update infer cost computation for types

compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs

@@ -602,41 +602,58 @@ impl<'a, 'tcx> FindInferSourceVisitor<'a, 'tcx> {
     /// Sources with a small cost are prefer and should result
     /// in a clearer and idiomatic suggestion.
     fn source_cost(&self, source: &InferSource<'tcx>) -> usize {
-        let tcx = self.infcx.tcx;
-
-        fn arg_cost<'tcx>(arg: GenericArg<'tcx>) -> usize {
-            match arg.unpack() {
-                GenericArgKind::Lifetime(_) => 0, // erased
-                GenericArgKind::Type(ty) => ty_cost(ty),
-                GenericArgKind::Const(_) => 3, // some non-zero value
-            }
+        #[derive(Clone, Copy)]
+        struct CostCtxt<'tcx> {
+            tcx: TyCtxt<'tcx>,
         }
-        fn ty_cost<'tcx>(ty: Ty<'tcx>) -> usize {
-            match ty.kind() {
-                ty::Closure(..) => 100,
-                ty::FnDef(..) => 20,
-                ty::FnPtr(..) => 10,
-                ty::Infer(..) => 0,
-                _ => 1,
+        impl<'tcx> CostCtxt<'tcx> {
+            fn arg_cost(self, arg: GenericArg<'tcx>) -> usize {
+                match arg.unpack() {
+                    GenericArgKind::Lifetime(_) => 0, // erased
+                    GenericArgKind::Type(ty) => self.ty_cost(ty),
+                    GenericArgKind::Const(_) => 3, // some non-zero value
+                }
+            }
+            fn ty_cost(self, ty: Ty<'tcx>) -> usize {
+                match ty.kind() {
+                    ty::Closure(..) => 1000,
+                    ty::FnDef(..) => 150,
+                    ty::FnPtr(..) => 30,
+                    ty::Adt(def, substs) => {
+                        5 + self
+                            .tcx
+                            .generics_of(def.did())
+                            .own_substs_no_defaults(self.tcx, substs)
+                            .iter()
+                            .map(|&arg| self.arg_cost(arg))
+                            .sum::<usize>()
+                    }
+                    ty::Tuple(args) => 5 + args.iter().map(|arg| self.ty_cost(arg)).sum::<usize>(),
+                    ty::Infer(..) => 0,
+                    _ => 1,
+                }
             }
         }
 
         // The sources are listed in order of preference here.
+        let tcx = self.infcx.tcx;
+        let ctx = CostCtxt { tcx };
         match source.kind {
-            InferSourceKind::LetBinding { ty, .. } => ty_cost(ty),
-            InferSourceKind::ClosureArg { ty, .. } => 5 + ty_cost(ty),
+            InferSourceKind::LetBinding { ty, .. } => ctx.ty_cost(ty),
+            InferSourceKind::ClosureArg { ty, .. } => ctx.ty_cost(ty),
             InferSourceKind::GenericArg { def_id, generic_args, .. } => {
                 let variant_cost = match tcx.def_kind(def_id) {
-                    DefKind::Variant | DefKind::Ctor(CtorOf::Variant, _) => 15, // `None::<u32>` and friends are ugly.
-                    _ => 12,
+                    // `None::<u32>` and friends are ugly.
+                    DefKind::Variant | DefKind::Ctor(CtorOf::Variant, _) => 15,
+                    _ => 10,
                 };
-                variant_cost + generic_args.iter().map(|&arg| arg_cost(arg)).sum::<usize>()
+                variant_cost + generic_args.iter().map(|&arg| ctx.arg_cost(arg)).sum::<usize>()
             }
             InferSourceKind::FullyQualifiedMethodCall { substs, .. } => {
-                20 + substs.iter().map(|arg| arg_cost(arg)).sum::<usize>()
+                20 + substs.iter().map(|arg| ctx.arg_cost(arg)).sum::<usize>()
             }
             InferSourceKind::ClosureReturn { ty, should_wrap_expr, .. } => {
-                30 + ty_cost(ty) + if should_wrap_expr.is_some() { 10 } else { 0 }
+                30 + ctx.ty_cost(ty) + if should_wrap_expr.is_some() { 10 } else { 0 }
             }
         }
     }
@@ -646,6 +663,7 @@ impl<'a, 'tcx> FindInferSourceVisitor<'a, 'tcx> {
     #[instrument(level = "debug", skip(self))]
     fn update_infer_source(&mut self, new_source: InferSource<'tcx>) {
         let cost = self.source_cost(&new_source) + self.attempt;
+        debug!(?cost);
         self.attempt += 1;
         if cost < self.infer_source_cost {
             self.infer_source_cost = cost;

src/test/ui/inference/need_type_info/channel.rs

@@ -0,0 +1,19 @@
+// Test that we suggest specifying the generic argument of `channel`
+// instead of the return type of that function, which is a lot more
+// complex.
+use std::sync::mpsc::channel;
+
+fn no_tuple() {
+    let _data =
+        channel(); //~ ERROR type annotations needed
+}
+
+fn tuple() {
+    let (_sender, _receiver) =
+        channel(); //~ ERROR type annotations needed
+}
+
+fn main() {
+    no_tuple();
+    tuple();
+}

src/test/ui/inference/need_type_info/channel.stderr

@@ -0,0 +1,25 @@
+error[E0282]: type annotations needed
+  --> $DIR/channel.rs:8:9
+   |
+LL |         channel();
+   |         ^^^^^^^ cannot infer type of the type parameter `T` declared on the function `channel`
+   |
+help: consider specifying the generic argument
+   |
+LL |         channel::<T>();
+   |                +++++
+
+error[E0282]: type annotations needed
+  --> $DIR/channel.rs:13:9
+   |
+LL |         channel();
+   |         ^^^^^^^ cannot infer type of the type parameter `T` declared on the function `channel`
+   |
+help: consider specifying the generic argument
+   |
+LL |         channel::<T>();
+   |                +++++
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0282`.

src/test/ui/issues/issue-25368.rs

@@ -5,10 +5,10 @@ use std::marker::PhantomData;
 struct Foo<T> {foo: PhantomData<T>}
 
 fn main() {
-    let (tx, rx) = //~ ERROR type annotations needed
+    let (tx, rx) =
         channel();
-    // FIXME(#89862): Suggest adding a generic argument to `channel` instead
     spawn(move || {
         tx.send(Foo{ foo: PhantomData });
+        //~^ ERROR type annotations needed
     });
 }

src/test/ui/issues/issue-25368.stderr

@@ -1,13 +1,13 @@
-error[E0282]: type annotations needed for `(Sender<Foo<T>>, std::sync::mpsc::Receiver<Foo<T>>)`
-  --> $DIR/issue-25368.rs:8:9
+error[E0282]: type annotations needed
+  --> $DIR/issue-25368.rs:11:27
    |
-LL |     let (tx, rx) =
-   |         ^^^^^^^^
+LL |         tx.send(Foo{ foo: PhantomData });
+   |                           ^^^^^^^^^^^ cannot infer type of the type parameter `T` declared on the struct `PhantomData`
    |
-help: consider giving this pattern a type, where the type for type parameter `T` is specified
+help: consider specifying the generic argument
    |
-LL |     let (tx, rx): (Sender<Foo<T>>, std::sync::mpsc::Receiver<Foo<T>>) =
-   |                 +++++++++++++++++++++++++++++++++++++++++++++++++++++
+LL |         tx.send(Foo{ foo: PhantomData::<T> });
+   |                                      +++++
 
 error: aborting due to previous error
 

src/test/ui/type/type-check/cannot_infer_local_or_vec_in_tuples.stderr

@@ -1,13 +1,15 @@
-error[E0282]: type annotations needed for `(Vec<T>,)`
-  --> $DIR/cannot_infer_local_or_vec_in_tuples.rs:2:9
+error[E0282]: type annotations needed
+  --> $DIR/cannot_infer_local_or_vec_in_tuples.rs:2:18
    |
 LL |     let (x, ) = (vec![], );
-   |         ^^^^^
+   |                  ^^^^^^ cannot infer type of the type parameter `T` declared on the struct `Vec`
    |
-help: consider giving this pattern a type, where the type for type parameter `T` is specified
+   = note: this error originates in the macro `vec` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: consider specifying the generic argument
+  --> $SRC_DIR/alloc/src/macros.rs:LL:COL
    |
-LL |     let (x, ): (Vec<T>,) = (vec![], );
-   |              +++++++++++
+LL |         $crate::__rust_force_expr!($crate::vec::Vec::<T>::new())
+   |                                                    +++++
 
 error: aborting due to previous error