Auto merge of #135031 - RalfJung:intrinsics-without-body, r=oli-obk

rustc_intrinsic: support functions without body

We synthesize a HIR body `loop {}` but such bodyless intrinsics.

Most of the diff is due to turning `ItemKind::Fn` into a brace (named-field) enum variant, because it carries a `bool`-typed field now. This is to remember whether the function has a body. MIR building panics to avoid ever translating the fake `loop {}` body, and the intrinsic logic uses the lack of a body to implicitly mark that intrinsic as must-be-overridden.

I first tried actually having no body rather than generating the fake body, but there's a *lot* of code that assumes that all function items have HIR and MIR, so this didn't work very well. Then I noticed that even `rustc_intrinsic_must_be_overridden` intrinsics have MIR generated (they are filled with an `Unreachable` terminator) so I guess I am not the first to discover this. ;)

r? `@oli-obk`

src/librustdoc/clean/mod.rs

@@ -2810,7 +2810,7 @@ fn clean_maybe_renamed_item<'tcx>(
             }),
             ItemKind::Macro(_, macro_kind) => clean_proc_macro(item, &mut name, macro_kind, cx),
             // proc macros can have a name set by attributes
-            ItemKind::Fn(ref sig, generics, body_id) => {
+            ItemKind::Fn { ref sig, generics, body: body_id, .. } => {
                 clean_fn_or_proc_macro(item, sig, generics, body_id, &mut name, cx)
             }
             ItemKind::Trait(_, _, generics, bounds, item_ids) => {

src/librustdoc/html/render/span_map.rs

@@ -245,7 +245,7 @@ impl<'tcx> Visitor<'tcx> for SpanMapVisitor<'tcx> {
         match item.kind {
             ItemKind::Static(_, _, _)
             | ItemKind::Const(_, _, _)
-            | ItemKind::Fn(_, _, _)
+            | ItemKind::Fn { .. }
             | ItemKind::Macro(_, _)
             | ItemKind::TyAlias(_, _)
             | ItemKind::Enum(_, _)

src/librustdoc/visit_ast.rs

@@ -499,7 +499,7 @@ impl<'a, 'tcx> RustdocVisitor<'a, 'tcx> {
             hir::ItemKind::Mod(m) => {
                 self.enter_mod(item.owner_id.def_id, m, name, renamed, import_id);
             }
-            hir::ItemKind::Fn(..)
+            hir::ItemKind::Fn { .. }
             | hir::ItemKind::ExternCrate(..)
             | hir::ItemKind::Enum(..)
             | hir::ItemKind::Struct(..)

src/tools/clippy/clippy_lints/src/arbitrary_source_item_ordering.rs

@@ -464,7 +464,7 @@ fn convert_module_item_kind(value: &ItemKind<'_>) -> SourceItemOrderingModuleIte
         ItemKind::Use(..) => Use,
         ItemKind::Static(..) => Static,
         ItemKind::Const(..) => Const,
-        ItemKind::Fn(..) => Fn,
+        ItemKind::Fn{ .. } => Fn,
         ItemKind::Macro(..) => Macro,
         ItemKind::Mod(..) => Mod,
         ItemKind::ForeignMod { .. } => ForeignMod,

src/tools/clippy/clippy_lints/src/attrs/utils.rs

@@ -21,7 +21,7 @@ pub(super) fn is_lint_level(symbol: Symbol, attr_id: AttrId) -> bool {
 }
 
 pub(super) fn is_relevant_item(cx: &LateContext<'_>, item: &Item<'_>) -> bool {
-    if let ItemKind::Fn(_, _, eid) = item.kind {
+    if let ItemKind::Fn { body: eid, .. } = item.kind {
         is_relevant_expr(cx, cx.tcx.typeck_body(eid), cx.tcx.hir().body(eid).value)
     } else {
         true

src/tools/clippy/clippy_lints/src/doc/mod.rs

@@ -639,7 +639,7 @@ impl<'tcx> LateLintPass<'tcx> for Documentation {
                     self.check_private_items,
                 );
                 match item.kind {
-                    ItemKind::Fn(sig, _, body_id) => {
+                    ItemKind::Fn { sig, body: body_id, .. } => {
                         if !(is_entrypoint_fn(cx, item.owner_id.to_def_id())
                             || in_external_macro(cx.tcx.sess, item.span))
                         {

src/tools/clippy/clippy_lints/src/doc/too_long_first_doc_paragraph.rs

@@ -25,7 +25,7 @@ pub(super) fn check(
             // page. So associated items or impl blocks are not part of this list.
             ItemKind::Static(..)
                 | ItemKind::Const(..)
-                | ItemKind::Fn(..)
+                | ItemKind::Fn{ .. }
                 | ItemKind::Macro(..)
                 | ItemKind::Mod(..)
                 | ItemKind::TyAlias(..)

src/tools/clippy/clippy_lints/src/exit.rs

@@ -48,7 +48,7 @@ impl<'tcx> LateLintPass<'tcx> for Exit {
             && let Some(def_id) = cx.qpath_res(path, path_expr.hir_id).opt_def_id()
             && cx.tcx.is_diagnostic_item(sym::process_exit, def_id)
             && let parent = cx.tcx.hir().get_parent_item(e.hir_id)
-            && let OwnerNode::Item(Item{kind: ItemKind::Fn(..), ..}) = cx.tcx.hir_owner_node(parent)
+            && let OwnerNode::Item(Item{kind: ItemKind::Fn{ .. }, ..}) = cx.tcx.hir_owner_node(parent)
             // If the next item up is a function we check if it is an entry point
             // and only then emit a linter warning
             && !is_entrypoint_fn(cx, parent.to_def_id())

src/tools/clippy/clippy_lints/src/extra_unused_type_parameters.rs

@@ -253,7 +253,11 @@ fn is_empty_body(cx: &LateContext<'_>, body: BodyId) -> bool {
 
 impl<'tcx> LateLintPass<'tcx> for ExtraUnusedTypeParameters {
     fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx Item<'tcx>) {
-        if let ItemKind::Fn(_, generics, body_id) = item.kind
+        if let ItemKind::Fn {
+            generics,
+            body: body_id,
+            ..
+        } = item.kind
             && !generics.params.is_empty()
             && !is_empty_body(cx, body_id)
             && (!self.avoid_breaking_exported_api || !cx.effective_visibilities.is_exported(item.owner_id.def_id))

src/tools/clippy/clippy_lints/src/functions/must_use.rs

@@ -24,7 +24,12 @@ use super::{DOUBLE_MUST_USE, MUST_USE_CANDIDATE, MUST_USE_UNIT};
 pub(super) fn check_item<'tcx>(cx: &LateContext<'tcx>, item: &'tcx hir::Item<'_>) {
     let attrs = cx.tcx.hir().attrs(item.hir_id());
     let attr = cx.tcx.get_attr(item.owner_id, sym::must_use);
-    if let hir::ItemKind::Fn(ref sig, _generics, ref body_id) = item.kind {
+    if let hir::ItemKind::Fn {
+        ref sig,
+        body: ref body_id,
+        ..
+    } = item.kind
+    {
         let is_public = cx.effective_visibilities.is_exported(item.owner_id.def_id);
         let fn_header_span = item.span.with_hi(sig.decl.output.span().hi());
         if let Some(attr) = attr {

src/tools/clippy/clippy_lints/src/functions/result.rs

@@ -36,7 +36,7 @@ fn result_err_ty<'tcx>(
 }
 
 pub(super) fn check_item<'tcx>(cx: &LateContext<'tcx>, item: &hir::Item<'tcx>, large_err_threshold: u64, msrv: &Msrv) {
-    if let hir::ItemKind::Fn(ref sig, _generics, _) = item.kind
+    if let hir::ItemKind::Fn { ref sig, .. } = item.kind
         && let Some((hir_ty, err_ty)) = result_err_ty(cx, sig.decl, item.owner_id.def_id, item.span)
     {
         if cx.effective_visibilities.is_exported(item.owner_id.def_id) {

src/tools/clippy/clippy_lints/src/implicit_hasher.rs

@@ -149,7 +149,12 @@ impl<'tcx> LateLintPass<'tcx> for ImplicitHasher {
                     );
                 }
             },
-            ItemKind::Fn(ref sig, generics, body_id) => {
+            ItemKind::Fn {
+                ref sig,
+                generics,
+                body: body_id,
+                ..
+            } => {
                 let body = cx.tcx.hir().body(body_id);
 
                 for ty in sig.decl.inputs {

src/tools/clippy/clippy_lints/src/lifetimes.rs

@@ -95,7 +95,13 @@ declare_lint_pass!(Lifetimes => [NEEDLESS_LIFETIMES, EXTRA_UNUSED_LIFETIMES]);
 
 impl<'tcx> LateLintPass<'tcx> for Lifetimes {
     fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx Item<'_>) {
-        if let ItemKind::Fn(ref sig, generics, id) = item.kind {
+        if let ItemKind::Fn {
+            ref sig,
+            generics,
+            body: id,
+            ..
+        } = item.kind
+        {
             check_fn_inner(cx, sig, Some(id), None, generics, item.span, true);
         } else if let ItemKind::Impl(impl_) = item.kind {
             if !item.span.from_expansion() {

src/tools/clippy/clippy_lints/src/loops/infinite_loop.rs

@@ -75,7 +75,11 @@ fn get_parent_fn_ret_ty<'tcx>(cx: &LateContext<'tcx>, expr: &Expr<'_>) -> Option
                 ..
             }) => (),
             Node::Item(hir::Item {
-                kind: hir::ItemKind::Fn(FnSig { decl, .. }, _, _),
+                kind:
+                    hir::ItemKind::Fn {
+                        sig: FnSig { decl, .. },
+                        ..
+                    },
                 ..
             })
             | Node::TraitItem(hir::TraitItem {

src/tools/clippy/clippy_lints/src/matches/needless_match.rs

@@ -133,7 +133,7 @@ fn expr_ty_matches_p_ty(cx: &LateContext<'_>, expr: &Expr<'_>, p_expr: &Expr<'_>
         },
         // compare match_expr ty with RetTy in `fn foo() -> RetTy`
         Node::Item(item) => {
-            if let ItemKind::Fn(..) = item.kind {
+            if let ItemKind::Fn{ .. } = item.kind {
                 let output = cx
                     .tcx
                     .fn_sig(item.owner_id)

src/tools/clippy/clippy_lints/src/methods/unnecessary_to_owned.rs

@@ -496,7 +496,7 @@ fn can_change_type<'a>(cx: &LateContext<'a>, mut expr: &'a Expr<'a>, mut ty: Ty<
             Node::Stmt(_) => return true,
             Node::Block(..) => continue,
             Node::Item(item) => {
-                if let ItemKind::Fn(_, _, body_id) = &item.kind
+                if let ItemKind::Fn { body: body_id, .. } = &item.kind
                     && let output_ty = return_ty(cx, item.owner_id)
                     && rustc_hir_typeck::can_coerce(cx.tcx, cx.param_env, item.owner_id.def_id, ty, output_ty)
                 {

src/tools/clippy/clippy_lints/src/missing_doc.rs

@@ -192,7 +192,7 @@ impl<'tcx> LateLintPass<'tcx> for MissingDoc {
 
     fn check_item(&mut self, cx: &LateContext<'tcx>, it: &'tcx hir::Item<'_>) {
         match it.kind {
-            hir::ItemKind::Fn(..) => {
+            hir::ItemKind::Fn{ .. } => {
                 // ignore main()
                 if it.ident.name == sym::main {
                     let at_root = cx.tcx.local_parent(it.owner_id.def_id) == CRATE_DEF_ID;

src/tools/clippy/clippy_lints/src/missing_inline.rs

@@ -96,7 +96,7 @@ impl<'tcx> LateLintPass<'tcx> for MissingInline {
             return;
         }
         match it.kind {
-            hir::ItemKind::Fn(..) => {
+            hir::ItemKind::Fn{ .. } => {
                 let desc = "a function";
                 let attrs = cx.tcx.hir().attrs(it.hir_id());
                 check_missing_inline_attrs(cx, attrs, it.span, desc);

src/tools/clippy/clippy_lints/src/mut_key.rs

@@ -76,7 +76,7 @@ impl_lint_pass!(MutableKeyType<'_> => [ MUTABLE_KEY_TYPE ]);
 
 impl<'tcx> LateLintPass<'tcx> for MutableKeyType<'tcx> {
     fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::Item<'tcx>) {
-        if let hir::ItemKind::Fn(ref sig, ..) = item.kind {
+        if let hir::ItemKind::Fn { ref sig, .. } = item.kind {
             self.check_sig(cx, item.owner_id.def_id, sig.decl);
         }
     }

src/tools/clippy/clippy_lints/src/no_effect.rs

@@ -144,7 +144,7 @@ impl NoEffect {
                     |diag| {
                         for parent in cx.tcx.hir().parent_iter(stmt.hir_id) {
                             if let Node::Item(item) = parent.1
-                                && let ItemKind::Fn(..) = item.kind
+                                && let ItemKind::Fn{ .. } = item.kind
                                 && let Node::Block(block) = cx.tcx.parent_hir_node(stmt.hir_id)
                                 && let [.., final_stmt] = block.stmts
                                 && final_stmt.hir_id == stmt.hir_id

src/tools/clippy/clippy_lints/src/no_mangle_with_rust_abi.rs

@@ -37,7 +37,7 @@ declare_lint_pass!(NoMangleWithRustAbi => [NO_MANGLE_WITH_RUST_ABI]);
 
 impl<'tcx> LateLintPass<'tcx> for NoMangleWithRustAbi {
     fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx Item<'tcx>) {
-        if let ItemKind::Fn(fn_sig, _, _) = &item.kind {
+        if let ItemKind::Fn { sig: fn_sig, .. } = &item.kind {
             let attrs = cx.tcx.hir().attrs(item.hir_id());
             let mut app = Applicability::MaybeIncorrect;
             let fn_snippet = snippet_with_applicability(cx, fn_sig.span.with_hi(item.ident.span.lo()), "..", &mut app);

src/tools/clippy/clippy_lints/src/ptr.rs

@@ -189,7 +189,7 @@ impl<'tcx> LateLintPass<'tcx> for Ptr {
         let mut parents = hir.parent_iter(body.value.hir_id);
         let (item_id, sig, is_trait_item) = match parents.next() {
             Some((_, Node::Item(i))) => {
-                if let ItemKind::Fn(sig, ..) = &i.kind {
+                if let ItemKind::Fn { sig, .. } = &i.kind {
                     (i.owner_id, sig, false)
                 } else {
                     return;

src/tools/clippy/clippy_lints/src/returns.rs

@@ -204,7 +204,7 @@ impl<'tcx> LateLintPass<'tcx> for Return {
 
             // Ensure this is not the final stmt, otherwise removing it would cause a compile error
             && let OwnerNode::Item(item) = cx.tcx.hir_owner_node(cx.tcx.hir().get_parent_item(expr.hir_id))
-            && let ItemKind::Fn(_, _, body) = item.kind
+            && let ItemKind::Fn { body, .. } = item.kind
             && let block = cx.tcx.hir().body(body).value
             && let ExprKind::Block(block, _) = block.kind
             && !is_inside_let_else(cx.tcx, expr)

src/tools/clippy/clippy_lints/src/unnecessary_box_returns.rs

@@ -130,9 +130,9 @@ impl LateLintPass<'_> for UnnecessaryBoxReturns {
     }
 
     fn check_item(&mut self, cx: &LateContext<'_>, item: &Item<'_>) {
-        let ItemKind::Fn(signature, ..) = &item.kind else {
+        let ItemKind::Fn { sig, .. } = &item.kind else {
             return;
         };
-        self.check_fn_item(cx, signature.decl, item.owner_id.def_id, item.ident.name);
+        self.check_fn_item(cx, sig.decl, item.owner_id.def_id, item.ident.name);
     }
 }

src/tools/clippy/clippy_utils/src/check_proc_macro.rs

@@ -245,7 +245,7 @@ fn item_search_pat(item: &Item<'_>) -> (Pat, Pat) {
         ItemKind::ExternCrate(_) => (Pat::Str("extern"), Pat::Str(";")),
         ItemKind::Static(..) => (Pat::Str("static"), Pat::Str(";")),
         ItemKind::Const(..) => (Pat::Str("const"), Pat::Str(";")),
-        ItemKind::Fn(sig, ..) => (fn_header_search_pat(sig.header), Pat::Str("")),
+        ItemKind::Fn { sig, .. } => (fn_header_search_pat(sig.header), Pat::Str("")),
         ItemKind::ForeignMod { .. } => (Pat::Str("extern"), Pat::Str("}")),
         ItemKind::TyAlias(..) => (Pat::Str("type"), Pat::Str(";")),
         ItemKind::Enum(..) => (Pat::Str("enum"), Pat::Str("}")),

src/tools/clippy/clippy_utils/src/lib.rs

@@ -1397,7 +1397,7 @@ pub fn get_enclosing_block<'tcx>(cx: &LateContext<'tcx>, hir_id: HirId) -> Optio
     enclosing_node.and_then(|node| match node {
         Node::Block(block) => Some(block),
         Node::Item(&Item {
-            kind: ItemKind::Fn(_, _, eid),
+            kind: ItemKind::Fn { body: eid, .. },
             ..
         })
         | Node::ImplItem(&ImplItem {
@@ -2565,7 +2565,7 @@ pub fn is_in_test_function(tcx: TyCtxt<'_>, id: HirId) -> bool {
             // function scope
             .any(|(_id, node)| {
                 if let Node::Item(item) = node {
-                    if let ItemKind::Fn(_, _, _) = item.kind {
+                    if let ItemKind::Fn { .. } = item.kind {
                         // Note that we have sorted the item names in the visitor,
                         // so the binary_search gets the same as `contains`, but faster.
                         return names.binary_search(&item.ident.name).is_ok();
@@ -2722,7 +2722,7 @@ impl<'tcx> ExprUseCtxt<'tcx> {
             }) => ExprUseNode::ConstStatic(owner_id),
 
             Node::Item(&Item {
-                kind: ItemKind::Fn(..),
+                kind: ItemKind::Fn { .. },
                 owner_id,
                 ..
             })

src/tools/miri/src/bin/miri.rs

@@ -242,7 +242,7 @@ impl rustc_driver::Callbacks for MiriBeRustCompilerCalls {
                             let is_reachable_non_generic = matches!(
                                 tcx.hir_node_by_def_id(local_def_id),
                                 Node::Item(&hir::Item {
-                                    kind: hir::ItemKind::Static(..) | hir::ItemKind::Fn(..),
+                                    kind: hir::ItemKind::Static(..) | hir::ItemKind::Fn{ .. },
                                     ..
                                 }) | Node::ImplItem(&hir::ImplItem {
                                     kind: hir::ImplItemKind::Fn(..),