1 files changed, 100 insertions, 0 deletions
diff --git a/src/tools/clippy/clippy_lints/src/functions/result.rs b/src/tools/clippy/clippy_lints/src/functions/result.rs
new file mode 100644
index 000000000..9591405cb
--- /dev/null
+++ b/src/tools/clippy/clippy_lints/src/functions/result.rs
@@ -0,0 +1,100 @@
+use rustc_errors::Diagnostic;
+use rustc_hir as hir;
+use rustc_lint::{LateContext, LintContext};
+use rustc_middle::lint::in_external_macro;
+use rustc_middle::ty::{self, Ty};
+use rustc_span::{sym, Span};
+
+use clippy_utils::diagnostics::{span_lint_and_help, span_lint_and_then};
+use clippy_utils::trait_ref_of_method;
+use clippy_utils::ty::{approx_ty_size, is_type_diagnostic_item};
+
+use super::{RESULT_LARGE_ERR, RESULT_UNIT_ERR};
+
+/// The type of the `Err`-variant in a `std::result::Result` returned by the
+/// given `FnDecl`
+fn result_err_ty<'tcx>(
+    cx: &LateContext<'tcx>,
+    decl: &hir::FnDecl<'tcx>,
+    id: hir::def_id::LocalDefId,
+    item_span: Span,
+) -> Option<(&'tcx hir::Ty<'tcx>, Ty<'tcx>)> {
+    if !in_external_macro(cx.sess(), item_span)
+        && let hir::FnRetTy::Return(hir_ty) = decl.output
+        && let ty = cx.tcx.erase_late_bound_regions(cx.tcx.fn_sig(id).output())
+        && is_type_diagnostic_item(cx, ty, sym::Result)
+        && let ty::Adt(_, substs) = ty.kind()
+    {
+        let err_ty = substs.type_at(1);
+        Some((hir_ty, err_ty))
+    } else {
+        None
+    }
+}
+
+pub(super) fn check_item<'tcx>(cx: &LateContext<'tcx>, item: &hir::Item<'tcx>, large_err_threshold: u64) {
+    if let hir::ItemKind::Fn(ref sig, _generics, _) = item.kind
+        && let Some((hir_ty, err_ty)) = result_err_ty(cx, sig.decl, item.def_id, item.span)
+    {
+        if cx.access_levels.is_exported(item.def_id) {
+            let fn_header_span = item.span.with_hi(sig.decl.output.span().hi());
+            check_result_unit_err(cx, err_ty, fn_header_span);
+        }
+        check_result_large_err(cx, err_ty, hir_ty.span, large_err_threshold);
+    }
+}
+
+pub(super) fn check_impl_item<'tcx>(cx: &LateContext<'tcx>, item: &hir::ImplItem<'tcx>, large_err_threshold: u64) {
+    // Don't lint if method is a trait's implementation, we can't do anything about those
+    if let hir::ImplItemKind::Fn(ref sig, _) = item.kind
+        && let Some((hir_ty, err_ty)) = result_err_ty(cx, sig.decl, item.def_id, item.span)
+        && trait_ref_of_method(cx, item.def_id).is_none()
+    {
+        if cx.access_levels.is_exported(item.def_id) {
+            let fn_header_span = item.span.with_hi(sig.decl.output.span().hi());
+            check_result_unit_err(cx, err_ty, fn_header_span);
+        }
+        check_result_large_err(cx, err_ty, hir_ty.span, large_err_threshold);
+    }
+}
+
+pub(super) fn check_trait_item<'tcx>(cx: &LateContext<'tcx>, item: &hir::TraitItem<'tcx>, large_err_threshold: u64) {
+    if let hir::TraitItemKind::Fn(ref sig, _) = item.kind {
+        let fn_header_span = item.span.with_hi(sig.decl.output.span().hi());
+        if let Some((hir_ty, err_ty)) = result_err_ty(cx, sig.decl, item.def_id, item.span) {
+            if cx.access_levels.is_exported(item.def_id) {
+                check_result_unit_err(cx, err_ty, fn_header_span);
+            }
+            check_result_large_err(cx, err_ty, hir_ty.span, large_err_threshold);
+        }
+    }
+}
+
+fn check_result_unit_err(cx: &LateContext<'_>, err_ty: Ty<'_>, fn_header_span: Span) {
+    if err_ty.is_unit() {
+        span_lint_and_help(
+            cx,
+            RESULT_UNIT_ERR,
+            fn_header_span,
+            "this returns a `Result<_, ()>`",
+            None,
+            "use a custom `Error` type instead",
+        );
+    }
+}
+
+fn check_result_large_err<'tcx>(cx: &LateContext<'tcx>, err_ty: Ty<'tcx>, hir_ty_span: Span, large_err_threshold: u64) {
+    let ty_size = approx_ty_size(cx, err_ty);
+    if ty_size >= large_err_threshold {
+        span_lint_and_then(
+            cx,
+            RESULT_LARGE_ERR,
+            hir_ty_span,
+            "the `Err`-variant returned from this function is very large",
+            |diag: &mut Diagnostic| {
+                diag.span_label(hir_ty_span, format!("the `Err`-variant is at least {ty_size} bytes"));
+                diag.help(format!("try reducing the size of `{err_ty}`, for example by boxing large elements or replacing it with `Box<{err_ty}>`"));
+            },
+        );
+    }
+}