Normalize c/Roc callconv parameters when C returns by pointer

Closes #2413

compiler/gen_llvm/src/llvm/build.rs

@@ -3600,11 +3600,14 @@ fn expose_function_to_host_help_c_abi_v2<'a, 'ctx, 'env>(
 
     let param_types = Vec::from_iter_in(roc_function.get_type().get_param_types(), env.arena);
 
-    // drop the "return pointer" if it exists on the roc function
+    let (params, param_types) = match (&roc_return, &cc_return) {
+        // Drop the "return pointer" if it exists on the roc function
         // and the c function does not return via pointer
-    let param_types = match (&roc_return, &cc_return) {
-        (RocReturn::ByPointer, CCReturn::Return) => &param_types[1..],
-        _ => &param_types,
+        (RocReturn::ByPointer, CCReturn::Return) => (&params[..], &param_types[1..]),
+        // Drop the return pointer the other way, if the C function returns by pointer but Roc
+        // doesn't
+        (RocReturn::Return, CCReturn::ByPointer) => (&params[1..], &param_types[..]),
+        _ => (&params[..], &param_types[..]),
     };
 
     debug_assert_eq!(params.len(), param_types.len());

compiler/load/tests/test_load.rs

@@ -648,4 +648,42 @@ mod test_load {
             Ok(_) => unreachable!("we expect failure here"),
         }
     }
+
+    #[test]
+    // See https://github.com/rtfeldman/roc/issues/2413
+    fn platform_exposes_main_return_by_pointer_issue() {
+        let modules = vec![
+            (
+                "platform/Package-Config.roc",
+                indoc!(
+                    r#"
+                    platform "examples/hello-world"
+                        requires {} { main : { content: Str, other: Str } }
+                        exposes []
+                        packages {}
+                        imports []
+                        provides [ mainForHost ]
+
+                    mainForHost : { content: Str, other: Str }
+                    mainForHost = main
+                    "#
+                ),
+            ),
+            (
+                "Main",
+                indoc!(
+                    r#"
+                    app "hello-world"
+                        packages { pf: "platform" }
+                        imports []
+                        provides [ main ] to pf
+
+                    main = { content: "Hello, World!\n", other: "" }
+                    "#
+                ),
+            ),
+        ];
+
+        assert!(multiple_modules(modules).is_ok());
+    }
 }