Add fast-path for Char.IsDigit in WellKnownFunctions

Co-authored-by: baronfel <[email protected]>

Author: Copilot

src/Build.UnitTests/Evaluation/Expander_Tests.cs

@@ -4741,6 +4741,40 @@ public void PropertyFunctionStringArrayGetValue()
             TestPropertyFunction("$(X.Split($([System.Convert]::ToString(`.`).ToCharArray())).GetValue($([System.Convert]::ToInt32(0))))", "X", "ab.cd", "ab");
         }
 
+        /// <summary>
+        /// Test that Char.IsDigit fast-path works correctly
+        /// </summary>
+        [Fact]
+        public void PropertyFunctionCharIsDigit()
+        {
+            PropertyDictionary<ProjectPropertyInstance> pg = new PropertyDictionary<ProjectPropertyInstance>();
+
+            Expander<ProjectPropertyInstance, ProjectItemInstance> expander = new Expander<ProjectPropertyInstance, ProjectItemInstance>(pg, FileSystems.Default);
+
+            // Test with digit characters
+            string result = expander.ExpandIntoStringLeaveEscaped("$([System.Char]::IsDigit('5'))", ExpanderOptions.ExpandProperties, MockElementLocation.Instance);
+            Assert.Equal("True", result);
+
+            result = expander.ExpandIntoStringLeaveEscaped("$([System.Char]::IsDigit('0'))", ExpanderOptions.ExpandProperties, MockElementLocation.Instance);
+            Assert.Equal("True", result);
+
+            result = expander.ExpandIntoStringLeaveEscaped("$([System.Char]::IsDigit('9'))", ExpanderOptions.ExpandProperties, MockElementLocation.Instance);
+            Assert.Equal("True", result);
+
+            // Test with non-digit characters
+            result = expander.ExpandIntoStringLeaveEscaped("$([System.Char]::IsDigit('a'))", ExpanderOptions.ExpandProperties, MockElementLocation.Instance);
+            Assert.Equal("False", result);
+
+            result = expander.ExpandIntoStringLeaveEscaped("$([System.Char]::IsDigit(' '))", ExpanderOptions.ExpandProperties, MockElementLocation.Instance);
+            Assert.Equal("False", result);
+
+            result = expander.ExpandIntoStringLeaveEscaped("$([System.Char]::IsDigit('/'))", ExpanderOptions.ExpandProperties, MockElementLocation.Instance);
+            Assert.Equal("False", result);
+
+            result = expander.ExpandIntoStringLeaveEscaped("$([System.Char]::IsDigit(':'))", ExpanderOptions.ExpandProperties, MockElementLocation.Instance);
+            Assert.Equal("False", result);
+        }
+
         private void TestPropertyFunction(string expression, string propertyName, string propertyValue, string expected)
         {
             var properties = new PropertyDictionary<ProjectPropertyInstance>();

src/Build.UnitTests/Scanner_Tests.cs

@@ -562,26 +562,5 @@ public void NegativeTests()
             Scanner lexer = new Scanner("'$(DEBUG) == true", ParserOptions.AllowAll);
             Assert.False(lexer.Advance());
         }
-
-        /// <summary>
-        /// Tests the fast-path IsDigit method in CharacterUtilities.
-        /// </summary>
-        [Fact]
-        public void CharacterUtilities_IsDigit_WorksCorrectly()
-        {
-            // Test ASCII digits
-            for (char c = '0'; c <= '9'; c++)
-            {
-                Assert.True(CharacterUtilities.IsDigit(c), $"Character '{c}' should be identified as a digit");
-            }
-
-            // Test non-digit characters
-            Assert.False(CharacterUtilities.IsDigit('/'), "Character '/' should not be identified as a digit");
-            Assert.False(CharacterUtilities.IsDigit(':'), "Character ':' should not be identified as a digit");
-            Assert.False(CharacterUtilities.IsDigit('a'), "Character 'a' should not be identified as a digit");
-            Assert.False(CharacterUtilities.IsDigit('A'), "Character 'A' should not be identified as a digit");
-            Assert.False(CharacterUtilities.IsDigit(' '), "Character ' ' should not be identified as a digit");
-            Assert.False(CharacterUtilities.IsDigit('\0'), "Character '\\0' should not be identified as a digit");
-        }
     }
 }

src/Build/Evaluation/Conditionals/CharacterUtilities.cs

@@ -9,7 +9,7 @@ internal static class CharacterUtilities
     {
         internal static bool IsNumberStart(char candidate)
         {
-            return candidate == '+' || candidate == '-' || candidate == '.' || IsDigit(candidate);
+            return candidate == '+' || candidate == '-' || candidate == '.' || char.IsDigit(candidate);
         }
 
         internal static bool IsSimpleStringStart(char candidate)
@@ -19,17 +19,12 @@ internal static bool IsSimpleStringStart(char candidate)
 
         internal static bool IsSimpleStringChar(char candidate)
         {
-            return IsSimpleStringStart(candidate) || IsDigit(candidate);
-        }
-
-        internal static bool IsDigit(char candidate)
-        {
-            return candidate >= '0' && candidate <= '9';
+            return IsSimpleStringStart(candidate) || char.IsDigit(candidate);
         }
 
         internal static bool IsHexDigit(char candidate)
         {
-            return IsDigit(candidate) || ((uint)((candidate | 0x20) - 'a') <= 'f' - 'a');
+            return char.IsDigit(candidate) || ((uint)((candidate | 0x20) - 'a') <= 'f' - 'a');
         }
     }
 }

src/Build/Evaluation/Conditionals/Scanner.cs

@@ -772,7 +772,7 @@ private void SkipWhiteSpace()
         }
         private void SkipDigits()
         {
-            while (_parsePoint < _expression.Length && CharacterUtilities.IsDigit(_expression[_parsePoint]))
+            while (_parsePoint < _expression.Length && char.IsDigit(_expression[_parsePoint]))
             {
                 _parsePoint++;
             }

src/Build/Evaluation/Expander/WellKnownFunctions.cs

@@ -875,6 +875,18 @@ internal static bool TryExecuteWellKnownFunction(string methodName, Type receive
                         }
                     }
                 }
+                else if (receiverType == typeof(char))
+                {
+                    if (string.Equals(methodName, nameof(char.IsDigit), StringComparison.OrdinalIgnoreCase))
+                    {
+                        if (ParseArgs.TryGetArg(args, out string? arg0) && arg0?.Length == 1)
+                        {
+                            char c = arg0[0];
+                            returnVal = c >= '0' && c <= '9';
+                            return true;
+                        }
+                    }
+                }
                 else if (string.Equals(methodName, nameof(Regex.Replace), StringComparison.OrdinalIgnoreCase) && args.Length == 3)
                 {
                     if (ParseArgs.TryGetArgs(args, out string? arg1, out string? arg2, out string? arg3) && arg1 != null && arg2 != null && arg3 != null)