Auto merge of #120675 - oli-obk:intrinsics3.0, r=pnkfelix

Add a scheme for moving away from `extern "rust-intrinsic"` entirely

All `rust-intrinsic`s can become free functions now, either with a fallback body, or with a dummy body and an attribute, requiring backends to actually implement the intrinsic.

This PR demonstrates the dummy-body scheme with the `vtable_size` intrinsic.

cc https://github.com/rust-lang/rust/issues/63585

follow-up to #120500

MCP at https://github.com/rust-lang/compiler-team/issues/720

compiler/rustc_middle/src/query/erase.rs

@@ -241,7 +241,7 @@ trivial! {
     Option<rustc_target::abi::FieldIdx>,
     Option<rustc_target::spec::PanicStrategy>,
     Option<usize>,
-    Option<rustc_span::Symbol>,
+    Option<rustc_middle::ty::IntrinsicDef>,
     Result<(), rustc_errors::ErrorGuaranteed>,
     Result<(), rustc_middle::traits::query::NoSolution>,
     Result<rustc_middle::traits::EvaluationResult, rustc_middle::traits::OverflowError>,

compiler/rustc_middle/src/query/mod.rs

@@ -1760,7 +1760,7 @@ rustc_queries! {
         separate_provide_extern
     }
     /// Whether the function is an intrinsic
-    query intrinsic(def_id: DefId) -> Option<Symbol> {
+    query intrinsic(def_id: DefId) -> Option<rustc_middle::ty::IntrinsicDef> {
         desc { |tcx| "fetch intrinsic name if `{}` is an intrinsic", tcx.def_path_str(def_id) }
         separate_provide_extern
     }

compiler/rustc_middle/src/ty/intrinsic.rs

@@ -0,0 +1,17 @@
+use rustc_span::{def_id::DefId, Symbol};
+
+use super::TyCtxt;
+
+#[derive(Copy, Clone, Debug, Decodable, Encodable, HashStable)]
+pub struct IntrinsicDef {
+    pub name: Symbol,
+    /// Whether the intrinsic has no meaningful body and all backends need to shim all calls to it.
+    pub must_be_overridden: bool,
+}
+
+impl TyCtxt<'_> {
+    pub fn is_intrinsic(self, def_id: DefId, name: Symbol) -> bool {
+        let Some(i) = self.intrinsic(def_id) else { return false };
+        i.name == name
+    }
+}

compiler/rustc_middle/src/ty/mod.rs

@@ -30,6 +30,7 @@ pub use adt::*;
 pub use assoc::*;
 pub use generic_args::*;
 pub use generics::*;
+pub use intrinsic::IntrinsicDef;
 use rustc_ast as ast;
 use rustc_ast::node_id::NodeMap;
 pub use rustc_ast_ir::{Movability, Mutability};
@@ -149,6 +150,7 @@ mod generic_args;
 mod generics;
 mod impls_ty;
 mod instance;
+mod intrinsic;
 mod list;
 mod opaque_types;
 mod parameterized;

compiler/rustc_middle/src/ty/parameterized.rs

@@ -75,6 +75,7 @@ trivially_parameterized_over_tcx! {
     ty::Visibility<DefIndex>,
     ty::adjustment::CoerceUnsizedInfo,
     ty::fast_reject::SimplifiedType,
+    ty::IntrinsicDef,
     rustc_ast::Attribute,
     rustc_ast::DelimArgs,
     rustc_ast::expand::StrippedCfgItem<rustc_hir::def_id::DefIndex>,

compiler/rustc_middle/src/ty/util.rs

@@ -19,7 +19,7 @@ use rustc_hir::def_id::{CrateNum, DefId, LocalDefId};
 use rustc_index::bit_set::GrowableBitSet;
 use rustc_macros::HashStable;
 use rustc_session::Limit;
-use rustc_span::{sym, Symbol};
+use rustc_span::sym;
 use rustc_target::abi::{Integer, IntegerType, Primitive, Size};
 use rustc_target::spec::abi::Abi;
 use smallvec::SmallVec;
@@ -1641,12 +1641,19 @@ pub fn is_doc_notable_trait(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
         .any(|items| items.iter().any(|item| item.has_name(sym::notable_trait)))
 }
 
-/// Determines whether an item is an intrinsic by Abi. or by whether it has a `rustc_intrinsic` attribute
-pub fn intrinsic(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Option<Symbol> {
+/// Determines whether an item is an intrinsic (which may be via Abi or via the `rustc_intrinsic` attribute)
+pub fn intrinsic(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Option<ty::IntrinsicDef> {
+    match tcx.def_kind(def_id) {
+        DefKind::Fn | DefKind::AssocFn => {}
+        _ => return None,
+    }
     if matches!(tcx.fn_sig(def_id).skip_binder().abi(), Abi::RustIntrinsic)
         || tcx.has_attr(def_id, sym::rustc_intrinsic)
     {
-        Some(tcx.item_name(def_id.into()))
+        Some(ty::IntrinsicDef {
+            name: tcx.item_name(def_id.into()),
+            must_be_overridden: tcx.has_attr(def_id, sym::rustc_intrinsic_must_be_overridden),
+        })
     } else {
         None
     }