Auto merge of #99943 - compiler-errors:tuple-trait, r=jackh726

Implement `std::marker::Tuple`, use it in `extern "rust-call"` and `Fn`-family traits

Implements rust-lang/compiler-team#537

I made a few opinionated decisions in this implementation, specifically:
1. Enforcing `extern "rust-call"` on fn items during wfcheck,
2. Enforcing this for all functions (not just ones that have bodies),
3. Gating this `Tuple` marker trait behind its own feature, instead of grouping it into (e.g.) `unboxed_closures`.

Still needing to be done:
1. Enforce that `extern "rust-call"` `fn`-ptrs are well-formed only if they have 1/2 args and the second one implements `Tuple`. (Doing this would fix ICE in #66696.)
2. Deny all explicit/user `impl`s of the `Tuple` trait, kinda like `Sized`.
3. Fixing `Tuple` trait built-in impl for chalk, so that chalkification tests are un-broken.

Open questions:
1. Does this need t-lang or t-libs signoff?

Fixes #99820

8 files changed, 72 insertions, 46 deletions

diff --git a/compiler/rustc_hir_analysis/src/check/wfcheck.rs b/compiler/rustc_hir_analysis/src/check/wfcheck.rs
index 0117bdd0ba8..416b555db5c 100644
--- a/compiler/rustc_hir_analysis/src/check/wfcheck.rs
+++ b/compiler/rustc_hir_analysis/src/check/wfcheck.rs
@@ -21,6 +21,7 @@ use rustc_middle::ty::{GenericArgKind, InternalSubsts};
 use rustc_session::parse::feature_err;
 use rustc_span::symbol::{sym, Ident, Symbol};
 use rustc_span::{Span, DUMMY_SP};
+use rustc_target::spec::abi::Abi;
 use rustc_trait_selection::autoderef::Autoderef;
 use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt;
 use rustc_trait_selection::traits::outlives_bounds::InferCtxtExt as _;
@@ -1542,6 +1543,33 @@ fn check_fn_or_method<'tcx>(
         sig.output(),
         hir_decl.output.span(),
     );
+
+    if sig.abi == Abi::RustCall {
+        let span = tcx.def_span(def_id);
+        let has_implicit_self = hir_decl.implicit_self != hir::ImplicitSelfKind::None;
+        let mut inputs = sig.inputs().iter().skip(if has_implicit_self { 1 } else { 0 });
+        // Check that the argument is a tuple
+        if let Some(ty) = inputs.next() {
+            wfcx.register_bound(
+                ObligationCause::new(span, wfcx.body_id, ObligationCauseCode::RustCall),
+                wfcx.param_env,
+                *ty,
+                tcx.require_lang_item(hir::LangItem::Tuple, Some(span)),
+            );
+        } else {
+            tcx.sess.span_err(
+                hir_decl.inputs.last().map_or(span, |input| input.span),
+                "functions with the \"rust-call\" ABI must take a single non-self tuple argument",
+            );
+        }
+        // No more inputs other than the `self` type and the tuple type
+        if inputs.next().is_some() {
+            tcx.sess.span_err(
+                hir_decl.inputs.last().map_or(span, |input| input.span),
+                "functions with the \"rust-call\" ABI must take a single non-self tuple argument",
+            );
+        }
+    }
 }
 
 /// Basically `check_associated_type_bounds`, but separated for now and should be
diff --git a/compiler/rustc_hir_typeck/src/callee.rs b/compiler/rustc_hir_typeck/src/callee.rs
index 000f4c9b684..2b019c8c9b7 100644
--- a/compiler/rustc_hir_typeck/src/callee.rs
+++ b/compiler/rustc_hir_typeck/src/callee.rs
@@ -468,6 +468,22 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             def_id,
         );
 
+        if fn_sig.abi == abi::Abi::RustCall {
+            let sp = arg_exprs.last().map_or(call_expr.span, |expr| expr.span);
+            if let Some(ty) = fn_sig.inputs().last().copied() {
+                self.register_bound(
+                    ty,
+                    self.tcx.require_lang_item(hir::LangItem::Tuple, Some(sp)),
+                    traits::ObligationCause::new(sp, self.body_id, traits::RustCall),
+                );
+            } else {
+                self.tcx.sess.span_err(
+                        sp,
+                        "functions with the \"rust-call\" ABI must take a single non-self tuple argument",
+                    );
+            }
+        }
+
         fn_sig.output()
     }
 
diff --git a/compiler/rustc_hir_typeck/src/check.rs b/compiler/rustc_hir_typeck/src/check.rs
index 80147d90091..37af6e79c3e 100644
--- a/compiler/rustc_hir_typeck/src/check.rs
+++ b/compiler/rustc_hir_typeck/src/check.rs
@@ -6,13 +6,11 @@ use rustc_hir as hir;
 use rustc_hir::def::DefKind;
 use rustc_hir::intravisit::Visitor;
 use rustc_hir::lang_items::LangItem;
-use rustc_hir::{ImplicitSelfKind, ItemKind, Node};
 use rustc_hir_analysis::check::fn_maybe_err;
 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use rustc_infer::infer::RegionVariableOrigin;
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_span::def_id::LocalDefId;
-use rustc_target::spec::abi::Abi;
 use rustc_trait_selection::traits;
 use std::cell::RefCell;
 
@@ -56,41 +54,6 @@ pub(super) fn check_fn<'a, 'tcx>(
 
     fn_maybe_err(tcx, span, fn_sig.abi);
 
-    if fn_sig.abi == Abi::RustCall {
-        let expected_args = if let ImplicitSelfKind::None = decl.implicit_self { 1 } else { 2 };
-
-        let err = || {
-            let item = match tcx.hir().get(fn_id) {
-                Node::Item(hir::Item { kind: ItemKind::Fn(header, ..), .. }) => Some(header),
-                Node::ImplItem(hir::ImplItem {
-                    kind: hir::ImplItemKind::Fn(header, ..), ..
-                }) => Some(header),
-                Node::TraitItem(hir::TraitItem {
-                    kind: hir::TraitItemKind::Fn(header, ..),
-                    ..
-                }) => Some(header),
-                // Closures are RustCall, but they tuple their arguments, so shouldn't be checked
-                Node::Expr(hir::Expr { kind: hir::ExprKind::Closure { .. }, .. }) => None,
-                node => bug!("Item being checked wasn't a function/closure: {:?}", node),
-            };
-
-            if let Some(header) = item {
-                tcx.sess.span_err(header.span, "functions with the \"rust-call\" ABI must take a single non-self argument that is a tuple");
-            }
-        };
-
-        if fn_sig.inputs().len() != expected_args {
-            err()
-        } else {
-            // FIXME(CraftSpider) Add a check on parameter expansion, so we don't just make the ICE happen later on
-            //   This will probably require wide-scale changes to support a TupleKind obligation
-            //   We can't resolve this without knowing the type of the param
-            if !matches!(fn_sig.inputs()[expected_args - 1].kind(), ty::Tuple(_) | ty::Param(_)) {
-                err()
-            }
-        }
-    }
-
     if body.generator_kind.is_some() && can_be_generator.is_some() {
         let yield_ty = fcx
             .next_ty_var(TypeVariableOrigin { kind: TypeVariableOriginKind::TypeInference, span });
diff --git a/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs b/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs
index e1955d838f2..4066cca8a4b 100644
--- a/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs
+++ b/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs
@@ -136,6 +136,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             tuple_arguments,
             Some(method.def_id),
         );
+
         method.sig.output()
     }
 
@@ -214,7 +215,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         "cannot use call notation; the first type parameter \
                          for the function trait is neither a tuple nor unit"
                     )
-                    .emit();
+                    .delay_as_bug();
                     (self.err_args(provided_args.len()), None)
                 }
             }
diff --git a/compiler/rustc_hir_typeck/src/lib.rs b/compiler/rustc_hir_typeck/src/lib.rs
index d1762598a52..624443d9594 100644
--- a/compiler/rustc_hir_typeck/src/lib.rs
+++ b/compiler/rustc_hir_typeck/src/lib.rs
@@ -458,7 +458,7 @@ fn report_unexpected_variant_res(tcx: TyCtxt<'_>, res: Res, qpath: &hir::QPath<'
 /// # fn f(x: (isize, isize)) {}
 /// f((1, 2));
 /// ```
-#[derive(Clone, Eq, PartialEq)]
+#[derive(Copy, Clone, Eq, PartialEq)]
 enum TupleArgumentsFlag {
     DontTupleArguments,
     TupleArguments,
diff --git a/compiler/rustc_middle/src/traits/mod.rs b/compiler/rustc_middle/src/traits/mod.rs
index 07ee758b32c..a29f0722ff7 100644
--- a/compiler/rustc_middle/src/traits/mod.rs
+++ b/compiler/rustc_middle/src/traits/mod.rs
@@ -438,6 +438,8 @@ pub enum ObligationCauseCode<'tcx> {
     },
 
     AscribeUserTypeProvePredicate(Span),
+
+    RustCall,
 }
 
 /// The 'location' at which we try to perform HIR-based wf checking.
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
index dacce5cd2f6..54281f91205 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
@@ -700,6 +700,25 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                             }
                         }
 
+                        if Some(trait_ref.def_id()) == tcx.lang_items().tuple_trait() {
+                            match obligation.cause.code().peel_derives() {
+                                ObligationCauseCode::RustCall => {
+                                    err.set_primary_message("functions with the \"rust-call\" ABI must take a single non-self tuple argument");
+                                }
+                                ObligationCauseCode::BindingObligation(def_id, _)
+                                | ObligationCauseCode::ItemObligation(def_id)
+                                    if ty::ClosureKind::from_def_id(tcx, *def_id).is_some() =>
+                                {
+                                    err.code(rustc_errors::error_code!(E0059));
+                                    err.set_primary_message(format!(
+                                        "type parameter to bare `{}` trait must be a tuple",
+                                        tcx.def_path_str(*def_id)
+                                    ));
+                                }
+                                _ => {}
+                            }
+                        }
+
                         if Some(trait_ref.def_id()) == tcx.lang_items().drop_trait()
                             && predicate_is_const
                         {
@@ -848,12 +867,8 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                             );
                         }
 
-                        let is_fn_trait = [
-                            self.tcx.lang_items().fn_trait(),
-                            self.tcx.lang_items().fn_mut_trait(),
-                            self.tcx.lang_items().fn_once_trait(),
-                        ]
-                        .contains(&Some(trait_ref.def_id()));
+                        let is_fn_trait =
+                            ty::ClosureKind::from_def_id(tcx, trait_ref.def_id()).is_some();
                         let is_target_feature_fn = if let ty::FnDef(def_id, _) =
                             *trait_ref.skip_binder().self_ty().kind()
                         {
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
index d7606d88803..0f4aa87b43f 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
@@ -2407,7 +2407,8 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             | ObligationCauseCode::CheckAssociatedTypeBounds { .. }
             | ObligationCauseCode::LetElse
             | ObligationCauseCode::BinOp { .. }
-            | ObligationCauseCode::AscribeUserTypeProvePredicate(..) => {}
+            | ObligationCauseCode::AscribeUserTypeProvePredicate(..)
+            | ObligationCauseCode::RustCall => {}
             ObligationCauseCode::SliceOrArrayElem => {
                 err.note("slice and array elements must have `Sized` type");
             }