1 files changed, 64 insertions, 0 deletions
diff --git a/src/tools/clippy/clippy_lints/src/types/vec_box.rs b/src/tools/clippy/clippy_lints/src/types/vec_box.rs
new file mode 100644
index 000000000..b2f536ca7
--- /dev/null
+++ b/src/tools/clippy/clippy_lints/src/types/vec_box.rs
@@ -0,0 +1,64 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::last_path_segment;
+use clippy_utils::source::snippet;
+use if_chain::if_chain;
+use rustc_errors::Applicability;
+use rustc_hir::{self as hir, def_id::DefId, GenericArg, QPath, TyKind};
+use rustc_lint::LateContext;
+use rustc_middle::ty::layout::LayoutOf;
+use rustc_middle::ty::TypeVisitable;
+use rustc_span::symbol::sym;
+use rustc_typeck::hir_ty_to_ty;
+
+use super::VEC_BOX;
+
+pub(super) fn check(
+    cx: &LateContext<'_>,
+    hir_ty: &hir::Ty<'_>,
+    qpath: &QPath<'_>,
+    def_id: DefId,
+    box_size_threshold: u64,
+) -> bool {
+    if cx.tcx.is_diagnostic_item(sym::Vec, def_id) {
+        if_chain! {
+            // Get the _ part of Vec<_>
+            if let Some(last) = last_path_segment(qpath).args;
+            if let Some(ty) = last.args.iter().find_map(|arg| match arg {
+                GenericArg::Type(ty) => Some(ty),
+                _ => None,
+            });
+            // ty is now _ at this point
+            if let TyKind::Path(ref ty_qpath) = ty.kind;
+            let res = cx.qpath_res(ty_qpath, ty.hir_id);
+            if let Some(def_id) = res.opt_def_id();
+            if Some(def_id) == cx.tcx.lang_items().owned_box();
+            // At this point, we know ty is Box<T>, now get T
+            if let Some(last) = last_path_segment(ty_qpath).args;
+            if let Some(boxed_ty) = last.args.iter().find_map(|arg| match arg {
+                GenericArg::Type(ty) => Some(ty),
+                _ => None,
+            });
+            let ty_ty = hir_ty_to_ty(cx.tcx, boxed_ty);
+            if !ty_ty.has_escaping_bound_vars();
+            if ty_ty.is_sized(cx.tcx.at(ty.span), cx.param_env);
+            if let Ok(ty_ty_size) = cx.layout_of(ty_ty).map(|l| l.size.bytes());
+            if ty_ty_size <= box_size_threshold;
+            then {
+                span_lint_and_sugg(
+                    cx,
+                    VEC_BOX,
+                    hir_ty.span,
+                    "`Vec<T>` is already on the heap, the boxing is unnecessary",
+                    "try",
+                    format!("Vec<{}>", snippet(cx, boxed_ty.span, "..")),
+                    Applicability::MachineApplicable,
+                );
+                true
+            } else {
+                false
+            }
+        }
+    } else {
+        false
+    }
+}