Merge #8987

8987: Fix lowering of FnOnce() without return type r=flodiebold a=flodiebold

This should result in an implicit `-> ()`, not leaving out the binding.

Co-authored-by: Florian Diebold <[email protected]>

2 files changed, 28 insertions, 4 deletions

diff --git a/crates/hir_ty/src/display.rs b/crates/hir_ty/src/display.rs
index 637bbc634..44f843bf3 100644
--- a/crates/hir_ty/src/display.rs
+++ b/crates/hir_ty/src/display.rs
@@ -778,8 +778,10 @@ fn write_bounds_like_dyn_trait(
             }
             WhereClause::AliasEq(alias_eq) if is_fn_trait => {
                 is_fn_trait = false;
-                write!(f, " -> ")?;
-                alias_eq.ty.hir_fmt(f)?;
+                if !alias_eq.ty.is_unit() {
+                    write!(f, " -> ")?;
+                    alias_eq.ty.hir_fmt(f)?;
+                }
             }
             WhereClause::AliasEq(AliasEq { ty, alias }) => {
                 // in types in actual Rust, these will always come
diff --git a/crates/hir_ty/src/tests/traits.rs b/crates/hir_ty/src/tests/traits.rs
index 71905baeb..6cd8786ea 100644
--- a/crates/hir_ty/src/tests/traits.rs
+++ b/crates/hir_ty/src/tests/traits.rs
@@ -3078,7 +3078,7 @@ fn infer_fn_trait_arg() {
 
 #[test]
 fn infer_box_fn_arg() {
-    // The type mismatch is a bug
+    // The type mismatch is because we don't define Unsize and CoerceUnsized
     check_infer_with_mismatches(
         r#"
 //- /lib.rs deps:std
@@ -3138,7 +3138,7 @@ fn foo() {
             555..557 'ps': {unknown}
             559..561 '{}': ()
             568..569 'f': Box<dyn FnOnce(&Option<i32>)>
-            568..573 'f(&s)': FnOnce::Output<dyn FnOnce(&Option<i32>), (&Option<i32>,)>
+            568..573 'f(&s)': ()
             570..572 '&s': &Option<i32>
             571..572 's': Option<i32>
             549..562: expected Box<dyn FnOnce(&Option<i32>)>, got Box<|{unknown}| -> ()>
@@ -3608,3 +3608,25 @@ fn main() {
         "#]],
     )
 }
+
+#[test]
+fn fn_returning_unit() {
+    check_infer_with_mismatches(
+        r#"
+#[lang = "fn_once"]
+trait FnOnce<Args> {
+    type Output;
+}
+
+fn test<F: FnOnce()>(f: F) {
+    let _: () = f();
+}"#,
+        expect![[r#"
+            82..83 'f': F
+            88..112 '{     ...f(); }': ()
+            98..99 '_': ()
+            106..107 'f': F
+            106..109 'f()': ()
+        "#]],
+    );
+}