rustc: Panic by default in DefIdTree::parent

Only crate root def-ids don't have a parent, and in majority of cases the argument of `DefIdTree::parent` cannot be a crate root.
So we now panic by default in `parent` and introduce a new non-panicing function `opt_parent` for cases where the argument can be a crate root.

Same applies to `local_parent`/`opt_local_parent`.

Author: petrochenkov

clippy_lints/src/matches/redundant_pattern_match.rs

@@ -193,7 +193,7 @@ fn find_sugg_for_if_let<'tcx>(
         PatKind::TupleStruct(ref qpath, [sub_pat], _) => {
             if let PatKind::Wild = sub_pat.kind {
                 let res = cx.typeck_results().qpath_res(qpath, check_pat.hir_id);
-                let Some(id) = res.opt_def_id().and_then(|ctor_id| cx.tcx.parent(ctor_id)) else { return };
+                let Some(id) = res.opt_def_id().map(|ctor_id| cx.tcx.parent(ctor_id)) else { return };
                 let lang_items = cx.tcx.lang_items();
                 if Some(id) == lang_items.result_ok_variant() {
                     ("is_ok()", try_get_generic_ty(op_ty, 0).unwrap_or(op_ty))

clippy_lints/src/methods/bind_instead_of_map.rs

@@ -42,7 +42,7 @@ pub(crate) trait BindInsteadOfMap {
 
     fn no_op_msg(cx: &LateContext<'_>) -> Option<String> {
         let variant_id = cx.tcx.lang_items().require(Self::VARIANT_LANG_ITEM).ok()?;
-        let item_id = cx.tcx.parent(variant_id)?;
+        let item_id = cx.tcx.parent(variant_id);
         Some(format!(
             "using `{}.{}({})`, which is a no-op",
             cx.tcx.item_name(item_id),
@@ -53,7 +53,7 @@ pub(crate) trait BindInsteadOfMap {
 
     fn lint_msg(cx: &LateContext<'_>) -> Option<String> {
         let variant_id = cx.tcx.lang_items().require(Self::VARIANT_LANG_ITEM).ok()?;
-        let item_id = cx.tcx.parent(variant_id)?;
+        let item_id = cx.tcx.parent(variant_id);
         Some(format!(
             "using `{}.{}(|x| {}(y))`, which is more succinctly expressed as `{}(|x| y)`",
             cx.tcx.item_name(item_id),
@@ -145,7 +145,7 @@ pub(crate) trait BindInsteadOfMap {
         if_chain! {
             if let Some(adt) = cx.typeck_results().expr_ty(recv).ty_adt_def();
             if let Ok(vid) = cx.tcx.lang_items().require(Self::VARIANT_LANG_ITEM);
-            if Some(adt.did()) == cx.tcx.parent(vid);
+            if adt.did() == cx.tcx.parent(vid);
             then {} else { return false; }
         }
 
@@ -182,7 +182,7 @@ pub(crate) trait BindInsteadOfMap {
     fn is_variant(cx: &LateContext<'_>, res: Res) -> bool {
         if let Res::Def(DefKind::Ctor(CtorOf::Variant, CtorKind::Fn), id) = res {
             if let Ok(variant_id) = cx.tcx.lang_items().require(Self::VARIANT_LANG_ITEM) {
-                return cx.tcx.parent(id) == Some(variant_id);
+                return cx.tcx.parent(id) == variant_id;
             }
         }
         false

clippy_lints/src/methods/chars_cmp.rs

@@ -19,7 +19,7 @@ pub(super) fn check(
     if_chain! {
         if let Some(args) = method_chain_args(info.chain, chain_methods);
         if let hir::ExprKind::Call(fun, [arg_char]) = info.other.kind;
-        if let Some(id) = path_def_id(cx, fun).and_then(|ctor_id| cx.tcx.parent(ctor_id));
+        if let Some(id) = path_def_id(cx, fun).map(|ctor_id| cx.tcx.parent(ctor_id));
         if Some(id) == cx.tcx.lang_items().option_some_variant();
         then {
             let mut applicability = Applicability::MachineApplicable;

clippy_lints/src/methods/option_map_or_none.rs

@@ -75,7 +75,7 @@ pub(super) fn check<'tcx>(
             let arg_snippet = snippet(cx, span, "..");
             let body = cx.tcx.hir().body(id);
                 if let Some((func, [arg_char])) = reduce_unit_expression(&body.value);
-                if let Some(id) = path_def_id(cx, func).and_then(|ctor_id| cx.tcx.parent(ctor_id));
+                if let Some(id) = path_def_id(cx, func).map(|ctor_id| cx.tcx.parent(ctor_id));
                 if Some(id) == cx.tcx.lang_items().option_some_variant();
                 then {
                     let func_snippet = snippet(cx, arg_char.span, "..");

clippy_lints/src/missing_doc.rs

@@ -114,7 +114,7 @@ impl<'tcx> LateLintPass<'tcx> for MissingDoc {
             hir::ItemKind::Fn(..) => {
                 // ignore main()
                 if it.ident.name == sym::main {
-                    let at_root = cx.tcx.local_parent(it.def_id) == Some(CRATE_DEF_ID);
+                    let at_root = cx.tcx.local_parent(it.def_id) == CRATE_DEF_ID;
                     if at_root {
                         return;
                     }

clippy_utils/src/lib.rs

@@ -235,7 +235,7 @@ pub fn is_lang_ctor(cx: &LateContext<'_>, qpath: &QPath<'_>, lang_item: LangItem
     if let QPath::Resolved(_, path) = qpath {
         if let Res::Def(DefKind::Ctor(..), ctor_id) = path.res {
             if let Ok(item_id) = cx.tcx.lang_items().require(lang_item) {
-                return cx.tcx.parent(ctor_id) == Some(item_id);
+                return cx.tcx.parent(ctor_id) == item_id;
             }
         }
     }