Switch C type printing to prefer east const/volatile when possible

Consistency be damned, it's what looks nice. Clang does it too.

Author: DougGregor

Sources/JExtractSwift/CTypes/CType.swift

@@ -132,33 +132,33 @@ extension CType: CustomStringConvertible {
       result += "*"
 
     case .qualified(const: let isConst, volatile: let isVolatile, type: let underlying):
-      if isConst || isVolatile {
-        hasEmptyPlaceholder = false
+      func printQualifier(_ qualifier: String, if condition: Bool) {
+        if condition {
+          result += qualifier
+          hasEmptyPlaceholder = false
+
+          spaceBeforePlaceHolder(
+            hasEmptyPlaceholder: hasEmptyPlaceholder,
+            result: &result
+          )
+        }
       }
 
-      underlying.printBefore(hasEmptyPlaceholder: &hasEmptyPlaceholder, result: &result)
+      let canPrefixQualifiers = underlying.canPrefixQualifiers
+      if canPrefixQualifiers {
+        printQualifier("const", if: isConst)
+        printQualifier("volatile", if: isVolatile)
+      }
 
-      // FIXME: "east const" is easier to print correctly, so do that. We could
-      // follow Clang and decide when it's correct to print "west const" by
-      // splitting the qualifiers before we get here.
-      if isConst {
-        result += "const"
+      if (isConst || isVolatile) && !canPrefixQualifiers {
         hasEmptyPlaceholder = false
-
-        spaceBeforePlaceHolder(
-          hasEmptyPlaceholder: hasEmptyPlaceholder,
-          result: &result
-        )
-
       }
-      if isVolatile {
-        result += "volatile"
-        hasEmptyPlaceholder = false
 
-        spaceBeforePlaceHolder(
-          hasEmptyPlaceholder: hasEmptyPlaceholder,
-          result: &result
-        )
+      underlying.printBefore(hasEmptyPlaceholder: &hasEmptyPlaceholder, result: &result)
+
+      if !canPrefixQualifiers {
+        printQualifier("const", if: isConst)
+        printQualifier("volatile", if: isVolatile)
       }
 
     case .tag(let tag):
@@ -265,6 +265,7 @@ extension CType: CustomStringConvertible {
     print(placeholder: nil)
   }
 
+  /// Print a space before the placeholder in a declarator.
   private func spaceBeforePlaceHolder(
     hasEmptyPlaceholder: Bool,
     result: inout String
@@ -273,6 +274,16 @@ extension CType: CustomStringConvertible {
       result += " "
     }
   }
+
+  /// Determine whether qualifiers can be printed before the given type
+  /// (`const int`) vs. having to be afterward to maintain semantics.
+  var canPrefixQualifiers: Bool {
+    switch self {
+    case .floating, .integral, .tag, .void: true
+    case .function, .pointer: false
+    case .qualified(const: _, volatile: _, type: let type): type.canPrefixQualifiers
+    }
+  }
 }
 
 extension CType {

Tests/JExtractSwiftTests/CTypeTests.swift

@@ -58,8 +58,8 @@ struct CTypeTests {
       ],
       isVariadic: true
     )
-    #expect(snprintf.description == "int32_t snprintf(int8_t *str, size_t size, int8_t const *format, ...)")
-    #expect(snprintf.functionType.description == "int32_t (int8_t *, size_t, int8_t const *, ...)")
+    #expect(snprintf.description == "int32_t snprintf(int8_t *str, size_t size, const int8_t *format, ...)")
+    #expect(snprintf.functionType.description == "int32_t (int8_t *, size_t, const int8_t *, ...)")
 
     let rand = CFunction(
       resultType: .integral(.signed(bits: 32)),
@@ -91,5 +91,16 @@ struct CTypeTests {
       isVariadic: false
     )
     #expect(doit.description == "void doit(void (*body)(_Bool))")
+
+    let ptrptr = CType.pointer(
+      .qualified(
+        const: true,
+        volatile: false,
+        type: .pointer(
+          .qualified(const: false, volatile: true, type: .void)
+        )
+      )
+    ) 
+    #expect(ptrptr.description == "volatile void *const *")
   }
 }

Tests/JExtractSwiftTests/FunctionLoweringTests.swift

@@ -30,7 +30,7 @@ final class FunctionLoweringTests {
         f(x: x, y: y, z: UnsafeBufferPointer<Bool>(start: z_pointer.assumingMemoryBound(to: Bool.self), count: z_count))
       }
       """,
-      expectedCFunction: "void c_f(ptrdiff_t x, float y, void const *z_pointer, ptrdiff_t z_count)"
+      expectedCFunction: "void c_f(ptrdiff_t x, float y, const void *z_pointer, ptrdiff_t z_count)"
     )
   }
 
@@ -45,7 +45,7 @@ final class FunctionLoweringTests {
         return f(t: (t_0, (t_1_0, t_1_1)), z: z_pointer.assumingMemoryBound(to: Int.self))
       }
       """,
-      expectedCFunction: "ptrdiff_t c_f(ptrdiff_t t_0, float t_1_0, double t_1_1, void const *z_pointer)"
+      expectedCFunction: "ptrdiff_t c_f(ptrdiff_t t_0, float t_1_0, double t_1_1, const void *z_pointer)"
     )
   }
 
@@ -82,7 +82,7 @@ final class FunctionLoweringTests {
         _result.assumingMemoryBound(to: Point.self).initialize(to: self.assumingMemoryBound(to: Point.self).pointee.shifted(by: (delta_0, delta_1)))
       }
       """,
-      expectedCFunction: "void c_shifted(double delta_0, double delta_1, void const *self, void *_result)"
+      expectedCFunction: "void c_shifted(double delta_0, double delta_1, const void *self, void *_result)"
     )
   }
 
@@ -120,7 +120,7 @@ final class FunctionLoweringTests {
         unsafeBitCast(self, to: Point.self).shift(by: (delta_0, delta_1))
       }
       """,
-      expectedCFunction: "void c_shift(double delta_0, double delta_1, void const *self)"
+      expectedCFunction: "void c_shift(double delta_0, double delta_1, const void *self)"
     )
   }
 
@@ -155,7 +155,7 @@ final class FunctionLoweringTests {
         return unsafeBitCast(Person.randomPerson(seed: seed), to: UnsafeRawPointer.self)
       }
       """,
-      expectedCFunction: "void const *c_randomPerson(double seed)"
+      expectedCFunction: "const void *c_randomPerson(double seed)"
     )
   }
 
@@ -190,7 +190,7 @@ final class FunctionLoweringTests {
         return unsafeBitCast(Person(seed: seed), to: UnsafeRawPointer.self)
       }
       """,
-      expectedCFunction: "void const *c_init(double seed)"
+      expectedCFunction: "const void *c_init(double seed)"
     )
   }
 
@@ -205,7 +205,7 @@ final class FunctionLoweringTests {
         f(t: unsafeBitCast(t, to: Int.self))
       }
       """,
-      expectedCFunction: "void c_f(void const *t)"
+      expectedCFunction: "void c_f(const void *t)"
     )
 
     try assertLoweredFunction("""
@@ -217,7 +217,7 @@ final class FunctionLoweringTests {
         return unsafeBitCast(f(), to: UnsafeRawPointer.self)
       }
       """,
-      expectedCFunction: "void const *c_f(void)"
+      expectedCFunction: "const void *c_f(void)"
     )
   }
 
@@ -236,7 +236,7 @@ final class FunctionLoweringTests {
         return unsafeBitCast(unsafeBitCast(self, to: Point.self).shifted(by: (delta_0, delta_1)), to: UnsafeRawPointer.self)
       }
       """,
-      expectedCFunction: "void const *c_shifted(double delta_0, double delta_1, void const *self)"
+      expectedCFunction: "const void *c_shifted(double delta_0, double delta_1, const void *self)"
     )
   }
 
@@ -254,7 +254,7 @@ final class FunctionLoweringTests {
         return UnsafeRawPointer(getPointer())
       }
       """,
-      expectedCFunction: "void const *c_getPointer(void)"
+      expectedCFunction: "const void *c_getPointer(void)"
     )
   }