Add Minimum Supported Swift Version Policy (#316)

This commit introduces a
"Minimum Supported Swift Version" policy to help users understand
whether their code is guaranteed to work on a particular version of
Swift.

As a starting point, we are supporting Swift 6.0+ .

Starting at `6.0` allows us to begin leveraging Swift's new ownership
features in order to automatically enforce certain safety requirements.

See the `Safety` chapter in the internal book for more information about
safety requirements that, by leveraging Swift 6.0, we will be able to
automatically enforce.

README.md

@@ -164,6 +164,25 @@ to see if any peak your interest.
 
 These issues come with step-by-step instructions that should help guide you towards implementing your first patch.
 
+## Minimum Supported Swift Version (MSSV)
+
+`swift-bridge` currently guarantees that the Swift code that it generates will work on Swift `6.0` and later.
+This is known the project's "Minimum Supported Swift Version" (MSSV).
+
+`swift-bridge`'s current policy is that the minimum required Swift version can be increased at any time to
+any Swift version that is at least one month old.
+
+For instance, if Swift `9.10.11` is released on April 5, 2035, then on May 5, 2035 the `swift-bridge` project is allowed
+to begin emitting Swift code that relies on Swift `9.10.11`.
+
+We will increase our support windows when one or both of the following happen:
+
+- We are no longer waiting for Swift features that increase the safety, performance and ergonomics of the Swift code that `swift-bridge` emits.
+  - For instance, Swift recently introduced the `~Copyable` protocol, which we plan to use enforce ownership when Swift code uses opaque Rust types.
+
+- The short support window is disrupting projects that use `swift-bridge` today.
+  - Please open an issue if our MSSV policy impacts your project
+
 ## Acknowledgements
 
 - [cxx](https://github.com/dtolnay/cxx) inspired the idea of using a bridge module to describe the FFI boundary.

crates/swift-bridge-ir/src/codegen/codegen_tests.rs

@@ -27,30 +27,30 @@ use crate::test_utils::{
     assert_trimmed_generated_equals_trimmed_expected, parse_ok,
 };
 
-mod already_declared_attribute_codegen_tests;
-mod argument_label_codegen_tests;
-mod async_function_codegen_tests;
-mod boxed_fnonce_codegen_tests;
-mod built_in_tuple_codegen_tests;
-mod c_header_declaration_order_codegen_tests;
-mod conditional_compilation_codegen_tests;
-mod derive_attribute_codegen_tests;
-mod derive_struct_attribute_codegen_tests;
-mod extern_rust_function_opaque_rust_type_argument_codegen_tests;
-mod extern_rust_function_opaque_rust_type_return_codegen_tests;
-mod extern_rust_method_swift_class_placement_codegen_tests;
-mod function_attribute_codegen_tests;
-mod generic_opaque_rust_type_codegen_tests;
-mod opaque_rust_type_codegen_tests;
-mod opaque_swift_type_codegen_tests;
-mod option_codegen_tests;
-mod result_codegen_tests;
-mod return_into_attribute_codegen_tests;
-mod single_representation_type_elision_codegen_tests;
-mod string_codegen_tests;
-mod transparent_enum_codegen_tests;
-mod transparent_struct_codegen_tests;
-mod vec_codegen_tests;
+mod already_declared_attribute;
+mod argument_label;
+mod async_function;
+mod boxed_fnonce;
+mod built_in_tuple;
+mod c_header_declaration_order;
+mod conditional_compilation;
+mod derive_attribute;
+mod derive_struct_attribute;
+mod extern_rust_function_opaque_rust_type_argument;
+mod extern_rust_function_opaque_rust_type_return;
+mod extern_rust_method_swift_class_placement;
+mod function_attribute;
+mod generic_opaque_rust_type;
+mod opaque_rust_type;
+mod opaque_swift_type;
+mod option;
+mod result;
+mod return_into_attribute;
+mod single_representation_type_elision;
+mod string;
+mod transparent_enum;
+mod transparent_struct;
+mod vec;
 
 struct CodegenTest {
     bridge_module: BridgeModule,