apicompat: treat IntPtr as nint and UIntPtr as nuint (#27601)

Previously, ApiCompat would consider `IntPtr` and `nint` (as well as `UIntPtr` and `nuint`) as different types, emitting diagnostics where there shouldn't have been. This change updates the display string used by the SymbolComparer to replace all instances of `IntPtr` with `nint` and `UIntPtr` with `nuint`.

Fixes #25566

src/ApiCompat/Microsoft.DotNet.ApiCompatibility/Extensions/SymbolExtensions.cs

@@ -29,7 +29,10 @@ private static SymbolDisplayFormat GetSymbolDisplayFormat()
             return format.WithParameterOptions(format.ParameterOptions & ~SymbolDisplayParameterOptions.IncludeParamsRefOut);
         }
 
-        internal static string ToComparisonDisplayString(this ISymbol symbol) => symbol.ToDisplayString(Format);
+        internal static string ToComparisonDisplayString(this ISymbol symbol) =>
+            symbol.ToDisplayString(Format)
+                  .Replace("System.IntPtr", "nint") // Treat IntPtr and nint as the same
+                  .Replace("System.UIntPtr", "nuint"); // Treat UIntPtr and nuint as the same
 
         internal static IEnumerable<ITypeSymbol> GetAllBaseTypes(this ITypeSymbol type)
         {

src/Tests/Microsoft.DotNet.ApiCompatibility.Tests/Rules/MembersMustExistTests.cs

@@ -466,5 +466,39 @@ private First() { }
             };
             Assert.Equal(expected, differences);
         }
+
+        [Fact]
+        public void NumericPtrNotFlagged()
+        {
+            string leftSyntax = @"
+namespace CompatTests
+{
+  using System;
+
+  public class First
+  {
+    public void F(IntPtr p) {}
+    public void G(UIntPtr p) {}
+  }
+}
+";
+            string rightSyntax = @"
+namespace CompatTests
+{
+  public class First
+  {
+    public void F(nint p) {}
+    public void G(nuint p) {}
+  }
+}
+";
+            IAssemblySymbol left = SymbolFactory.GetAssemblyFromSyntax(leftSyntax);
+            IAssemblySymbol right = SymbolFactory.GetAssemblyFromSyntax(rightSyntax);
+            ApiComparer differ = new(s_ruleFactory);
+
+            IEnumerable<CompatDifference> differences = differ.GetDifferences(left, right);
+
+            Assert.Empty(differences);
+        }
     }
 }