will trigger on cases where the `default`
-    /// code comes from a macro that does something different based on
-    /// e.g. target operating system.
-    ///
     /// ### Example
     /// ```rust
     /// let x: Box<String> = Box::new(Default::default());
@@ -41,21 +42,88 @@ impl LateLintPass<'_> for BoxDefault {
     fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {
         if let ExprKind::Call(box_new, [arg]) = expr.kind
             && let ExprKind::Path(QPath::TypeRelative(ty, seg)) = box_new.kind
-            && let ExprKind::Call(..) = arg.kind
+            && let ExprKind::Call(arg_path, ..) = arg.kind
             && !in_external_macro(cx.sess(), expr.span)
-            && expr.span.eq_ctxt(arg.span)
+            && (expr.span.eq_ctxt(arg.span) || is_vec_expn(cx, arg))
             && seg.ident.name == sym::new
-            && path_def_id(cx, ty) == cx.tcx.lang_items().owned_box()
+            && path_def_id(cx, ty).map_or(false, |id| Some(id) == cx.tcx.lang_items().owned_box())
             && is_default_equivalent(cx, arg)
         {
-            span_lint_and_help(
+            let arg_ty = cx.typeck_results().expr_ty(arg);
+            span_lint_and_sugg(
                 cx,
                 BOX_DEFAULT,
                 expr.span,
                 "`Box::new(_)` of default value",
-                None,
-                "use `Box::default()` instead",
+                "try",
+                if is_plain_default(arg_path) || given_type(cx, expr) {
+                    "Box::default()".into()
+                } else {
+                    format!("Box::<{arg_ty}>::default()")
+                },
+                Applicability::MachineApplicable
             );
         }
     }
 }
+
+fn is_plain_default(arg_path: &Expr<'_>) -> bool {
+    // we need to match the actual path so we don't match e.g. "u8::default"
+    if let ExprKind::Path(QPath::Resolved(None, path)) = &arg_path.kind {
+        // avoid generic parameters
+        match_path(path, &paths::DEFAULT_TRAIT_METHOD) && path.segments.iter().all(|seg| seg.args.is_none())
+    } else {
+        false
+    }
+}
+
+fn is_vec_expn(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
+    macro_backtrace(expr.span)
+        .next()
+        .map_or(false, |call| cx.tcx.is_diagnostic_item(sym::vec_macro, call.def_id))
+}
+
+#[derive(Default)]
+struct InferVisitor(bool);
+
+impl<'tcx> Visitor<'tcx> for InferVisitor {
+    fn visit_ty(&mut self, t: &rustc_hir::Ty<'_>) {
+        self.0 |= matches!(t.kind, TyKind::Infer);
+        if !self.0 {
+            walk_ty(self, t);
+        }
+    }
+}
+
+fn given_type(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
+    match get_parent_node(cx.tcx, expr.hir_id) {
+        Some(Node::Local(Local { ty: Some(ty), .. })) => {
+            let mut v = InferVisitor::default();
+            v.visit_ty(ty);
+            !v.0
+        },
+        Some(
+            Node::Expr(Expr {
+                kind: ExprKind::Call(path, args),
+                ..
+            }) | Node::Block(Block {
+                expr:
+                    Some(Expr {
+                        kind: ExprKind::Call(path, args),
+                        ..
+                    }),
+                ..
+            }),
+        ) => {
+            if let Some(index) = args.iter().position(|arg| arg.hir_id == expr.hir_id) &&
+                let Some(sig) = expr_sig(cx, path) &&
+                let Some(input) = sig.input(index)
+            {
+                input.no_bound_vars().is_some()
+            } else {
+                false
+            }
+        },
+        _ => false,
+    }
+}