Stop new projects starting with the word Pulumi

Fixes: #4013

Without this, we are in a situation where we have a cyclic import

CHANGELOG.md

@@ -4,8 +4,8 @@ CHANGELOG
 ## HEAD (Unreleased)
 * Fix Kubernetes YAML parsing error in .NET.
   [#4023](https://github.com/pulumi/pulumi/pull/4023)
-
----
+- Avoid projects beginning with `Pulumi` to stop cyclic imports
+  [#4013](https://github.com/pulumi/pulumi/pull/4013)
 
 ## 1.12.0 (2020-03-04)
 - Avoid Configuring providers which are not used during preview.

pkg/workspace/templates.go

@@ -522,6 +522,12 @@ func ValidateProjectName(s string) error {
 		return errors.New("A project name may only contain alphanumeric, hyphens, underscores, and periods")
 	}
 
+	// This is needed to stop cyclic imports in DotNet projects
+	if strings.ToLower(s) == "pulumi" || strings.HasPrefix(strings.ToLower(s), "pulumi.") {
+		return errors.New("A project name must not be `Pulumi` and must not start with the prefix `Pulumi.` " +
+			"to avoid collision with standard libraries")
+	}
+
 	return nil
 }
 

pkg/workspace/templates_test.go

@@ -241,3 +241,58 @@ func TestRetrieveFileTemplate(t *testing.T) {
 		})
 	}
 }
+
+func TestProjectNames(t *testing.T) {
+	tests := []struct {
+		testName    string
+		projectName string
+		expectError bool
+	}{
+		{
+			testName:    "Correct Project Name",
+			projectName: "SampleProject",
+			expectError: false,
+		},
+		{
+			testName:    "Project Name with unsupported punctuation",
+			projectName: "SampleProject!",
+			expectError: true,
+		},
+		{
+			testName:    "Project Name starting with the word Pulumi",
+			projectName: "PulumiProject",
+			expectError: false,
+		},
+		{
+			testName:    "Project Name greater than 100 characters",
+			projectName: "cZClTe6xrjgKzH5QS8rFEPqYK1z4bbMeMr6n89n87djq9emSAlznQXXkkCEpBBCaZAFNlCvbfqVcqoifYlfPl11hvekIDjXVIY7m1",
+			expectError: true,
+		},
+		{
+			testName:    "Project Name is Pulumi",
+			projectName: "Pulumi",
+			expectError: true,
+		},
+		{
+			testName:    "Project Name is Pulumi - mixed case",
+			projectName: "pUlumI",
+			expectError: true,
+		},
+		{
+			testName:    "Project Name is Pulumi.Test",
+			projectName: "Pulumi.Test",
+			expectError: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.testName, func(t *testing.T) {
+			err := ValidateProjectName(tt.projectName)
+			if tt.expectError {
+				assert.Error(t, err)
+			} else {
+				assert.Nil(t, err)
+			}
+		})
+	}
+}