Fix printing unit return ty, don't elaborate FnOnce unless we see it

11 files changed, 40 insertions, 31 deletions

diff --git a/compiler/rustc_middle/src/ty/print/pretty.rs b/compiler/rustc_middle/src/ty/print/pretty.rs
index aa175baa8c7..175295b3199 100644
--- a/compiler/rustc_middle/src/ty/print/pretty.rs
+++ b/compiler/rustc_middle/src/ty/print/pretty.rs
@@ -839,19 +839,25 @@ pub trait PrettyPrinter<'tcx>:
                         p!(print(ty));
                     }
 
-                    p!(") -> ", print(return_ty), write("{}", if paren_needed { ")" } else { "" }));
+                    p!(")");
+                    if !return_ty.skip_binder().is_unit() {
+                        p!("-> ", print(return_ty));
+                    }
+                    p!(write("{}", if paren_needed { ")" } else { "" }));
 
                     first = false;
                 }
                 // If we got here, we can't print as a `impl Fn(A, B) -> C`. Just record the
                 // trait_refs we collected in the OpaqueFnEntry as normal trait refs.
                 _ => {
-                    traits.entry(fn_once_trait_ref).or_default().extend(
-                        // Group the return ty with its def id, if we had one.
-                        entry
-                            .return_ty
-                            .map(|ty| (self.tcx().lang_items().fn_once_output().unwrap(), ty)),
-                    );
+                    if entry.has_fn_once {
+                        traits.entry(fn_once_trait_ref).or_default().extend(
+                            // Group the return ty with its def id, if we had one.
+                            entry
+                                .return_ty
+                                .map(|ty| (self.tcx().lang_items().fn_once_output().unwrap(), ty)),
+                        );
+                    }
                     if let Some(trait_ref) = entry.fn_mut_trait_ref {
                         traits.entry(trait_ref).or_default();
                     }
@@ -943,6 +949,7 @@ pub trait PrettyPrinter<'tcx>:
                 if let Some((_, ty)) = proj_ty {
                     entry.return_ty = Some(ty);
                 }
+                entry.has_fn_once = true;
                 return;
             } else if Some(trait_def_id) == self.tcx().lang_items().fn_mut_trait() {
                 let super_trait_ref = crate::traits::util::supertraits(self.tcx(), trait_ref)
@@ -2695,6 +2702,8 @@ pub fn provide(providers: &mut ty::query::Providers) {
 
 #[derive(Default)]
 pub struct OpaqueFnEntry<'tcx> {
+    // The trait ref is already stored as a key, so just track if we have it as a real predicate
+    has_fn_once: bool,
     fn_mut_trait_ref: Option<ty::PolyTraitRef<'tcx>>,
     fn_trait_ref: Option<ty::PolyTraitRef<'tcx>>,
     return_ty: Option<ty::Binder<'tcx, Ty<'tcx>>>,
diff --git a/src/test/mir-opt/inline/issue_78442.bar.Inline.diff b/src/test/mir-opt/inline/issue_78442.bar.Inline.diff
index 3e72d1c42fa..045bc720ca7 100644
--- a/src/test/mir-opt/inline/issue_78442.bar.Inline.diff
+++ b/src/test/mir-opt/inline/issue_78442.bar.Inline.diff
@@ -19,16 +19,16 @@
 +         _4 = hide_foo() -> [return: bb1, unwind: bb3]; // scope 0 at $DIR/issue-78442.rs:11:5: 11:15
                                            // mir::Constant
                                            // + span: $DIR/issue-78442.rs:11:5: 11:13
-                                           // + literal: Const { ty: fn() -> impl Fn() -> () {hide_foo}, val: Value(Scalar(<ZST>)) }
+                                           // + literal: Const { ty: fn() -> impl Fn() {hide_foo}, val: Value(Scalar(<ZST>)) }
       }
   
       bb1: {
           _3 = &_4;                        // scope 0 at $DIR/issue-78442.rs:11:5: 11:15
           StorageLive(_5);                 // scope 0 at $DIR/issue-78442.rs:11:5: 11:17
--         _2 = <impl Fn() -> () as Fn<()>>::call(move _3, move _5) -> [return: bb2, unwind: bb4]; // scope 0 at $DIR/issue-78442.rs:11:5: 11:17
+-         _2 = <impl Fn() as Fn<()>>::call(move _3, move _5) -> [return: bb2, unwind: bb4]; // scope 0 at $DIR/issue-78442.rs:11:5: 11:17
 -                                          // mir::Constant
 -                                          // + span: $DIR/issue-78442.rs:11:5: 11:15
--                                          // + literal: Const { ty: for<'r> extern "rust-call" fn(&'r impl Fn() -> (), ()) -> <impl Fn() -> () as std::ops::FnOnce<()>>::Output {<impl Fn() -> () as std::ops::Fn<()>>::call}, val: Value(Scalar(<ZST>)) }
+-                                          // + literal: Const { ty: for<'r> extern "rust-call" fn(&'r impl Fn(), ()) -> <impl Fn() as std::ops::FnOnce<()>>::Output {<impl Fn() as std::ops::Fn<()>>::call}, val: Value(Scalar(<ZST>)) }
 +         _2 = move (*_3)() -> [return: bb5, unwind: bb3]; // scope 1 at $DIR/issue-78442.rs:11:5: 11:17
       }
   
diff --git a/src/test/mir-opt/inline/issue_78442.bar.RevealAll.diff b/src/test/mir-opt/inline/issue_78442.bar.RevealAll.diff
index 622599a2d18..3190db52286 100644
--- a/src/test/mir-opt/inline/issue_78442.bar.RevealAll.diff
+++ b/src/test/mir-opt/inline/issue_78442.bar.RevealAll.diff
@@ -5,8 +5,8 @@
       debug _baz => _1;                    // in scope 0 at $DIR/issue-78442.rs:9:5: 9:9
       let mut _0: ();                      // return place in scope 0 at $DIR/issue-78442.rs:10:3: 10:3
       let _2: ();                          // in scope 0 at $DIR/issue-78442.rs:11:5: 11:17
--     let mut _3: &impl Fn() -> ();        // in scope 0 at $DIR/issue-78442.rs:11:5: 11:15
--     let _4: impl Fn() -> ();             // in scope 0 at $DIR/issue-78442.rs:11:5: 11:15
+-     let mut _3: &impl Fn();              // in scope 0 at $DIR/issue-78442.rs:11:5: 11:15
+-     let _4: impl Fn();                   // in scope 0 at $DIR/issue-78442.rs:11:5: 11:15
 +     let mut _3: &fn() {foo};             // in scope 0 at $DIR/issue-78442.rs:11:5: 11:15
 +     let _4: fn() {foo};                  // in scope 0 at $DIR/issue-78442.rs:11:5: 11:15
       let mut _5: ();                      // in scope 0 at $DIR/issue-78442.rs:11:5: 11:17
@@ -18,17 +18,17 @@
           _4 = hide_foo() -> [return: bb1, unwind: bb4]; // scope 0 at $DIR/issue-78442.rs:11:5: 11:15
                                            // mir::Constant
                                            // + span: $DIR/issue-78442.rs:11:5: 11:13
-                                           // + literal: Const { ty: fn() -> impl Fn() -> () {hide_foo}, val: Value(Scalar(<ZST>)) }
+                                           // + literal: Const { ty: fn() -> impl Fn() {hide_foo}, val: Value(Scalar(<ZST>)) }
       }
   
       bb1: {
           _3 = &_4;                        // scope 0 at $DIR/issue-78442.rs:11:5: 11:15
           StorageLive(_5);                 // scope 0 at $DIR/issue-78442.rs:11:5: 11:17
           nop;                             // scope 0 at $DIR/issue-78442.rs:11:5: 11:17
-          _2 = <impl Fn() -> () as Fn<()>>::call(move _3, move _5) -> [return: bb2, unwind: bb4]; // scope 0 at $DIR/issue-78442.rs:11:5: 11:17
+          _2 = <impl Fn() as Fn<()>>::call(move _3, move _5) -> [return: bb2, unwind: bb4]; // scope 0 at $DIR/issue-78442.rs:11:5: 11:17
                                            // mir::Constant
                                            // + span: $DIR/issue-78442.rs:11:5: 11:15
-                                           // + literal: Const { ty: for<'r> extern "rust-call" fn(&'r impl Fn() -> (), ()) -> <impl Fn() -> () as std::ops::FnOnce<()>>::Output {<impl Fn() -> () as std::ops::Fn<()>>::call}, val: Value(Scalar(<ZST>)) }
+                                           // + literal: Const { ty: for<'r> extern "rust-call" fn(&'r impl Fn(), ()) -> <impl Fn() as std::ops::FnOnce<()>>::Output {<impl Fn() as std::ops::Fn<()>>::call}, val: Value(Scalar(<ZST>)) }
       }
   
       bb2: {
diff --git a/src/test/ui/entry-point/imported_main_const_fn_item_type_forbidden.stderr b/src/test/ui/entry-point/imported_main_const_fn_item_type_forbidden.stderr
index 42c6de20127..c731c328322 100644
--- a/src/test/ui/entry-point/imported_main_const_fn_item_type_forbidden.stderr
+++ b/src/test/ui/entry-point/imported_main_const_fn_item_type_forbidden.stderr
@@ -21,7 +21,7 @@ LL |     type MainFn = impl Fn();
 LL |     pub const BAR: MainFn = bar;
    |                             ^^^ expected opaque type, found fn item
    |
-   = note: expected opaque type `impl Fn() -> ()`
+   = note: expected opaque type `impl Fn()`
                   found fn item `fn() {bar}`
 
 error: could not find defining uses
diff --git a/src/test/ui/impl-trait/auto-trait-leak2.stderr b/src/test/ui/impl-trait/auto-trait-leak2.stderr
index 2ca180029ae..37ae3c68029 100644
--- a/src/test/ui/impl-trait/auto-trait-leak2.stderr
+++ b/src/test/ui/impl-trait/auto-trait-leak2.stderr
@@ -2,16 +2,16 @@ error[E0277]: `Rc<Cell<i32>>` cannot be sent between threads safely
   --> $DIR/auto-trait-leak2.rs:13:10
    |
 LL | fn before() -> impl Fn(i32) {
-   |                ------------ within this `impl Fn(i32) -> ()`
+   |                ------------ within this `impl Fn(i32)`
 ...
 LL |     send(before());
    |     ---- ^^^^^^^^ `Rc<Cell<i32>>` cannot be sent between threads safely
    |     |
    |     required by a bound introduced by this call
    |
-   = help: within `impl Fn(i32) -> ()`, the trait `Send` is not implemented for `Rc<Cell<i32>>`
+   = help: within `impl Fn(i32)`, the trait `Send` is not implemented for `Rc<Cell<i32>>`
    = note: required because it appears within the type `[closure@$DIR/auto-trait-leak2.rs:7:5: 7:22]`
-   = note: required because it appears within the type `impl Fn(i32) -> ()`
+   = note: required because it appears within the type `impl Fn(i32)`
 note: required by a bound in `send`
   --> $DIR/auto-trait-leak2.rs:10:12
    |
@@ -27,11 +27,11 @@ LL |     send(after());
    |     required by a bound introduced by this call
 ...
 LL | fn after() -> impl Fn(i32) {
-   |               ------------ within this `impl Fn(i32) -> ()`
+   |               ------------ within this `impl Fn(i32)`
    |
-   = help: within `impl Fn(i32) -> ()`, the trait `Send` is not implemented for `Rc<Cell<i32>>`
+   = help: within `impl Fn(i32)`, the trait `Send` is not implemented for `Rc<Cell<i32>>`
    = note: required because it appears within the type `[closure@$DIR/auto-trait-leak2.rs:24:5: 24:22]`
-   = note: required because it appears within the type `impl Fn(i32) -> ()`
+   = note: required because it appears within the type `impl Fn(i32)`
 note: required by a bound in `send`
   --> $DIR/auto-trait-leak2.rs:10:12
    |
diff --git a/src/test/ui/impl-trait/issue-87450.stderr b/src/test/ui/impl-trait/issue-87450.stderr
index 9b7d225a53e..5019e544bd5 100644
--- a/src/test/ui/impl-trait/issue-87450.stderr
+++ b/src/test/ui/impl-trait/issue-87450.stderr
@@ -17,10 +17,10 @@ LL | fn foo() -> impl Fn() {
    |             ^^^^^^^^^ recursive opaque type
 ...
 LL |     wrap(wrap(wrap(wrap(wrap(wrap(wrap(foo())))))))
-   |     ----------------------------------------------- returning here with type `impl Fn() -> ()`
+   |     ----------------------------------------------- returning here with type `impl Fn()`
 ...
 LL | fn wrap(f: impl Fn()) -> impl Fn() {
-   |                          --------- returning this opaque type `impl Fn() -> ()`
+   |                          --------- returning this opaque type `impl Fn()`
 
 error: aborting due to previous error; 1 warning emitted
 
diff --git a/src/test/ui/impl-trait/issues/issue-86201.stderr b/src/test/ui/impl-trait/issues/issue-86201.stderr
index d65bddf7cfa..b1460096ded 100644
--- a/src/test/ui/impl-trait/issues/issue-86201.stderr
+++ b/src/test/ui/impl-trait/issues/issue-86201.stderr
@@ -7,7 +7,7 @@ LL |
 LL | static STATIC_FN: FunType = some_fn;
    |                             ^^^^^^^ expected opaque type, found fn item
    |
-   = note: expected opaque type `impl Fn<()> + FnOnce<()>`
+   = note: expected opaque type `impl Fn<()>`
                   found fn item `fn() {some_fn}`
 
 error: could not find defining uses
diff --git a/src/test/ui/issues/issue-59494.rs b/src/test/ui/issues/issue-59494.rs
index 65ef436129e..a53e28f7246 100644
--- a/src/test/ui/issues/issue-59494.rs
+++ b/src/test/ui/issues/issue-59494.rs
@@ -19,5 +19,5 @@ fn main() {
     let g = |(a, _)| a;
     let t7 = |env| |a| |b| t7p(f, g)(((env, a), b));
     let t8 = t8n(t7, t7p(f, g));
-    //~^ ERROR: expected a `Fn<(_,)>` closure, found `impl Fn(((_, _), _)) -> ()` [E0277]
+    //~^ ERROR: expected a `Fn<(_,)>` closure, found `impl Fn(((_, _), _))` [E0277]
 }
diff --git a/src/test/ui/issues/issue-59494.stderr b/src/test/ui/issues/issue-59494.stderr
index 0af54f567f0..a9284535e4d 100644
--- a/src/test/ui/issues/issue-59494.stderr
+++ b/src/test/ui/issues/issue-59494.stderr
@@ -1,12 +1,12 @@
-error[E0277]: expected a `Fn<(_,)>` closure, found `impl Fn(((_, _), _)) -> ()`
+error[E0277]: expected a `Fn<(_,)>` closure, found `impl Fn(((_, _), _))`
   --> $DIR/issue-59494.rs:21:22
    |
 LL |     let t8 = t8n(t7, t7p(f, g));
-   |              ---     ^^^^^^^^^ expected an `Fn<(_,)>` closure, found `impl Fn(((_, _), _)) -> ()`
+   |              ---     ^^^^^^^^^ expected an `Fn<(_,)>` closure, found `impl Fn(((_, _), _))`
    |              |
    |              required by a bound introduced by this call
    |
-   = help: the trait `Fn<(_,)>` is not implemented for `impl Fn(((_, _), _)) -> ()`
+   = help: the trait `Fn<(_,)>` is not implemented for `impl Fn(((_, _), _))`
 note: required by a bound in `t8n`
   --> $DIR/issue-59494.rs:5:45
    |
diff --git a/src/test/ui/lint/opaque-ty-ffi-unsafe.rs b/src/test/ui/lint/opaque-ty-ffi-unsafe.rs
index efdc4037b63..3a62b6a21a5 100644
--- a/src/test/ui/lint/opaque-ty-ffi-unsafe.rs
+++ b/src/test/ui/lint/opaque-ty-ffi-unsafe.rs
@@ -9,7 +9,7 @@ pub fn ret_closure() -> A {
 
 extern "C" {
     pub fn a(_: A);
-    //~^ ERROR `extern` block uses type `impl Fn() -> ()`, which is not FFI-safe [improper_ctypes]
+    //~^ ERROR `extern` block uses type `impl Fn()`, which is not FFI-safe [improper_ctypes]
 }
 
 fn main() {}
diff --git a/src/test/ui/lint/opaque-ty-ffi-unsafe.stderr b/src/test/ui/lint/opaque-ty-ffi-unsafe.stderr
index c77d8c0c18b..5afbef778b3 100644
--- a/src/test/ui/lint/opaque-ty-ffi-unsafe.stderr
+++ b/src/test/ui/lint/opaque-ty-ffi-unsafe.stderr
@@ -1,4 +1,4 @@
-error: `extern` block uses type `impl Fn() -> ()`, which is not FFI-safe
+error: `extern` block uses type `impl Fn()`, which is not FFI-safe
   --> $DIR/opaque-ty-ffi-unsafe.rs:11:17
    |
 LL |     pub fn a(_: A);