Refactor logic for which version to install

This does not change the version returned. In the case the the go mod
version is supported and the go env version is below goMinVersion, the
message now talks about go env version being unsupported instead of
it being less than go mod version. This seems more sensible to me.

go/extractor/cli/go-autobuilder/go-autobuilder.go

@@ -727,72 +727,46 @@ func outsideSupportedRange(version string) bool {
 	return semver.Compare(short, "v"+minGoVersion) < 0 || semver.Compare(short, "v"+maxGoVersion) > 0
 }
 
-// Check if `v.goModVersion` or `v.goEnvVersion` are outside of the supported range. If so, emit
-// a diagnostic and return an empty version to indicate that we should not attempt to install a
-// different version of Go.
-func checkForUnsupportedVersions(v versionInfo) (msg, version string) {
-	if v.goModVersionFound && outsideSupportedRange(v.goModVersion) {
+// Assuming `v.goModVersionFound` is false, emit a diagnostic and return the version to install,
+// or the empty string if we should not attempt to install a version of Go.
+func checkForGoModVersionNotFound(v versionInfo) (msg, version string) {
+	if !v.goEnvVersionFound {
+		msg = "No version of Go installed and no `go.mod` file found. Writing an environment " +
+			"file specifying the maximum supported version of Go (" + maxGoVersion + ")."
+		version = maxGoVersion
+		diagnostics.EmitNoGoModAndNoGoEnv(msg)
+	} else if outsideSupportedRange(v.goEnvVersion) {
+		msg = "No `go.mod` file found. The version of Go installed in the environment (" +
+			v.goEnvVersion + ") is outside of the supported range (" + minGoVersion + "-" +
+			maxGoVersion + "). Writing an environment file specifying the maximum supported " +
+			"version of Go (" + maxGoVersion + ")."
+		version = maxGoVersion
+		diagnostics.EmitNoGoModAndGoEnvUnsupported(msg)
+	} else {
+		msg = "No `go.mod` file found. Version " + v.goEnvVersion + " installed in the " +
+			"environment is supported. Writing an environment file not specifying any " +
+			"version of Go."
+		version = ""
+		diagnostics.EmitNoGoModAndGoEnvSupported(msg)
+	}
+
+	return msg, version
+}
+
+// Assuming `v.goModVersionFound` is true, emit a diagnostic and return the version to install,
+// or the empty string if we should not attempt to install a version of Go.
+func checkForGoModVersionFound(v versionInfo) (msg, version string) {
+	if outsideSupportedRange(v.goModVersion) {
 		msg = "The version of Go found in the `go.mod` file (" + v.goModVersion +
 			") is outside of the supported range (" + minGoVersion + "-" + maxGoVersion +
 			"). Writing an environment file not specifying any version of Go."
 		version = ""
 		diagnostics.EmitUnsupportedVersionGoMod(msg)
-	}
-
-	return msg, version
-}
-
-// Check if either `v.goEnvVersionFound` or `v.goModVersionFound` are false. If so, emit
-// a diagnostic and return the version to install, or the empty string if we should not attempt to
-// install a version of Go. We assume that `checkForUnsupportedVersions` has already been
-// called, so any versions that are found are within the supported range.
-func checkForVersionsNotFound(v versionInfo) (msg, version string) {
-	if !v.goEnvVersionFound && !v.goModVersionFound {
-		msg = "No version of Go installed and no `go.mod` file found. Writing an environment " +
-			"file specifying the maximum supported version of Go (" + maxGoVersion + ")."
-		version = maxGoVersion
-		diagnostics.EmitNoGoModAndNoGoEnv(msg)
-	}
-
-	if !v.goEnvVersionFound && v.goModVersionFound {
+	} else if !v.goEnvVersionFound {
 		msg = "No version of Go installed. Writing an environment file specifying the version " +
 			"of Go found in the `go.mod` file (" + v.goModVersion + ")."
 		version = v.goModVersion
 		diagnostics.EmitNoGoEnv(msg)
-	}
-
-	if v.goEnvVersionFound && !v.goModVersionFound {
-		if outsideSupportedRange(v.goEnvVersion) {
-			msg = "No `go.mod` file found. The version of Go installed in the environment (" +
-				v.goEnvVersion + ") is outside of the supported range (" + minGoVersion + "-" +
-				maxGoVersion + "). Writing an environment file specifying the maximum supported " +
-				"version of Go (" + maxGoVersion + ")."
-			version = maxGoVersion
-			diagnostics.EmitNoGoModAndGoEnvUnsupported(msg)
-		} else {
-			msg = "No `go.mod` file found. Version " + v.goEnvVersion + " installed in the " +
-				"environment is supported. Writing an environment file not specifying any " +
-				"version of Go."
-			version = ""
-			diagnostics.EmitNoGoModAndGoEnvSupported(msg)
-		}
-	}
-
-	return msg, version
-}
-
-// Compare `v.goModVersion` and `v.goEnvVersion`. emit a diagnostic and return the version to
-// install, or the empty string if we should not attempt to install a version of Go. We assume that
-// `checkForUnsupportedVersions` and `checkForVersionsNotFound` have already been called, so both
-// versions are found and are within the supported range.
-func compareVersions(v versionInfo) (msg, version string) {
-	if semver.Compare("v"+v.goModVersion, "v"+v.goEnvVersion) > 0 {
-		msg = "The version of Go installed in the environment (" + v.goEnvVersion +
-			") is lower than the version found in the `go.mod` file (" + v.goModVersion +
-			"). Writing an environment file specifying the version of Go from the `go.mod` " +
-			"file (" + v.goModVersion + ")."
-		version = v.goModVersion
-		diagnostics.EmitVersionGoModHigherVersionEnvironment(msg)
 	} else if outsideSupportedRange(v.goEnvVersion) {
 		msg = "The version of Go installed in the environment (" + v.goEnvVersion +
 			") is outside of the supported range (" + minGoVersion + "-" + maxGoVersion + "). " +
@@ -800,8 +774,14 @@ func compareVersions(v versionInfo) (msg, version string) {
 			v.goModVersion + ")."
 		version = v.goModVersion
 		diagnostics.EmitVersionGoModSupportedAndGoEnvUnsupported(msg)
+	} else if semver.Compare("v"+v.goModVersion, "v"+v.goEnvVersion) > 0 {
+		msg = "The version of Go installed in the environment (" + v.goEnvVersion +
+			") is lower than the version found in the `go.mod` file (" + v.goModVersion +
+			"). Writing an environment file specifying the version of Go from the `go.mod` " +
+			"file (" + v.goModVersion + ")."
+		version = v.goModVersion
+		diagnostics.EmitVersionGoModHigherVersionEnvironment(msg)
 	} else {
-
 		msg = "The version of Go installed in the environment (" + v.goEnvVersion +
 			") is supported and is high enough for the version found in the `go.mod` file (" +
 			v.goModVersion + "). Writing an environment file not specifying any version of Go."
@@ -815,18 +795,11 @@ func compareVersions(v versionInfo) (msg, version string) {
 // Check the versions of Go found in the environment and in the `go.mod` file, and return a
 // version to install. If the version is the empty string then no installation is required.
 func getVersionToInstall(v versionInfo) (msg, version string) {
-	msg, version = checkForUnsupportedVersions(v)
-	if msg != "" {
-		return msg, version
+	if !v.goModVersionFound {
+		return checkForGoModVersionNotFound(v)
 	}
 
-	msg, version = checkForVersionsNotFound(v)
-	if msg != "" {
-		return msg, version
-	}
-
-	msg, version = compareVersions(v)
-	return msg, version
+	return checkForGoModVersionFound(v)
 }
 
 // Write an environment file to the current directory. If `version` is the empty string then