Rollup merge of #103142 - fmease:fix-103052, r=oli-obk

Make diagnostic for unsatisfied `Termination` bounds more precise

Don't blindly emit a diagnostic claiming that “*`main` has an invalid return type*” if we encounter a type that should but doesn't implement `std::process::Termination` and isn't actually the return type of the program entry `main`.

Fixes #103052.

``@rustbot`` label A-diagnostics T-compiler T-libs
r? diagnostics

5 files changed, 63 insertions, 3 deletions

diff --git a/src/test/ui/rfc-1937-termination-trait/issue-103052-1.rs b/src/test/ui/rfc-1937-termination-trait/issue-103052-1.rs
new file mode 100644
index 00000000000..a75c91cc90d
--- /dev/null
+++ b/src/test/ui/rfc-1937-termination-trait/issue-103052-1.rs
@@ -0,0 +1,11 @@
+// Check that we don't blindly emit a diagnostic claiming that "`main` has an invalid return type"
+// if we encounter a type that doesn't implement `std::process::Termination` and is not actually
+// the return type of the program entry `main`.
+
+fn receive(_: impl std::process::Termination) {}
+
+struct Something;
+
+fn main() {
+    receive(Something); //~ ERROR the trait bound `Something: Termination` is not satisfied
+}
diff --git a/src/test/ui/rfc-1937-termination-trait/issue-103052-1.stderr b/src/test/ui/rfc-1937-termination-trait/issue-103052-1.stderr
new file mode 100644
index 00000000000..409dede1a90
--- /dev/null
+++ b/src/test/ui/rfc-1937-termination-trait/issue-103052-1.stderr
@@ -0,0 +1,17 @@
+error[E0277]: the trait bound `Something: Termination` is not satisfied
+  --> $DIR/issue-103052-1.rs:10:13
+   |
+LL |     receive(Something);
+   |     ------- ^^^^^^^^^ the trait `Termination` is not implemented for `Something`
+   |     |
+   |     required by a bound introduced by this call
+   |
+note: required by a bound in `receive`
+  --> $DIR/issue-103052-1.rs:5:20
+   |
+LL | fn receive(_: impl std::process::Termination) {}
+   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `receive`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/src/test/ui/rfc-1937-termination-trait/issue-103052-2.rs b/src/test/ui/rfc-1937-termination-trait/issue-103052-2.rs
new file mode 100644
index 00000000000..fa9182b6dee
--- /dev/null
+++ b/src/test/ui/rfc-1937-termination-trait/issue-103052-2.rs
@@ -0,0 +1,18 @@
+#![feature(return_position_impl_trait_in_trait)]
+#![allow(incomplete_features)]
+
+mod child {
+    trait Main {
+        fn main() -> impl std::process::Termination;
+    }
+
+    struct Something;
+
+    impl Main for () {
+        fn main() -> Something { //~ ERROR the trait bound `Something: Termination` is not satisfied
+            Something
+        }
+    }
+}
+
+fn main() {}
diff --git a/src/test/ui/rfc-1937-termination-trait/issue-103052-2.stderr b/src/test/ui/rfc-1937-termination-trait/issue-103052-2.stderr
new file mode 100644
index 00000000000..a700c72ea68
--- /dev/null
+++ b/src/test/ui/rfc-1937-termination-trait/issue-103052-2.stderr
@@ -0,0 +1,15 @@
+error[E0277]: the trait bound `Something: Termination` is not satisfied
+  --> $DIR/issue-103052-2.rs:12:22
+   |
+LL |         fn main() -> Something {
+   |                      ^^^^^^^^^ the trait `Termination` is not implemented for `Something`
+   |
+note: required by a bound in `Main::main::{opaque#0}`
+  --> $DIR/issue-103052-2.rs:6:27
+   |
+LL |         fn main() -> impl std::process::Termination;
+   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `Main::main::{opaque#0}`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/src/test/ui/rfc-1937-termination-trait/termination-trait-test-wrong-type.stderr b/src/test/ui/rfc-1937-termination-trait/termination-trait-test-wrong-type.stderr
index 7c1390cdc64..6ee32314607 100644
--- a/src/test/ui/rfc-1937-termination-trait/termination-trait-test-wrong-type.stderr
+++ b/src/test/ui/rfc-1937-termination-trait/termination-trait-test-wrong-type.stderr
@@ -1,4 +1,4 @@
-error[E0277]: `main` has invalid return type `f32`
+error[E0277]: the trait bound `f32: Termination` is not satisfied
   --> $DIR/termination-trait-test-wrong-type.rs:6:1
    |
 LL |   #[test]
@@ -6,9 +6,8 @@ LL |   #[test]
 LL | / fn can_parse_zero_as_f32() -> Result<f32, ParseFloatError> {
 LL | |     "0".parse()
 LL | | }
-   | |_^ `main` can only return types that implement `Termination`
+   | |_^ the trait `Termination` is not implemented for `f32`
    |
-   = help: the trait `Termination` is not implemented for `f32`
    = note: required for `Result<f32, ParseFloatError>` to implement `Termination`
 note: required by a bound in `assert_test_result`
   --> $SRC_DIR/test/src/lib.rs:LL:COL