Auto merge of rust-lang#139582 - oli-obk:coercion-cleanups, r=<try>

bors · bors · commit 269623e84a16 · 2025-04-10T07:21:32.000Z
Various coercion cleanups

I think the commit order is the most reasonable one, but there's probably more ways to get to the same goal.

Essentially I got rid of the `simple` and `identity` helpers by adding a dedicated function for the common `identity` case and getting rid of the callbacks alltogether by realizing that all callbacks were of the pattern "use this fixed prefix list of adjustments, then add another adjustment with the unified type as the target type".

No behavioral changes intended
diff --git a/compiler/rustc_hir_typeck/src/coercion.rs b/compiler/rustc_hir_typeck/src/coercion.rs
@@ -103,15 +103,6 @@ fn coerce_mutbls<'tcx>(
     if from_mutbl >= to_mutbl { Ok(()) } else { Err(TypeError::Mutability) }
 }
 
-/// Do not require any adjustments, i.e. coerce `x -> x`.
-fn identity(_: Ty<'_>) -> Vec<Adjustment<'_>> {
-    vec![]
-}
-
-fn simple<'tcx>(kind: Adjust) -> impl FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>> {
-    move |target| vec![Adjustment { kind, target }]
-}
-
 /// This always returns `Ok(...)`.
 fn success<'tcx>(
     adj: Vec<Adjustment<'tcx>>,
@@ -131,7 +122,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         Coerce { fcx, cause, allow_two_phase, use_lub: false, coerce_never }
     }
 
-    fn unify(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> InferResult<'tcx, Ty<'tcx>> {
+    fn unify_raw(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> InferResult<'tcx, Ty<'tcx>> {
         debug!("unify(a: {:?}, b: {:?}, use_lub: {})", a, b, self.use_lub);
         self.commit_if_ok(|_| {
             let at = self.at(&self.cause, self.fcx.param_env);
@@ -161,13 +152,30 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         })
     }
 
+    /// Unify two types (using sub or lub).
+    fn unify(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
+        self.unify_raw(a, b)
+            .and_then(|InferOk { value: ty, obligations }| success(vec![], ty, obligations))
+    }
+
     /// Unify two types (using sub or lub) and produce a specific coercion.
-    fn unify_and<F>(&self, a: Ty<'tcx>, b: Ty<'tcx>, f: F) -> CoerceResult<'tcx>
-    where
-        F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
-    {
-        self.unify(a, b)
-            .and_then(|InferOk { value: ty, obligations }| success(f(ty), ty, obligations))
+    fn unify_and(
+        &self,
+        a: Ty<'tcx>,
+        b: Ty<'tcx>,
+        adjustments: impl IntoIterator<Item = Adjustment<'tcx>>,
+        final_adjustment: Adjust,
+    ) -> CoerceResult<'tcx> {
+        self.unify_raw(a, b).and_then(|InferOk { value: ty, obligations }| {
+            success(
+                adjustments
+                    .into_iter()
+                    .chain(std::iter::once(Adjustment { target: ty, kind: final_adjustment }))
+                    .collect(),
+                ty,
+                obligations,
+            )
+        })
     }
 
     #[instrument(skip(self))]
@@ -180,18 +188,22 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         // Coercing from `!` to any type is allowed:
         if a.is_never() {
             if self.coerce_never {
-                return success(simple(Adjust::NeverToAny)(b), b, PredicateObligations::new());
+                return success(
+                    vec![Adjustment { kind: Adjust::NeverToAny, target: b }],
+                    b,
+                    PredicateObligations::new(),
+                );
             } else {
                 // Otherwise the only coercion we can do is unification.
-                return self.unify_and(a, b, identity);
+                return self.unify(a, b);
             }
         }
 
         // Coercing *from* an unresolved inference variable means that
         // we have no information about the source type. This will always
         // ultimately fall back to some form of subtyping.
         if a.is_ty_var() {
-            return self.coerce_from_inference_variable(a, b, identity);
+            return self.coerce_from_inference_variable(a, b);
         }
 
         // Consider coercing the subtype to a DST
@@ -247,7 +259,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
             ty::FnPtr(a_sig_tys, a_hdr) => {
                 // We permit coercion of fn pointers to drop the
                 // unsafe qualifier.
-                self.coerce_from_fn_pointer(a, a_sig_tys.with(a_hdr), b)
+                self.coerce_from_fn_pointer(a_sig_tys.with(a_hdr), b)
             }
             ty::Closure(closure_def_id_a, args_a) => {
                 // Non-capturing closures are coercible to
@@ -257,20 +269,15 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
             }
             _ => {
                 // Otherwise, just use unification rules.
-                self.unify_and(a, b, identity)
+                self.unify(a, b)
             }
         }
     }
 
     /// Coercing *from* an inference variable. In this case, we have no information
     /// about the source type, so we can't really do a true coercion and we always
     /// fall back to subtyping (`unify_and`).
-    fn coerce_from_inference_variable(
-        &self,
-        a: Ty<'tcx>,
-        b: Ty<'tcx>,
-        make_adjustments: impl FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
-    ) -> CoerceResult<'tcx> {
+    fn coerce_from_inference_variable(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
         debug!("coerce_from_inference_variable(a={:?}, b={:?})", a, b);
         assert!(a.is_ty_var() && self.shallow_resolve(a) == a);
         assert!(self.shallow_resolve(b) == b);
@@ -298,12 +305,11 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                 "coerce_from_inference_variable: two inference variables, target_ty={:?}, obligations={:?}",
                 target_ty, obligations
             );
-            let adjustments = make_adjustments(target_ty);
-            InferResult::Ok(InferOk { value: (adjustments, target_ty), obligations })
+            success(vec![], target_ty, obligations)
         } else {
             // One unresolved type variable: just apply subtyping, we may be able
             // to do something useful.
-            self.unify_and(a, b, make_adjustments)
+            self.unify(a, b)
         }
     }
 
@@ -331,7 +337,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                 coerce_mutbls(mt_a.mutbl, mutbl_b)?;
                 (r_a, mt_a)
             }
-            _ => return self.unify_and(a, b, identity),
+            _ => return self.unify(a, b),
         };
 
         let span = self.cause.span;
@@ -437,7 +443,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                 referent_ty,
                 mutbl_b, // [1] above
             );
-            match self.unify(derefd_ty_a, b) {
+            match self.unify_raw(derefd_ty_a, b) {
                 Ok(ok) => {
                     found = Some(ok);
                     break;
@@ -579,13 +585,18 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         // We only have the latter, so we use an inference variable
         // for the former and let type inference do the rest.
         let coerce_target = self.next_ty_var(self.cause.span);
-        let mut coercion = self.unify_and(coerce_target, target, |target| {
-            let unsize = Adjustment { kind: Adjust::Pointer(PointerCoercion::Unsize), target };
-            match reborrow {
-                None => vec![unsize],
-                Some((ref deref, ref autoref)) => vec![deref.clone(), autoref.clone(), unsize],
+
+        let mut coercion = match reborrow {
+            None => {
+                self.unify_and(coerce_target, target, [], Adjust::Pointer(PointerCoercion::Unsize))?
             }
-        })?;
+            Some((ref deref, ref autoref)) => self.unify_and(
+                coerce_target,
+                target,
+                [deref.clone(), autoref.clone()],
+                Adjust::Pointer(PointerCoercion::Unsize),
+            )?,
+        };
 
         let mut selcx = traits::SelectionContext::new(self);
 
@@ -708,7 +719,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
             && let ty::Dynamic(b_data, _, ty::DynStar) = b.kind()
             && a_data.principal_def_id() == b_data.principal_def_id()
         {
-            return self.unify_and(a, b, |_| vec![]);
+            return self.unify(a, b);
         }
 
         // Check the obligations of the cast -- for example, when casting
@@ -808,23 +819,15 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
 
         // To complete the reborrow, we need to make sure we can unify the inner types, and if so we
         // add the adjustments.
-        self.unify_and(a, b, |_inner_ty| {
-            vec![Adjustment { kind: Adjust::ReborrowPin(mut_b), target: b }]
-        })
+        self.unify_and(a, b, [], Adjust::ReborrowPin(mut_b))
     }
 
-    fn coerce_from_safe_fn<F, G>(
+    fn coerce_from_safe_fn(
         &self,
-        a: Ty<'tcx>,
         fn_ty_a: ty::PolyFnSig<'tcx>,
         b: Ty<'tcx>,
-        to_unsafe: F,
-        normal: G,
-    ) -> CoerceResult<'tcx>
-    where
-        F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
-        G: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
-    {
+        adjustment: Option<Adjust>,
+    ) -> CoerceResult<'tcx> {
         self.commit_if_ok(|snapshot| {
             let outer_universe = self.infcx.universe();
 
@@ -833,9 +836,26 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                 && hdr_b.safety.is_unsafe()
             {
                 let unsafe_a = self.tcx.safe_to_unsafe_fn_ty(fn_ty_a);
-                self.unify_and(unsafe_a, b, to_unsafe)
+                match adjustment {
+                    Some(kind) => self.unify_and(
+                        unsafe_a,
+                        b,
+                        [Adjustment { kind, target: Ty::new_fn_ptr(self.tcx, fn_ty_a) }],
+                        Adjust::Pointer(PointerCoercion::UnsafeFnPointer),
+                    ),
+                    None => self.unify_and(
+                        unsafe_a,
+                        b,
+                        [],
+                        Adjust::Pointer(PointerCoercion::UnsafeFnPointer),
+                    ),
+                }
             } else {
-                self.unify_and(a, b, normal)
+                let a = Ty::new_fn_ptr(self.tcx, fn_ty_a);
+                match adjustment {
+                    Some(adjust) => self.unify_and(a, b, [], adjust),
+                    None => self.unify(a, b),
+                }
             };
 
             // FIXME(#73154): This is a hack. Currently LUB can generate
@@ -852,7 +872,6 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
 
     fn coerce_from_fn_pointer(
         &self,
-        a: Ty<'tcx>,
         fn_ty_a: ty::PolyFnSig<'tcx>,
         b: Ty<'tcx>,
     ) -> CoerceResult<'tcx> {
@@ -861,15 +880,9 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         //!
 
         let b = self.shallow_resolve(b);
-        debug!("coerce_from_fn_pointer(a={:?}, b={:?})", a, b);
-
-        self.coerce_from_safe_fn(
-            a,
-            fn_ty_a,
-            b,
-            simple(Adjust::Pointer(PointerCoercion::UnsafeFnPointer)),
-            identity,
-        )
+        debug!(?fn_ty_a, ?b, "coerce_from_fn_pointer");
+
+        self.coerce_from_safe_fn(fn_ty_a, b, None)
     }
 
     fn coerce_from_fn_item(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
@@ -916,30 +929,16 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                     self.at(&self.cause, self.param_env).normalize(a_sig);
                 obligations.extend(o1);
 
-                let a_fn_pointer = Ty::new_fn_ptr(self.tcx, a_sig);
                 let InferOk { value, obligations: o2 } = self.coerce_from_safe_fn(
-                    a_fn_pointer,
                     a_sig,
                     b,
-                    |unsafe_ty| {
-                        vec![
-                            Adjustment {
-                                kind: Adjust::Pointer(PointerCoercion::ReifyFnPointer),
-                                target: a_fn_pointer,
-                            },
-                            Adjustment {
-                                kind: Adjust::Pointer(PointerCoercion::UnsafeFnPointer),
-                                target: unsafe_ty,
-                            },
-                        ]
-                    },
-                    simple(Adjust::Pointer(PointerCoercion::ReifyFnPointer)),
+                    Some(Adjust::Pointer(PointerCoercion::ReifyFnPointer)),
                 )?;
 
                 obligations.extend(o2);
                 Ok(InferOk { value, obligations })
             }
-            _ => self.unify_and(a, b, identity),
+            _ => self.unify(a, b),
         }
     }
 
@@ -983,10 +982,11 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                 self.unify_and(
                     pointer_ty,
                     b,
-                    simple(Adjust::Pointer(PointerCoercion::ClosureFnPointer(safety))),
+                    [],
+                    Adjust::Pointer(PointerCoercion::ClosureFnPointer(safety)),
                 )
             }
-            _ => self.unify_and(a, b, identity),
+            _ => self.unify(a, b),
         }
     }
 
@@ -1001,7 +1001,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         let (is_ref, mt_a) = match *a.kind() {
             ty::Ref(_, ty, mutbl) => (true, ty::TypeAndMut { ty, mutbl }),
             ty::RawPtr(ty, mutbl) => (false, ty::TypeAndMut { ty, mutbl }),
-            _ => return self.unify_and(a, b, identity),
+            _ => return self.unify(a, b),
         };
         coerce_mutbls(mt_a.mutbl, mutbl_b)?;
 
@@ -1011,16 +1011,16 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         // representation, we still register an Adjust::DerefRef so that
         // regionck knows that the region for `a` must be valid here.
         if is_ref {
-            self.unify_and(a_raw, b, |target| {
-                vec![
-                    Adjustment { kind: Adjust::Deref(None), target: mt_a.ty },
-                    Adjustment { kind: Adjust::Borrow(AutoBorrow::RawPtr(mutbl_b)), target },
-                ]
-            })
+            self.unify_and(
+                a_raw,
+                b,
+                [Adjustment { kind: Adjust::Deref(None), target: mt_a.ty }],
+                Adjust::Borrow(AutoBorrow::RawPtr(mutbl_b)),
+            )
         } else if mt_a.mutbl != mutbl_b {
-            self.unify_and(a_raw, b, simple(Adjust::Pointer(PointerCoercion::MutToConstPointer)))
+            self.unify_and(a_raw, b, [], Adjust::Pointer(PointerCoercion::MutToConstPointer))
         } else {
-            self.unify_and(a_raw, b, identity)
+            self.unify(a_raw, b)
         }
     }
 }
@@ -1118,9 +1118,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let cause = self.cause(DUMMY_SP, ObligationCauseCode::ExprAssignable);
         // We don't ever need two-phase here since we throw out the result of the coercion.
         let coerce = Coerce::new(self, cause, AllowTwoPhase::No, true);
-        coerce
-            .autoderef(DUMMY_SP, expr_ty)
-            .find_map(|(ty, steps)| self.probe(|_| coerce.unify(ty, target)).ok().map(|_| steps))
+        coerce.autoderef(DUMMY_SP, expr_ty).find_map(|(ty, steps)| {
+            self.probe(|_| coerce.unify_raw(ty, target)).ok().map(|_| steps)
+        })
     }
 
     /// Given a type, this function will calculate and return the type given
