Make RustString an extern type to avoid improper_ctypes warnings

compiler/rustc_llvm/llvm-wrapper/LLVMWrapper.h

@@ -104,8 +104,9 @@ typedef struct OpaqueRustString *RustStringRef;
 typedef struct LLVMOpaqueTwine *LLVMTwineRef;
 typedef struct LLVMOpaqueSMDiagnostic *LLVMSMDiagnosticRef;
 
-extern "C" void LLVMRustStringWriteImpl(RustStringRef Str, const char *Ptr,
-                                        size_t Size);
+extern "C" void LLVMRustStringWriteImpl(RustStringRef buf,
+                                        const char *slice_ptr,
+                                        size_t slice_len);
 
 class RawRustStringOstream : public llvm::raw_ostream {
   RustStringRef Str;

compiler/rustc_llvm/src/lib.rs

@@ -2,42 +2,75 @@
 #![allow(internal_features)]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
+#![feature(extern_types)]
 #![feature(rustdoc_internals)]
 #![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
-// NOTE: This crate only exists to allow linking on mingw targets.
-
 use std::cell::RefCell;
-use std::slice;
+use std::{ptr, slice};
 
-use libc::{c_char, size_t};
+use libc::size_t;
 
-#[repr(C)]
-pub struct RustString {
-    pub bytes: RefCell<Vec<u8>>,
+unsafe extern "C" {
+    /// Opaque type that allows C++ code to write bytes to a Rust-side buffer,
+    /// in conjunction with `RawRustStringOstream`. Use this as `&RustString`
+    /// (Rust) and `RustStringRef` (C++) in FFI signatures.
+    pub type RustString;
 }
 
 impl RustString {
-    pub fn len(&self) -> usize {
-        self.bytes.borrow().len()
-    }
-
-    pub fn is_empty(&self) -> bool {
-        self.bytes.borrow().is_empty()
+    pub fn build_byte_buffer(closure: impl FnOnce(&Self)) -> Vec<u8> {
+        let buf = RustStringInner::default();
+        closure(buf.as_opaque());
+        buf.into_inner()
     }
 }
 
-/// Appending to a Rust string -- used by RawRustStringOstream.
+/// Underlying implementation of [`RustString`].
+///
+/// Having two separate types makes it possible to use the opaque [`RustString`]
+/// in FFI signatures without `improper_ctypes` warnings. This is a workaround
+/// for the fact that there is no way to opt out of `improper_ctypes` when
+/// _declaring_ a type (as opposed to using that type).
+#[derive(Default)]
+struct RustStringInner {
+    bytes: RefCell<Vec<u8>>,
+}
+
+impl RustStringInner {
+    fn as_opaque(&self) -> &RustString {
+        let ptr: *const RustStringInner = ptr::from_ref(self);
+        // We can't use `ptr::cast` here because extern types are `!Sized`.
+        let ptr = ptr as *const RustString;
+        unsafe { &*ptr }
+    }
+
+    fn from_opaque(opaque: &RustString) -> &Self {
+        // SAFETY: A valid `&RustString` must have been created via `as_opaque`.
+        let ptr: *const RustString = ptr::from_ref(opaque);
+        let ptr: *const RustStringInner = ptr.cast();
+        unsafe { &*ptr }
+    }
+
+    fn into_inner(self) -> Vec<u8> {
+        self.bytes.into_inner()
+    }
+}
+
+/// Appends the contents of a byte slice to a [`RustString`].
+///
+/// This function is implemented in `rustc_llvm` so that the C++ code in this
+/// crate can link to it directly, without an implied link-time dependency on
+/// `rustc_codegen_llvm`.
 #[unsafe(no_mangle)]
 pub unsafe extern "C" fn LLVMRustStringWriteImpl(
-    sr: &RustString,
-    ptr: *const c_char,
-    size: size_t,
+    buf: &RustString,
+    slice_ptr: *const u8, // Same ABI as `*const c_char`
+    slice_len: size_t,
 ) {
-    let slice = unsafe { slice::from_raw_parts(ptr as *const u8, size) };
-
-    sr.bytes.borrow_mut().extend_from_slice(slice);
+    let slice = unsafe { slice::from_raw_parts(slice_ptr, slice_len) };
+    RustStringInner::from_opaque(buf).bytes.borrow_mut().extend_from_slice(slice);
 }
 
 /// Initialize targets enabled by the build script via `cfg(llvm_component = "...")`.