Remove fail("Unreachable") from features' and action_configs' declaration statements

If toolchain A and B need feature f, but toolchain C doesn't, the generated declaration statement for feature f would be:
if ctx.attr.cpu == 'A.cpu':
   f = feature(...)
elif ctx.attr.cpu == "B.cpu" and ctx.attr.compiler == "B.compiler":
   f = feature(...)
else:
   fail("Unreachable")

This will break the rule implementation in the case of toolchain C because it will reach the  fail("Unreachable") although it doesn't need to declare feature f

This cl fixes that

Issue #5380

PiperOrigin-RevId: 232683639

tools/migration/crosstool_to_starlark_lib.go

@@ -347,7 +347,7 @@ func getStringStatement(crosstool *crosstoolpb.CrosstoolRelease,
 
 	mappedValuesToIds := getMappedStringValuesToIdentifiers(identifiers, fieldValues)
 	return getAssignmentStatement(field, mappedValuesToIds, crosstool,
-		cToolchainIdentifiers, depth /* isPlainString= */, true)
+		cToolchainIdentifiers, depth /* isPlainString= */, true, /* shouldFail= */ true)
 }
 
 func getFeatures(crosstool *crosstoolpb.CrosstoolRelease) (
@@ -392,7 +392,8 @@ func getFeaturesDeclaration(crosstool *crosstoolpb.CrosstoolRelease,
 				crosstool,
 				cToolchainIdentifiers,
 				depth,
-				/* isPlainString= */ false))
+				/* isPlainString= */ false,
+			  /* shouldFail= */ false))
 	}
 	return strings.Join(res, "")
 }
@@ -467,7 +468,8 @@ func getActionConfigsDeclaration(
 				crosstool,
 				cToolchainIdentifiers,
 				depth,
-				/* isPlainString= */ false))
+				/* isPlainString= */ false,
+			  /* shouldFail= */ false))
 	}
 	return strings.Join(res, "")
 }
@@ -650,7 +652,8 @@ func getArtifactNamePatterns(crosstool *crosstoolpb.CrosstoolRelease,
 			crosstool,
 			cToolchainIdentifiers,
 			depth,
-			/* isPlainString= */ false))
+			/* isPlainString= */ false,
+		  /* shouldFail= */ true))
 	return strings.Join(res, "\n")
 }
 
@@ -826,7 +829,8 @@ func getToolPaths(crosstool *crosstoolpb.CrosstoolRelease,
 			crosstool,
 			cToolchainIdentifiers,
 			depth,
-			/* isPlainString= */ false))
+			/* isPlainString= */ false,
+		  /* shouldFail= */ true))
 	return strings.Join(res, "\n")
 }
 
@@ -865,7 +869,8 @@ func getMakeVariables(crosstool *crosstoolpb.CrosstoolRelease,
 			crosstool,
 			cToolchainIdentifiers,
 			depth,
-			/* isPlainString= */ false))
+			/* isPlainString= */ false,
+		  /* shouldFail= */ true))
 	return strings.Join(res, "\n")
 }
 
@@ -1034,7 +1039,7 @@ func makeStringArr(arr []string, depth int, isPlainString bool) string {
 func getAssignmentStatement(field string, valToIds map[string][]string,
 	crosstool *crosstoolpb.CrosstoolRelease,
 	toCToolchainIdentifier map[string]CToolchainIdentifier,
-	depth int, isPlainString bool) string {
+	depth int, isPlainString, shouldFail bool) string {
 	var b bytes.Buffer
 	if len(valToIds) <= 1 {
 		// if there is only one possible value for this field, we don't need if statements
@@ -1063,11 +1068,13 @@ func getAssignmentStatement(field string, valToIds map[string][]string,
 					toCToolchainIdentifier, depth, isPlainString))
 			first = false
 		}
+		if shouldFail {
 			b.WriteString(
 				fmt.Sprintf(
 					"%selse:\n%sfail(\"Unreachable\")\n",
 					getTabs(depth), getTabs(depth+1)))
 		}
+	}
 	b.WriteString("\n")
 	return b.String()
 }

tools/migration/crosstool_to_starlark_lib_test.go

@@ -1080,6 +1080,73 @@ func TestAllAndNoneAvailableErrorsWhenMoreThanOneElement(t *testing.T) {
 	}
 }
 
+func TestNoFailUnreachableInFeaturesAndActionConfigsDeclaration(t *testing.T) {
+	toolchainFeatureAEnabled := getCToolchain("1", "cpuA", "compilerA",
+		[]string{getFeature([]string{"name: 'A'", "enabled: true"})},
+	)
+	toolchainFeatureADisabled := getCToolchain("2", "cpuA", "compilerB",
+		[]string{getFeature([]string{"name: 'A'", "enabled: false"})},
+	)
+
+	toolchainWithoutFeatureA := getCToolchain("3", "cpuA", "compilerC", []string{})
+
+	toolchainActionConfigAEnabled := getCToolchain("4", "cpuA", "compilerD",
+		[]string{getActionConfig([]string{
+			"config_name: 'A'",
+			"action_name: 'A'",
+			"enabled: true",
+		})})
+
+	toolchainActionConfigADisabled := getCToolchain("5", "cpuA", "compilerE",
+		[]string{getActionConfig([]string{
+			"config_name: 'A'",
+			"action_name: 'A'",
+		})})
+
+	toolchainWithoutActionConfigA := getCToolchain("6", "cpuA", "compilerF", []string{})
+
+	testCases := []struct {
+		field        string
+		toolchains   []string
+		expectedText string
+	}{
+		{field: "features",
+			toolchains: []string{
+				toolchainFeatureAEnabled, toolchainFeatureADisabled, toolchainWithoutFeatureA},
+			expectedText: `
+    if (ctx.attr.cpu == "cpuA" and ctx.attr.compiler == "compilerB"):
+        a_feature = feature(name = "A")
+    elif (ctx.attr.cpu == "cpuA" and ctx.attr.compiler == "compilerA"):
+        a_feature = feature(name = "A", enabled = True)
+
+` /* empty line after the elif means there's no else statement */},
+		{field: "action_config",
+			toolchains: []string{
+				toolchainActionConfigAEnabled, toolchainActionConfigADisabled, toolchainWithoutActionConfigA},
+			expectedText: `
+    if (ctx.attr.cpu == "cpuA" and ctx.attr.compiler == "compilerE"):
+        a_action = action_config(action_name = "A")
+    elif (ctx.attr.cpu == "cpuA" and ctx.attr.compiler == "compilerD"):
+        a_action = action_config(action_name = "A", enabled = True)
+
+` /* empty line after the elif means there's no else statement */ },
+	}
+
+	for _, tc := range testCases {
+		crosstool := makeCrosstool(tc.toolchains)
+		got, err := Transform(crosstool)
+		if err != nil {
+			t.Fatalf("CROSSTOOL conversion failed: %v", err)
+		}
+		if !strings.Contains(got, tc.expectedText) {
+			t.Errorf("Failed to correctly convert '%s' field, expected to contain:\n%v\n",
+				tc.field, tc.expectedText)
+			t.Fatalf("Tested CROSSTOOL:\n%v\n\nGenerated rule:\n%v\n",
+				strings.Join(tc.toolchains, "\n"), got)
+		}
+	}
+}
+
 func TestActionConfigDeclaration(t *testing.T) {
 	toolchainEmpty1 := getCToolchain("1", "cpuA", "compilerA", []string{})
 	toolchainEmpty2 := getCToolchain("2", "cpuB", "compilerA", []string{})