Fix cross-DLL panics under MSVC

Author: Amanieu

src/doc/unstable-book/src/language-features/lang-items.md

@@ -248,7 +248,6 @@ the source code.
   - `eh_personality`: `libpanic_unwind/gcc.rs` (GNU)
   - `eh_personality`: `libpanic_unwind/seh.rs` (SEH)
   - `eh_unwind_resume`: `libpanic_unwind/gcc.rs` (GCC)
-  - `eh_catch_typeinfo`: `libpanic_unwind/seh.rs` (SEH)
   - `eh_catch_typeinfo`: `libpanic_unwind/emcc.rs` (EMCC)
   - `panic`: `libcore/panicking.rs`
   - `panic_bounds_check`: `libcore/panicking.rs`

src/libpanic_unwind/seh.rs

@@ -167,6 +167,9 @@ pub struct _TypeDescriptor {
 
 // Note that we intentionally ignore name mangling rules here: we don't want C++
 // to be able to catch Rust panics by simply declaring a `struct rust_panic`.
+//
+// When modifying, make sure that the type name string exactly matches
+// the one used in src/librustc_codegen_llvm/intrinsic.rs.
 const TYPE_NAME: [u8; 11] = *b"rust_panic\0";
 
 static mut THROW_INFO: _ThrowInfo = _ThrowInfo {
@@ -199,12 +202,12 @@ extern "C" {
     static TYPE_INFO_VTABLE: *const u8;
 }
 
-// We use #[lang = "eh_catch_typeinfo"] here as this is the type descriptor which
-// we'll use in LLVM's `catchpad` instruction which ends up also being passed as
-// an argument to the C++ personality function.
+// This type descriptor is only used when throwing an exception. The catch part
+// is handled by the try intrinsic, which generates its own TypeDescriptor.
 //
-// Again, I'm not entirely sure what this is describing, it just seems to work.
-#[cfg_attr(not(test), lang = "eh_catch_typeinfo")]
+// This is fine since the MSVC runtime uses string comparison on the type name
+// to match TypeDescriptors rather than pointer equality.
+#[cfg_attr(bootstrap, lang = "eh_catch_typeinfo")]
 static mut TYPE_DESCRIPTOR: _TypeDescriptor = _TypeDescriptor {
     pVFTable: unsafe { &TYPE_INFO_VTABLE } as *const _ as *const _,
     spare: core::ptr::null_mut(),

src/librustc_codegen_llvm/intrinsic.rs

@@ -954,19 +954,39 @@ fn codegen_msvc_try(
         let cs = catchswitch.catch_switch(None, None, 1);
         catchswitch.add_handler(cs, catchpad.llbb());
 
+        // We can't use the TypeDescriptor defined in libpanic_unwind because it
+        // might be in another DLL and the SEH encoding only supports specifying
+        // a TypeDescriptor from the current module.
+        //
+        // However this isn't an issue since the MSVC runtime uses string
+        // comparison on the type name to match TypeDescriptors rather than
+        // pointer equality.
+        //
+        // So instead we generate a new TypeDescriptor in each module that uses
+        // `try` and let the linker merge duplicate definitions in the same
+        // module.
+        //
+        // When modifying, make sure that the type_name string exactly matches
+        // the one used in src/libpanic_unwind/seh.rs.
+        let type_info_vtable = bx.declare_global("??_7type_info@@6B@", bx.type_i8p());
+        let type_name = bx.const_bytes(b"rust_panic\0");
+        let type_info =
+            bx.const_struct(&[type_info_vtable, bx.const_null(bx.type_i8p()), type_name], false);
+        let tydesc = bx.declare_global("__rust_panic_type_info", bx.val_ty(type_info));
+        unsafe {
+            llvm::LLVMRustSetLinkage(tydesc, llvm::Linkage::LinkOnceODRLinkage);
+            llvm::SetUniqueComdat(bx.llmod, tydesc);
+            llvm::LLVMSetInitializer(tydesc, type_info);
+        }
+
         // The flag value of 8 indicates that we are catching the exception by
         // reference instead of by value. We can't use catch by value because
         // that requires copying the exception object, which we don't support
         // since our exception object effectively contains a Box.
         //
         // Source: MicrosoftCXXABI::getAddrOfCXXCatchHandlerType in clang
         let flags = bx.const_i32(8);
-        let tydesc = match bx.tcx().lang_items().eh_catch_typeinfo() {
-            Some(did) => bx.get_static(did),
-            None => bug!("eh_catch_typeinfo not defined, but needed for SEH unwinding"),
-        };
         let funclet = catchpad.catch_pad(cs, &[tydesc, flags, slot]);
-
         let i64_align = bx.tcx().data_layout.i64_align.abi;
         let payload_ptr = catchpad.load(slot, ptr_align);
         let payload = catchpad.load(payload_ptr, i64_align);