From c3ba03a6fb22362f3ec643efcb083bb54f4cd6c7 Mon Sep 17 00:00:00 2001
From: Alex Hoppen <ahoppen@apple.com>
Date: Fri, 28 Apr 2023 15:52:09 -0700
Subject: [PATCH] Add a conditional compilation flag to disable preconditions
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

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
---
 Sources/SwiftSyntax/Assert.swift | 33 ++++++++++++++++++++++++++++++--
 1 file changed, 31 insertions(+), 2 deletions(-)

diff --git a/Sources/SwiftSyntax/Assert.swift b/Sources/SwiftSyntax/Assert.swift
index 003947160de..a7607c65abb 100644
--- a/Sources/SwiftSyntax/Assert.swift
+++ b/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
+}