Make sure that we suggest turbofishing the right type arg for never suggestion

compiler/rustc_hir_typeck/src/fallback.rs

@@ -621,7 +621,11 @@ impl<'tcx> AnnotateUnitFallbackVisitor<'_, 'tcx> {
                 .iter()
                 .filter(|arg| matches!(arg.unpack(), ty::GenericArgKind::Type(_)))
                 .count();
-            for (idx, arg) in args.iter().enumerate() {
+            for (idx, arg) in args
+                .iter()
+                .filter(|arg| matches!(arg.unpack(), ty::GenericArgKind::Type(_)))
+                .enumerate()
+            {
                 if let Some(ty) = arg.as_type()
                     && let Some(vid) = self.fcx.root_vid(ty)
                     && self.reachable_vids.contains(&vid)

tests/ui/editions/never-type-fallback-breaking.e2021.fixed

@@ -0,0 +1,52 @@
+//@ revisions: e2021 e2024
+//
+//@[e2021] edition: 2021
+//@[e2024] edition: 2024
+//@[e2024] compile-flags: -Zunstable-options
+//
+//@[e2021] run-pass
+//@[e2021] run-rustfix
+//@[e2024] check-fail
+
+fn main() {
+    m();
+    q();
+    let _ = meow();
+}
+
+fn m() {
+    //[e2021]~^ this function depends on never type fallback being `()`
+    //[e2021]~| this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+    let x: () = match true {
+        true => Default::default(),
+        //[e2024]~^ error: the trait bound `!: Default` is not satisfied
+        false => panic!("..."),
+    };
+
+    dbg!(x);
+}
+
+fn q() -> Option<()> {
+    //[e2021]~^ this function depends on never type fallback being `()`
+    //[e2021]~| this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+    fn deserialize<T: Default>() -> Option<T> {
+        Some(T::default())
+    }
+
+    deserialize::<()>()?;
+    //[e2024]~^ error: the trait bound `!: Default` is not satisfied
+
+    None
+}
+
+// Make sure we turbofish the right argument
+fn help<'a: 'a, T: Into<()>, U>(_: U) -> Result<T, ()> {
+    Err(())
+}
+fn meow() -> Result<(), ()> {
+    //[e2021]~^ this function depends on never type fallback being `()`
+    //[e2021]~| this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+    help::<(), _>(1)?;
+    //[e2024]~^ error: the trait bound `(): From<!>` is not satisfied
+    Ok(())
+}

tests/ui/editions/never-type-fallback-breaking.e2021.stderr

@@ -1,5 +1,5 @@
 warning: this function depends on never type fallback being `()`
-  --> $DIR/never-type-fallback-breaking.rs:15:1
+  --> $DIR/never-type-fallback-breaking.rs:17:1
    |
 LL | fn m() {
    | ^^^^^^
@@ -8,7 +8,7 @@ LL | fn m() {
    = note: for more information, see issue #123748 <https://github.com/rust-lang/rust/issues/123748>
    = help: specify the types explicitly
 note: in edition 2024, the requirement `!: Default` will fail
-  --> $DIR/never-type-fallback-breaking.rs:19:17
+  --> $DIR/never-type-fallback-breaking.rs:21:17
    |
 LL |         true => Default::default(),
    |                 ^^^^^^^^^^^^^^^^^^
@@ -19,7 +19,7 @@ LL |     let x: () = match true {
    |          ++++
 
 warning: this function depends on never type fallback being `()`
-  --> $DIR/never-type-fallback-breaking.rs:27:1
+  --> $DIR/never-type-fallback-breaking.rs:29:1
    |
 LL | fn q() -> Option<()> {
    | ^^^^^^^^^^^^^^^^^^^^
@@ -28,7 +28,7 @@ LL | fn q() -> Option<()> {
    = note: for more information, see issue #123748 <https://github.com/rust-lang/rust/issues/123748>
    = help: specify the types explicitly
 note: in edition 2024, the requirement `!: Default` will fail
-  --> $DIR/never-type-fallback-breaking.rs:34:5
+  --> $DIR/never-type-fallback-breaking.rs:36:5
    |
 LL |     deserialize()?;
    |     ^^^^^^^^^^^^^
@@ -37,5 +37,24 @@ help: use `()` annotations to avoid fallback changes
 LL |     deserialize::<()>()?;
    |                ++++++
 
-warning: 2 warnings emitted
+warning: this function depends on never type fallback being `()`
+  --> $DIR/never-type-fallback-breaking.rs:46:1
+   |
+LL | fn meow() -> Result<(), ()> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #123748 <https://github.com/rust-lang/rust/issues/123748>
+   = help: specify the types explicitly
+note: in edition 2024, the requirement `(): From<!>` will fail
+  --> $DIR/never-type-fallback-breaking.rs:49:5
+   |
+LL |     help(1)?;
+   |     ^^^^^^^
+help: use `()` annotations to avoid fallback changes
+   |
+LL |     help::<(), _>(1)?;
+   |         +++++++++
+
+warning: 3 warnings emitted
 

tests/ui/editions/never-type-fallback-breaking.e2024.stderr

@@ -1,5 +1,5 @@
 error[E0277]: the trait bound `!: Default` is not satisfied
-  --> $DIR/never-type-fallback-breaking.rs:19:17
+  --> $DIR/never-type-fallback-breaking.rs:21:17
    |
 LL |         true => Default::default(),
    |                 ^^^^^^^^^^^^^^^^^^ the trait `Default` is not implemented for `!`
@@ -8,19 +8,42 @@ LL |         true => Default::default(),
    = help: did you intend to use the type `()` here instead?
 
 error[E0277]: the trait bound `!: Default` is not satisfied
-  --> $DIR/never-type-fallback-breaking.rs:34:5
+  --> $DIR/never-type-fallback-breaking.rs:36:5
    |
 LL |     deserialize()?;
    |     ^^^^^^^^^^^^^ the trait `Default` is not implemented for `!`
    |
    = note: this error might have been caused by changes to Rust's type-inference algorithm (see issue #48950 <https://github.com/rust-lang/rust/issues/48950> for more information)
    = help: did you intend to use the type `()` here instead?
 note: required by a bound in `deserialize`
-  --> $DIR/never-type-fallback-breaking.rs:30:23
+  --> $DIR/never-type-fallback-breaking.rs:32:23
    |
 LL |     fn deserialize<T: Default>() -> Option<T> {
    |                       ^^^^^^^ required by this bound in `deserialize`
 
-error: aborting due to 2 previous errors
+error[E0277]: the trait bound `(): From<!>` is not satisfied
+  --> $DIR/never-type-fallback-breaking.rs:49:5
+   |
+LL |     help(1)?;
+   |     ^^^^^^^ the trait `From<!>` is not implemented for `()`
+   |
+   = help: the following other types implement trait `From<T>`:
+             `(T, T)` implements `From<[T; 2]>`
+             `(T, T, T)` implements `From<[T; 3]>`
+             `(T, T, T, T)` implements `From<[T; 4]>`
+             `(T, T, T, T, T)` implements `From<[T; 5]>`
+             `(T, T, T, T, T, T)` implements `From<[T; 6]>`
+             `(T, T, T, T, T, T, T)` implements `From<[T; 7]>`
+             `(T, T, T, T, T, T, T, T)` implements `From<[T; 8]>`
+             `(T, T, T, T, T, T, T, T, T)` implements `From<[T; 9]>`
+           and 4 others
+   = note: required for `!` to implement `Into<()>`
+note: required by a bound in `help`
+  --> $DIR/never-type-fallback-breaking.rs:43:20
+   |
+LL | fn help<'a: 'a, T: Into<()>, U>(_: U) -> Result<T, ()> {
+   |                    ^^^^^^^^ required by this bound in `help`
+
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0277`.

tests/ui/editions/never-type-fallback-breaking.rs

@@ -5,11 +5,13 @@
 //@[e2024] compile-flags: -Zunstable-options
 //
 //@[e2021] run-pass
+//@[e2021] run-rustfix
 //@[e2024] check-fail
 
 fn main() {
     m();
     q();
+    let _ = meow();
 }
 
 fn m() {
@@ -36,3 +38,15 @@ fn q() -> Option<()> {
 
     None
 }
+
+// Make sure we turbofish the right argument
+fn help<'a: 'a, T: Into<()>, U>(_: U) -> Result<T, ()> {
+    Err(())
+}
+fn meow() -> Result<(), ()> {
+    //[e2021]~^ this function depends on never type fallback being `()`
+    //[e2021]~| this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+    help(1)?;
+    //[e2024]~^ error: the trait bound `(): From<!>` is not satisfied
+    Ok(())
+}