Import _CSwiftSyntax using @_implementationOnly.

Sources/SwiftSyntax/AtomicCounter.swift

@@ -10,36 +10,12 @@
 //
 //===----------------------------------------------------------------------===//
 
-// This is using the C function from '_CSwiftSyntax/src/atomic-counter.c' by
-// dynamically loading it using `dlsym`. The reason this mechanism is used
-// instead of importing a header is so that we preserve the property that
-// SwiftSyntax is a self-contained Swift module.
-// (also see '_CSwiftSyntax/include/README.md')
-
-#if os(Linux)
-import Glibc
-#else
-import Darwin.C
-#endif
-
-#if os(macOS) || os(iOS) || os(watchOS) || os(tvOS)
-  fileprivate let RTLD_DEFAULT = UnsafeMutableRawPointer(bitPattern: -2)
-#elseif os(Linux)
-  fileprivate let RTLD_DEFAULT = UnsafeMutableRawPointer(bitPattern: 0)
-#endif
-
-fileprivate typealias get_unique_counter_ty = (@convention(c) ()->UInt64)
-fileprivate let get_unique_counter_str = "cswiftsyntax_get_unique_counter"
+@_implementationOnly import _CSwiftSyntax
 
 /// Provides API to get an atomically increasing counter value.
 struct AtomicCounter {
   /// Get an atomically increasing counter value.
   static func next() -> UInt64 {
-    return AtomicCounter.cswiftsyntax_get_unique_counter()
+    return cswiftsyntax_get_unique_counter()
   }
-
-  static fileprivate let cswiftsyntax_get_unique_counter: get_unique_counter_ty = { ()->get_unique_counter_ty in
-    let ptr = dlsym(RTLD_DEFAULT, get_unique_counter_str)!
-    return unsafeBitCast(ptr, to: get_unique_counter_ty.self)
-  }()
 }

Sources/_CSwiftSyntax/README.md

@@ -0,0 +1,9 @@
+This module is an implementation detail of SwiftSyntax and does not export
+any public API. As such, any uses of it inside SwiftSyntax should import it
+with the `@_implementationOnly` attribute (available since Swift 5.1).
+
+This preserves the property that SwiftSyntax is a self-contained Swift
+module, meaning that when distributing it, we do not also have to bundle
+these private headers or use any dynamic symbol loading tricks that require
+additional linker flags to be passed on some platforms (see
+[SR-11293](https://bugs.swift.org/browse/SR-11293), for example).

Sources/_CSwiftSyntax/include/atomic-counter.h

@@ -0,0 +1,3 @@
+#include <stdint.h>
+
+uint64_t cswiftsyntax_get_unique_counter(void);

Sources/_CSwiftSyntax/src/atomic-counter.c

@@ -1,4 +1,4 @@
-#include <stdint.h>
+#include "atomic-counter.h"
 
 uint64_t cswiftsyntax_get_unique_counter(void) {
   static _Atomic uint64_t counter = 0;