Point out when a callable is not actually callable because its return is not sized

compiler/rustc_trait_selection/src/infer.rs

@@ -24,6 +24,13 @@ fn type_is_copy_modulo_regions(
         span: Span,
     ) -> bool;
 
+    fn type_is_sized_modulo_regions(
+        &self,
+        param_env: ty::ParamEnv<'tcx>,
+        ty: Ty<'tcx>,
+        span: Span,
+    ) -> bool;
+
     fn partially_normalize_associated_types_in<T>(
         &self,
         cause: ObligationCause<'tcx>,
@@ -74,6 +81,16 @@ fn type_is_copy_modulo_regions(
         traits::type_known_to_meet_bound_modulo_regions(self, param_env, ty, copy_def_id, span)
     }
 
+    fn type_is_sized_modulo_regions(
+        &self,
+        param_env: ty::ParamEnv<'tcx>,
+        ty: Ty<'tcx>,
+        span: Span,
+    ) -> bool {
+        let lang_item = self.tcx.require_lang_item(LangItem::Sized, None);
+        traits::type_known_to_meet_bound_modulo_regions(self, param_env, ty, lang_item, span)
+    }
+
     /// Normalizes associated types in `value`, potentially returning
     /// new obligations that must further be processed.
     fn partially_normalize_associated_types_in<T>(

compiler/rustc_typeck/src/check/callee.rs

@@ -1,5 +1,5 @@
 use super::method::MethodCallee;
-use super::{Expectation, FnCtxt, TupleArgumentsFlag};
+use super::{DefIdOrName, Expectation, FnCtxt, TupleArgumentsFlag};
 use crate::type_error_struct;
 
 use rustc_errors::{struct_span_err, Applicability, Diagnostic};
@@ -24,7 +24,8 @@
 use rustc_span::Span;
 use rustc_target::spec::abi;
 use rustc_trait_selection::autoderef::Autoderef;
-use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
+use rustc_trait_selection::infer::InferCtxtExt as _;
+use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt as _;
 
 use std::iter;
 
@@ -471,7 +472,25 @@ fn confirm_builtin_call(
                 };
 
                 if !self.maybe_suggest_bad_array_definition(&mut err, call_expr, callee_expr) {
-                    err.span_label(call_expr.span, "call expression requires function");
+                    if let Some((maybe_def, output_ty, _)) = self.extract_callable_info(callee_expr, callee_ty)
+                        && !self.type_is_sized_modulo_regions(self.param_env, output_ty, callee_expr.span)
+                    {
+                        let descr = match maybe_def {
+                            DefIdOrName::DefId(def_id) => self.tcx.def_kind(def_id).descr(def_id),
+                            DefIdOrName::Name(name) => name,
+                        };
+                        err.span_label(
+                            callee_expr.span,
+                            format!("this {descr} returns an unsized value `{output_ty}`, so it cannot be called")
+                        );
+                        if let DefIdOrName::DefId(def_id) = maybe_def
+                            && let Some(def_span) = self.tcx.hir().span_if_local(def_id)
+                        {
+                            err.span_label(def_span, "the callable type is defined here");
+                        }
+                    } else {
+                        err.span_label(call_expr.span, "call expression requires function");
+                    }
                 }
 
                 if let Some(span) = self.tcx.hir().res_span(def) {

compiler/rustc_typeck/src/check/cast.rs

@@ -35,7 +35,6 @@
 use hir::def_id::LOCAL_CRATE;
 use rustc_errors::{struct_span_err, Applicability, DiagnosticBuilder, ErrorGuaranteed};
 use rustc_hir as hir;
-use rustc_hir::lang_items::LangItem;
 use rustc_middle::mir::Mutability;
 use rustc_middle::ty::adjustment::AllowTwoPhase;
 use rustc_middle::ty::cast::{CastKind, CastTy};
@@ -47,7 +46,6 @@
 use rustc_span::symbol::sym;
 use rustc_span::Span;
 use rustc_trait_selection::infer::InferCtxtExt;
-use rustc_trait_selection::traits;
 use rustc_trait_selection::traits::error_reporting::report_object_safety_error;
 
 /// Reifies a cast check to be checked once we have full type information for
@@ -97,7 +95,7 @@ fn pointer_kind(
             return Err(reported);
         }
 
-        if self.type_is_known_to_be_sized_modulo_regions(t, span) {
+        if self.type_is_sized_modulo_regions(self.param_env, t, span) {
             return Ok(Some(PointerKind::Thin));
         }
 
@@ -705,7 +703,7 @@ pub fn check(mut self, fcx: &FnCtxt<'a, 'tcx>) {
 
         debug!("check_cast({}, {:?} as {:?})", self.expr.hir_id, self.expr_ty, self.cast_ty);
 
-        if !fcx.type_is_known_to_be_sized_modulo_regions(self.cast_ty, self.span)
+        if !fcx.type_is_sized_modulo_regions(fcx.param_env, self.cast_ty, self.span)
             && !self.cast_ty.has_infer_types()
         {
             self.report_cast_to_unsized_type(fcx);
@@ -1084,10 +1082,3 @@ fn fuzzy_provenance_int2ptr_lint(&self, fcx: &FnCtxt<'a, 'tcx>) {
         );
     }
 }
-
-impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
-    fn type_is_known_to_be_sized_modulo_regions(&self, ty: Ty<'tcx>, span: Span) -> bool {
-        let lang_item = self.tcx.require_lang_item(LangItem::Sized, None);
-        traits::type_known_to_meet_bound_modulo_regions(self, self.param_env, ty, lang_item, span)
-    }
-}

compiler/rustc_typeck/src/check/fn_ctxt/suggestions.rs

@@ -144,7 +144,10 @@ pub(crate) fn suggest_fn_call(
         false
     }
 
-    fn extract_callable_info(
+    /// Extracts information about a callable type for diagnostics. This is a
+    /// heuristic -- it doesn't necessarily mean that a type is always callable,
+    /// because the callable type must also be well-formed to be called.
+    pub(in super::super) fn extract_callable_info(
         &self,
         expr: &Expr<'_>,
         found: Ty<'tcx>,
@@ -1130,7 +1133,7 @@ pub(crate) fn consider_removing_semicolon(
     }
 }
 
-enum DefIdOrName {
+pub enum DefIdOrName {
     DefId(DefId),
     Name(&'static str),
 }

src/test/ui/issues/issue-41139.stderr

@@ -5,9 +5,7 @@ LL | fn get_function<'a>() -> &'a dyn Fn() -> dyn Trait {
    | -------------------------------------------------- `get_function` defined here returns `&dyn Fn() -> (dyn Trait + 'static)`
 ...
 LL |     let t: &dyn Trait = &get_function()();
-   |                          ^^^^^^^^^^^^^^--
+   |                          ^^^^^^^^^^^^^^ this trait object returns an unsized value `(dyn Trait + 'static)`, so it cannot be called
-   |                          |
-   |                          call expression requires function
 
 error: aborting due to previous error