Respond to PR feedback.

Make the change smaller until @jaredpar gets back. Only make the minimal change required, which is to check DOTNET_HOST_PATH.

src/Compilers/Core/MSBuildTaskTests/TestUtilities/TaskTestUtil.cs

@@ -41,23 +41,7 @@ public static void AssertCommandLine(
 
             var message = engine.BuildMessages.OfType<TaskCommandLineEventArgs>().Single();
             var commandLine = message.CommandLine.Replace("  ", " ").Trim();
-
-            var dotnetPath = RuntimeHostInfo.GetDotNetPathOrDefault();
-            var expectedCommandLine = $@"{dotnetPath} exec ""{task.PathToManagedTool}"" {line}";
-
-            bool isOnlyFileName = Path.GetFileName(dotnetPath).Length == dotnetPath.Length;
-            if (isOnlyFileName)
-            {
-                // When ToolTask.GenerateFullPathToTool() returns only a file name (not a path to a file), MSBuild's ToolTask
-                // will search the %PATH% (see https://github.com/dotnet/msbuild/blob/5410bf323451e04e99e79bcffd158e6d8d378149/src/Utilities/ToolTask.cs#L494-L513)
-                // and log the full path to the exe. In this case, only assert that the commandLine ends with the expected
-                // command line, and ignore the full path at the beginning.
-                Assert.EndsWith(expectedCommandLine, commandLine);
-            }
-            else
-            {
-                Assert.Equal(expectedCommandLine, commandLine);
-            }
+            Assert.Equal($@"{RuntimeHostInfo.GetDotNetPathOrDefault()} exec ""{task.PathToManagedTool}"" {line}", commandLine);
 
             compilerTask.NoConfig = true;
             Assert.Equal("/noconfig", compilerTask.GenerateToolArguments());

src/Compilers/Shared/RuntimeHostInfo.cs

@@ -53,11 +53,39 @@ internal static (string processFilePath, string commandLineArguments, string too
 
         /// <summary>
         /// Get the path to the dotnet executable. In the case the .NET SDK did not provide this information
-        /// in the environment this will return simply "dotnet".
+        /// in the environment this tries to find "dotnet" on the PATH. In the case it is not found,
+        /// this will return simply "dotnet".
         /// </summary>
-        internal static string GetDotNetPathOrDefault() =>
-            Environment.GetEnvironmentVariable(DotNetHostPathEnvironmentName) ??
-                (PlatformInformation.IsWindows ? "dotnet.exe" : "dotnet");
+        internal static string GetDotNetPathOrDefault()
+        {
+            if (Environment.GetEnvironmentVariable(DotNetHostPathEnvironmentName) is string pathToDotNet)
+            {
+                return pathToDotNet;
+            }
+
+            var (fileName, sep) = PlatformInformation.IsWindows
+                ? ("dotnet.exe", ';')
+                : ("dotnet", ':');
+
+            var path = Environment.GetEnvironmentVariable("PATH") ?? "";
+            foreach (var item in path.Split(sep, StringSplitOptions.RemoveEmptyEntries))
+            {
+                try
+                {
+                    var filePath = Path.Combine(item, fileName);
+                    if (File.Exists(filePath))
+                    {
+                        return filePath;
+                    }
+                }
+                catch
+                {
+                    // If we can't read a directory for any reason just skip it
+                }
+            }
+
+            return fileName;
+        }
 #else
 
         internal static bool IsCoreClrRuntime => false;