Auto merge of #129317 - compiler-errors:expectation-subtyping, r=lcnr

Use equality when relating formal and expected type in arg checking

#129059 uncovered an interesting issue in argument checking. When we check arguments, we create three sets of types:
* Formals
* Expected
* Actuals

The **actuals** are the types of the argument expressions themselves. The **formals** are the types from the signature that we're checking. The **expected** types are the formal types, but passed through `expected_inputs_for_expected_outputs`:

a971212545/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs (L691-L725)

This method attempts to constrain the formal inputs by relating the [expectation](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir_typeck/expectation/enum.Expectation.html) of the call expression and the formal output.

When we check an argument, we get the expression's actual type, and then we first attempt to coerce it to the expected type:

a971212545/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs (L280-L293)

Then we subtype the expected type and the formal type:

a971212545/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs (L299-L305)

However, since we are now recording the right coercion target (since #129059), we now end up recording the expected type to the typeck results, rather than the actual.

Since that expected type was [fudged](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_infer/infer/struct.InferCtxt.html#method.fudge_inference_if_ok), it has fresh variables. And since the expected type is only subtyped against the formal type, if that expected type has a bivariant parameter, it will likely remain unconstrained since `Covariant * Bivariant = Bivariant` according to [xform](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/enum.Variance.html#method.xform). This leads to an unconstrained type variable in writeback.

AFAICT, there's no reason for us to be using subtyping here, though. The expected output is already related to the expectation by subtyping:

a971212545/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs (L713)

So the formals don't need "another" indirection of subtyping in the argument checking... So I've changed it to use equality here. We could alternatively fix this by requiring WF for all the expected types to constrain their bivariant parameters, but this seems a bit overkill.

Fixes #129286

compiler/rustc_hir_typeck/src/callee.rs

@@ -503,18 +503,12 @@ fn confirm_builtin_call(
         let fn_sig = self.instantiate_binder_with_fresh_vars(call_expr.span, infer::FnCall, fn_sig);
         let fn_sig = self.normalize(call_expr.span, fn_sig);
 
-        // Call the generic checker.
-        let expected_arg_tys = self.expected_inputs_for_expected_output(
-            call_expr.span,
-            expected,
-            fn_sig.output(),
-            fn_sig.inputs(),
-        );
         self.check_argument_types(
             call_expr.span,
             call_expr,
             fn_sig.inputs(),
-            expected_arg_tys,
+            fn_sig.output(),
+            expected,
             arg_exprs,
             fn_sig.c_variadic,
             TupleArgumentsFlag::DontTupleArguments,
@@ -866,19 +860,12 @@ fn confirm_deferred_closure_call(
         // don't know the full details yet (`Fn` vs `FnMut` etc), but we
         // do know the types expected for each argument and the return
         // type.
-
-        let expected_arg_tys = self.expected_inputs_for_expected_output(
-            call_expr.span,
-            expected,
-            fn_sig.output(),
-            fn_sig.inputs(),
-        );
-
         self.check_argument_types(
             call_expr.span,
             call_expr,
             fn_sig.inputs(),
-            expected_arg_tys,
+            fn_sig.output(),
+            expected,
             arg_exprs,
             fn_sig.c_variadic,
             TupleArgumentsFlag::TupleArguments,

compiler/rustc_hir_typeck/src/expr.rs

@@ -1673,15 +1673,22 @@ fn check_expr_struct_fields(
     ) {
         let tcx = self.tcx;
 
-        let expected_inputs =
+        let adt_ty = self.resolve_vars_with_obligations(adt_ty);
-            self.expected_inputs_for_expected_output(span, expected, adt_ty, &[adt_ty]);
+        let adt_ty_hint = expected.only_has_type(self).and_then(|expected| {
-        let adt_ty_hint = if let Some(expected_inputs) = expected_inputs {
+            self.fudge_inference_if_ok(|| {
-            expected_inputs.get(0).cloned().unwrap_or(adt_ty)
+                let ocx = ObligationCtxt::new(self);
-        } else {
+                ocx.sup(&self.misc(span), self.param_env, expected, adt_ty)?;
-            adt_ty
+                if !ocx.select_where_possible().is_empty() {
-        };
+                    return Err(TypeError::Mismatch);
-        // re-link the regions that EIfEO can erase.
+                }
-        self.demand_eqtype(span, adt_ty_hint, adt_ty);
+                Ok(self.resolve_vars_if_possible(adt_ty))
+            })
+            .ok()
+        });
+        if let Some(adt_ty_hint) = adt_ty_hint {
+            // re-link the variables that the fudging above can create.
+            self.demand_eqtype(span, adt_ty_hint, adt_ty);
+        }
 
         let ty::Adt(adt, args) = adt_ty.kind() else {
             span_bug!(span, "non-ADT passed to check_expr_struct_fields");

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -20,7 +20,6 @@
 use rustc_infer::infer::{DefineOpaqueTypes, InferResult};
 use rustc_lint::builtin::SELF_CONSTRUCTOR_FROM_OUTER_ITEM;
 use rustc_middle::ty::adjustment::{Adjust, Adjustment, AutoBorrow, AutoBorrowMutability};
-use rustc_middle::ty::error::TypeError;
 use rustc_middle::ty::fold::TypeFoldable;
 use rustc_middle::ty::visit::{TypeVisitable, TypeVisitableExt};
 use rustc_middle::ty::{
@@ -36,7 +35,7 @@
 use rustc_target::abi::FieldIdx;
 use rustc_trait_selection::error_reporting::infer::need_type_info::TypeAnnotationNeeded;
 use rustc_trait_selection::traits::{
-    self, NormalizeExt, ObligationCauseCode, ObligationCtxt, StructurallyNormalizeExt,
+    self, NormalizeExt, ObligationCauseCode, StructurallyNormalizeExt,
 };
 use tracing::{debug, instrument};
 
@@ -689,42 +688,6 @@ pub(crate) fn err_args(&self, len: usize, guar: ErrorGuaranteed) -> Vec<Ty<'tcx>
         vec![ty_error; len]
     }
 
-    /// Unifies the output type with the expected type early, for more coercions
-    /// and forward type information on the input expressions.
-    #[instrument(skip(self, call_span), level = "debug")]
-    pub(crate) fn expected_inputs_for_expected_output(
-        &self,
-        call_span: Span,
-        expected_ret: Expectation<'tcx>,
-        formal_ret: Ty<'tcx>,
-        formal_args: &[Ty<'tcx>],
-    ) -> Option<Vec<Ty<'tcx>>> {
-        let formal_ret = self.resolve_vars_with_obligations(formal_ret);
-        let ret_ty = expected_ret.only_has_type(self)?;
-
-        let expect_args = self
-            .fudge_inference_if_ok(|| {
-                let ocx = ObligationCtxt::new(self);
-
-                // Attempt to apply a subtyping relationship between the formal
-                // return type (likely containing type variables if the function
-                // is polymorphic) and the expected return type.
-                // No argument expectations are produced if unification fails.
-                let origin = self.misc(call_span);
-                ocx.sup(&origin, self.param_env, ret_ty, formal_ret)?;
-                if !ocx.select_where_possible().is_empty() {
-                    return Err(TypeError::Mismatch);
-                }
-
-                // Record all the argument types, with the args
-                // produced from the above subtyping unification.
-                Ok(Some(formal_args.iter().map(|&ty| self.resolve_vars_if_possible(ty)).collect()))
-            })
-            .unwrap_or_default();
-        debug!(?formal_args, ?formal_ret, ?expect_args, ?expected_ret);
-        expect_args
-    }
-
     pub(crate) fn resolve_lang_item_path(
         &self,
         lang_item: hir::LangItem,

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -17,6 +17,7 @@
 use rustc_index::IndexVec;
 use rustc_infer::infer::{DefineOpaqueTypes, InferOk, TypeTrace};
 use rustc_middle::ty::adjustment::AllowTwoPhase;
+use rustc_middle::ty::error::TypeError;
 use rustc_middle::ty::visit::TypeVisitableExt;
 use rustc_middle::ty::{self, IsSuggestable, Ty, TyCtxt};
 use rustc_middle::{bug, span_bug};
@@ -25,7 +26,7 @@
 use rustc_span::{sym, Span, DUMMY_SP};
 use rustc_trait_selection::error_reporting::infer::{FailureCode, ObligationCauseExt};
 use rustc_trait_selection::infer::InferCtxtExt;
-use rustc_trait_selection::traits::{self, ObligationCauseCode, SelectionContext};
+use rustc_trait_selection::traits::{self, ObligationCauseCode, ObligationCtxt, SelectionContext};
 use tracing::debug;
 use {rustc_ast as ast, rustc_hir as hir};
 
@@ -124,6 +125,7 @@ pub(in super::super) fn check_method_argument_types(
         };
         if let Err(guar) = has_error {
             let err_inputs = self.err_args(args_no_rcvr.len(), guar);
+            let err_output = Ty::new_error(self.tcx, guar);
 
             let err_inputs = match tuple_arguments {
                 DontTupleArguments => err_inputs,
@@ -134,28 +136,23 @@ pub(in super::super) fn check_method_argument_types(
                 sp,
                 expr,
                 &err_inputs,
-                None,
+                err_output,
+                NoExpectation,
                 args_no_rcvr,
                 false,
                 tuple_arguments,
                 method.ok().map(|method| method.def_id),
             );
-            return Ty::new_error(self.tcx, guar);
+            return err_output;
         }
 
         let method = method.unwrap();
-        // HACK(eddyb) ignore self in the definition (see above).
-        let expected_input_tys = self.expected_inputs_for_expected_output(
-            sp,
-            expected,
-            method.sig.output(),
-            &method.sig.inputs()[1..],
-        );
         self.check_argument_types(
             sp,
             expr,
             &method.sig.inputs()[1..],
-            expected_input_tys,
+            method.sig.output(),
+            expected,
             args_no_rcvr,
             method.sig.c_variadic,
             tuple_arguments,
@@ -175,8 +172,9 @@ pub(in super::super) fn check_argument_types(
         call_expr: &'tcx hir::Expr<'tcx>,
         // Types (as defined in the *signature* of the target function)
         formal_input_tys: &[Ty<'tcx>],
-        // More specific expected types, after unifying with caller output types
+        formal_output: Ty<'tcx>,
-        expected_input_tys: Option<Vec<Ty<'tcx>>>,
+        // Expected output from the parent expression or statement
+        expectation: Expectation<'tcx>,
         // The expressions for each provided argument
         provided_args: &'tcx [hir::Expr<'tcx>],
         // Whether the function is variadic, for example when imported from C
@@ -210,6 +208,40 @@ pub(in super::super) fn check_argument_types(
             );
         }
 
+        // First, let's unify the formal method signature with the expectation eagerly.
+        // We use this to guide coercion inference; it's output is "fudged" which means
+        // any remaining type variables are assigned to new, unrelated variables. This
+        // is because the inference guidance here is only speculative.
+        let formal_output = self.resolve_vars_with_obligations(formal_output);
+        let expected_input_tys: Option<Vec<_>> = expectation
+            .only_has_type(self)
+            .and_then(|expected_output| {
+                self.fudge_inference_if_ok(|| {
+                    let ocx = ObligationCtxt::new(self);
+
+                    // Attempt to apply a subtyping relationship between the formal
+                    // return type (likely containing type variables if the function
+                    // is polymorphic) and the expected return type.
+                    // No argument expectations are produced if unification fails.
+                    let origin = self.misc(call_span);
+                    ocx.sup(&origin, self.param_env, expected_output, formal_output)?;
+                    if !ocx.select_where_possible().is_empty() {
+                        return Err(TypeError::Mismatch);
+                    }
+
+                    // Record all the argument types, with the args
+                    // produced from the above subtyping unification.
+                    Ok(Some(
+                        formal_input_tys
+                            .iter()
+                            .map(|&ty| self.resolve_vars_if_possible(ty))
+                            .collect(),
+                    ))
+                })
+                .ok()
+            })
+            .unwrap_or_default();
+
         let mut err_code = E0061;
 
         // If the arguments should be wrapped in a tuple (ex: closures), unwrap them here
@@ -292,21 +324,20 @@ pub(in super::super) fn check_argument_types(
 
             let coerce_error =
                 self.coerce(provided_arg, checked_ty, coerced_ty, AllowTwoPhase::Yes, None).err();
-
             if coerce_error.is_some() {
                 return Compatibility::Incompatible(coerce_error);
             }
 
-            // 3. Check if the formal type is a supertype of the checked one
+            // 3. Check if the formal type is actually equal to the checked one
-            //    and register any such obligations for future type checks
+            //    and register any such obligations for future type checks.
-            let supertype_error = self.at(&self.misc(provided_arg.span), self.param_env).sup(
+            let formal_ty_error = self.at(&self.misc(provided_arg.span), self.param_env).eq(
                 DefineOpaqueTypes::Yes,
                 formal_input_ty,
                 coerced_ty,
             );
 
             // If neither check failed, the types are compatible
-            match supertype_error {
+            match formal_ty_error {
                 Ok(InferOk { obligations, value: () }) => {
                     self.register_predicates(obligations);
                     Compatibility::Compatible

tests/ui/coercion/constrain-expectation-in-arg.rs

@@ -0,0 +1,24 @@
+//@ check-pass
+
+// Regression test for for #129286.
+// Makes sure that we don't have unconstrained type variables that come from
+// bivariant type parameters due to the way that we construct expectation types
+// when checking call expressions in HIR typeck.
+
+trait Trait {
+    type Item;
+}
+
+struct Struct<A: Trait<Item = B>, B> {
+    pub field: A,
+}
+
+fn identity<T>(x: T) -> T {
+    x
+}
+
+fn test<A: Trait<Item = B>, B>(x: &Struct<A, B>) {
+    let x: &Struct<_, _> = identity(x);
+}
+
+fn main() {}