[Support] Add a basic C API for llvm::Error.

Summary:
The C-API supports consuming errors, converting an error to a string error
message, and querying an error's type. Other LLVM C APIs that wish to use
llvm::Error can supply error-type-id checkers and custom
error-to-structured-type converters for any custom errors they provide.

Reviewers: bogner, zturner, labath, dblaikie

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D50716

llvm-svn: 339802

2 files changed, 81 insertions, 1 deletions

diff --git a/llvm/include/llvm-c/Error.h b/llvm/include/llvm-c/Error.h
new file mode 100644
index 00000000000..00714ec950e
--- /dev/null
+++ b/llvm/include/llvm-c/Error.h
@@ -0,0 +1,65 @@
+/*===------- llvm-c/Error.h - llvm::Error class C Interface -------*- C -*-===*\
+|*                                                                            *|
+|*                     The LLVM Compiler Infrastructure                       *|
+|*                                                                            *|
+|* This file is distributed under the University of Illinois Open Source      *|
+|* License. See LICENSE.TXT for details.                                      *|
+|*                                                                            *|
+|*===----------------------------------------------------------------------===*|
+|*                                                                            *|
+|* This file defines the C interface to LLVM's Error class.                   *|
+|*                                                                            *|
+\*===----------------------------------------------------------------------===*/
+
+#ifndef LLVM_C_ERROR_H
+#define LLVM_C_ERROR_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * Opaque reference to an error instance. Null serves as the 'success' value.
+ */
+typedef struct LLVMOpaqueError *LLVMErrorRef;
+
+/**
+ * Error type identifier.
+ */
+typedef const void *LLVMErrorTypeId;
+
+/**
+ * Returns the type id for the given error instance, which must be a failure
+ * value (i.e. non-null).
+ */
+LLVMErrorTypeId LLVMGetErrorTypeId(LLVMErrorRef Err);
+
+/**
+ * Dispose of the given error without handling it. This operation consumes the
+ * error, and the given LLVMErrorRef value is not usable once this call returns.
+ * Note: This method *only* needs to be called if the error is not being passed
+ * to some other consuming operation, e.g. LLVMGetErrorMessage.
+ */
+void LLVMConsumeError(LLVMErrorRef Err);
+
+/**
+ * Returns the given string's error message. This operation consumes the error,
+ * and the given LLVMErrorRef value is not usable once this call returns.
+ */
+char *LLVMGetErrorMessage(LLVMErrorRef Err);
+
+/**
+ * Dispose of the given error message.
+ */
+void LLVMDisposeErrorMessage(char *ErrMsg);
+
+/**
+ * Returns the type id for llvm StringError.
+ */
+LLVMErrorTypeId LLVMGetStringErrorTypeId();
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif
diff --git a/llvm/include/llvm/Support/Error.h b/llvm/include/llvm/Support/Error.h
index 8015cab45a0..b82663d5d87 100644
--- a/llvm/include/llvm/Support/Error.h
+++ b/llvm/include/llvm/Support/Error.h
@@ -14,8 +14,9 @@
 #ifndef LLVM_SUPPORT_ERROR_H
 #define LLVM_SUPPORT_ERROR_H
 
-#include "llvm/ADT/SmallVector.h"
+#include "llvm-c/Error.h"
 #include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/Twine.h"
 #include "llvm/Config/abi-breaking.h"
@@ -167,6 +168,9 @@ class LLVM_NODISCARD Error {
   // error.
   template <typename T> friend class Expected;
 
+  // wrap needs to be able to steal the payload.
+  friend LLVMErrorRef wrap(Error);
+
 protected:
   /// Create a success value. Prefer using 'Error::success()' for readability
   Error() {
@@ -1183,6 +1187,17 @@ private:
   std::function<int(const Error &)> GetExitCode;
 };
 
+/// Conversion from Error to LLVMErrorRef for C error bindings.
+inline LLVMErrorRef wrap(Error Err) {
+  return reinterpret_cast<LLVMErrorRef>(Err.takePayload().release());
+}
+
+/// Conversion from LLVMErrorRef to Error for C error bindings.
+inline Error unwrap(LLVMErrorRef ErrRef) {
+  return Error(std::unique_ptr<ErrorInfoBase>(
+      reinterpret_cast<ErrorInfoBase *>(ErrRef)));
+}
+
 } // end namespace llvm
 
 #endif // LLVM_SUPPORT_ERROR_H