Add a conditional compilation flag to disable preconditions

In some situations, like an IDE that runs SwiftSyntax in process, crashing is the worst possible outcome and it’s preferrable to continue running, possibly accepting a misparse. These use cases can disable preconditions in SwiftSyntax using a conditional compilation flag

rdar://108666839

Author: ahoppen

Sources/SwiftSyntax/Assert.swift

@@ -29,7 +29,12 @@
 /// Use this instead of `precondition` in places where the assertion has a
 /// non-trivial cost but provides little value in release builds.
 @_transparent
-public func assert(_ condition: @autoclosure () -> Bool, _ message: @autoclosure () -> String = String(), file: StaticString = #file, line: UInt = #line) {
+public func assert(
+  _ condition: @autoclosure () -> Bool,
+  _ message: @autoclosure () -> String = String(),
+  file: StaticString = #file,
+  line: UInt = #line
+) {
   #if SWIFTSYNTAX_ENABLE_ASSERTIONS
   if !_fastPath(condition()) {
     fatalError(message(), file: file, line: line)
@@ -44,10 +49,34 @@ public func assert(_ condition: @autoclosure () -> Bool, _ message: @autoclosure
 /// requested by setting the `SWIFTSYNTAX_ENABLE_ASSERTIONS` conditional
 /// compilation flag.
 @_transparent
-public func assertionFailure(_ message: @autoclosure () -> String = String(), file: StaticString = #file, line: UInt = #line) {
+public func assertionFailure(
+  _ message: @autoclosure () -> String = String(),
+  file: StaticString = #file,
+  line: UInt = #line
+) {
   #if SWIFTSYNTAX_ENABLE_ASSERTIONS
   fatalError(message(), file: file, line: line)
   #else
   Swift.assertionFailure(message(), file: file, line: line)
   #endif
 }
+
+// MARK: - Precondition
+
+/// Override Swift’s `precondition` with slightly changed semantics.
+/// In release builds, it also emits the error message upon failure, like `fatalError`.
+/// It can also be disabled by setting the `SWIFTSYNTAX_DISABLE_PRECONDITION` conditional compilation flag.
+/// Note that `SWIFTSYNTAX_DISABLE_PRECONDITION` does not disable `preconditionFailure`.
+@_transparent
+public func precondition(
+  _ condition: @autoclosure () -> Bool,
+  _ message: @autoclosure () -> String = String(),
+  file: StaticString = #file,
+  line: UInt = #line
+) {
+  #if !SWIFTSYNTAX_DISABLE_PRECONDITIONS
+  if !_fastPath(condition()) {
+    fatalError(message(), file: file, line: line)
+  }
+  #endif
+}