Simplify the thunk names we generate for test functions. (#750)

Parameterized test functions with the same name but different argument
types currently get the same generated names from swift-syntax, so we do
some additional decorating to ensure uniqueness. This results in very
long symbol names that `swift-demangle` has trouble with.

This PR changes the decorating formula. Previously, we would include a
copy of the test function's signature (stripped of whitespace, Unicode,
and non-identifier-friendly ASCII) and, if the identifier contained
non-ASCII characters, the CRC32 of the identifier for further
uniqueness. This change drops the copy of the identifier name and always
includes the CRC32. Collisions are only possible for fully-qualified
test function names that are _identical_, and I naïvely judge the risk
of those collisions to be sufficiently low that we can make this change.

For `ZipTests.allElementsEqual😀(i:j:)` we currently generate a thunk
named:

```
$s12TestingTests03ZipB0V0022allElementsEqual_oxFJo4TestfMp_43funcallElementsEqual__i_Int_j_Int__5bcb7b35fMu_
```

This change would change it to:

```
$s12TestingTests03ZipB0V0022allElementsEqual_oxFJo4TestfMp_9Z5bcb7b35fMu_
```

Which demangles to:

```
$s12TestingTests03ZipB0V0022allElementsEqual_oxFJo4TestfMp_9Z5bcb7b35fMu_ ---> unique name #1 of Z5bcb7b35 in peer macro @test expansion #1 of allElementsEqual😀 in TestingTests.ZipTests
```

The benefit of the change is that the generated names are shorter and
easier to read when expanding a macro, and play better with the
demangler. There may also be some benefit on Windows where the linker
has a 65KB symbol name cap, although we're not exporting these symbols
so probably not.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Sources/TestingMacros/Support/Additions/MacroExpansionContextAdditions.swift

@@ -62,44 +62,18 @@ extension MacroExpansionContext {
       .tokens(viewMode: .fixedUp)
       .map(\.textWithoutBackticks)
       .joined()
+    let crcValue = crc32(identifierCharacters.utf8)
+    let suffix = String(crcValue, radix: 16, uppercase: false)
 
-    // Strip out any characters in the function's signature that won't play well
-    // in a generated symbol name.
-    let identifier = String(
-      identifierCharacters.map { character in
-        if character.isLetter || character.isWholeNumber {
-          return character
-        }
-        return "_"
-      }
-    )
-
-    // If there is a non-ASCII character in the identifier, we might be
-    // stripping it out above because we are only looking for letters and
-    // digits. If so, add in a hash of the identifier to improve entropy and
-    // reduce the risk of a collision.
-    //
-    // For example, the following function names will produce identical unique
-    // names without this mutation:
-    //
-    // @Test(arguments: [0]) func A(🙃: Int) {}
-    // @Test(arguments: [0]) func A(🙂: Int) {}
-    //
-    // Note the check here is not the same as the one above: punctuation like
-    // "(" should be replaced, but should not cause a hash to be emitted since
-    // it does not contribute any entropy to the makeUniqueName() algorithm.
-    //
-    // The intent here is not to produce a cryptographically strong hash, but to
-    // disambiguate between superficially similar function names. A collision
-    // may still occur, but we only need it to be _unlikely_. CRC-32 is good
-    // enough for our purposes.
-    if !identifierCharacters.allSatisfy(\.isASCII) {
-      let crcValue = crc32(identifierCharacters.utf8)
-      let suffix = String(crcValue, radix: 16, uppercase: false)
-      return makeUniqueName("\(prefix)\(identifier)_\(suffix)")
+    // If the caller did not specify a prefix and the CRC32 value starts with a
+    // digit, include a single-character prefix to ensure that Swift's name
+    // demangling still works correctly.
+    var prefix = prefix
+    if prefix.isEmpty, let firstSuffixCharacter = suffix.first, firstSuffixCharacter.isWholeNumber {
+      prefix = "Z"
     }
 
-    return makeUniqueName("\(prefix)\(identifier)")
+    return makeUniqueName("\(prefix)\(suffix)")
   }
 }
 

Tests/TestingMacrosTests/UniqueIdentifierTests.swift

@@ -27,50 +27,35 @@ struct UniqueIdentifierTests {
     return BasicMacroExpansionContext().makeUniqueName(thunking: functionDecl).text
   }
 
-  @Test("Thunk identifiers contain a function's name")
-  func thunkNameContainsFunctionName() throws {
-    let uniqueName = try makeUniqueName("func someDistinctFunctionName() async throws")
-    #expect(uniqueName.contains("someDistinctFunctionName"))
-  }
-
   @Test("Thunk identifiers do not contain backticks")
   func noBackticks() throws {
     let uniqueName = try makeUniqueName("func `someDistinctFunctionName`() async throws")
-    #expect(uniqueName.contains("someDistinctFunctionName"))
 
     #expect(!uniqueName.contains("`"))
   }
 
   @Test("Thunk identifiers do not contain arbitrary Unicode")
   func noArbitraryUnicode() throws {
     let uniqueName = try makeUniqueName("func someDistinctFunction🌮Name🐔() async throws")
-    #expect(uniqueName.contains("someDistinctFunction"))
 
     #expect(!uniqueName.contains("🌮"))
     #expect(!uniqueName.contains("🐔"))
-    #expect(uniqueName.contains("Name"))
   }
 
   @Test("Argument types influence generated identifiers")
   func argumentTypes() throws {
     let uniqueNameWithInt = try makeUniqueName("func someDistinctFunctionName(i: Int) async throws")
-    #expect(uniqueNameWithInt.contains("someDistinctFunctionName"))
     let uniqueNameWithUInt = try makeUniqueName("func someDistinctFunctionName(i: UInt) async throws")
-    #expect(uniqueNameWithUInt.contains("someDistinctFunctionName"))
 
     #expect(uniqueNameWithInt != uniqueNameWithUInt)
   }
 
   @Test("Effects influence generated identifiers")
   func effects() throws {
     let uniqueName = try makeUniqueName("func someDistinctFunctionName()")
-    #expect(uniqueName.contains("someDistinctFunctionName"))
     let uniqueNameAsync = try makeUniqueName("func someDistinctFunctionName() async")
-    #expect(uniqueNameAsync.contains("someDistinctFunctionName"))
     let uniqueNameThrows = try makeUniqueName("func someDistinctFunctionName() throws")
-    #expect(uniqueNameThrows.contains("someDistinctFunctionName"))
     let uniqueNameAsyncThrows = try makeUniqueName("func someDistinctFunctionName() async throws")
-    #expect(uniqueNameAsyncThrows.contains("someDistinctFunctionName"))
 
     #expect(uniqueName != uniqueNameAsync)
     #expect(uniqueName != uniqueNameThrows)
@@ -89,4 +74,11 @@ struct UniqueIdentifierTests {
     #expect(uniqueName1 != uniqueName3)
     #expect(uniqueName2 != uniqueName3)
   }
+
+  @Test("Body does not influence generated identifiers")
+  func body() throws {
+    let uniqueName1 = try makeUniqueName("func f() { abc() }")
+    let uniqueName2 = try makeUniqueName("func f() { def() }")
+    #expect(uniqueName1 == uniqueName2)
+  }
 }