Add check for invalid sketch src subfolder name case

Author: per1234

Diff for: check/checkconfigurations/checkconfigurations.go

@@ -1061,6 +1061,21 @@ var configurations = []Type{
 		ErrorModes:      []checkmode.Type{checkmode.Default},
 		CheckFunction:   checkfunctions.RecursiveLibraryWithUtilityFolder,
 	},
+	{
+		ProjectType:     projecttype.Sketch,
+		Category:        "structure",
+		Subcategory:     "",
+		ID:              "",
+		Brief:           "incorrect src folder case",
+		Description:     "",
+		MessageTemplate: "Incorrect src folder case. This will cause the source files under it to not be compiled on case-sensitive operating systems. See: https://arduino.github.io/arduino-cli/latest/sketch-specification/#src-subfolder",
+		DisableModes:    nil,
+		EnableModes:     []checkmode.Type{checkmode.Default},
+		InfoModes:       nil,
+		WarningModes:    []checkmode.Type{checkmode.Default},
+		ErrorModes:      []checkmode.Type{checkmode.Strict},
+		CheckFunction:   checkfunctions.IncorrectSketchSrcFolderNameCase,
+	},
 	{
 		ProjectType:     projecttype.Sketch,
 		Category:        "structure",

Diff for: check/checkfunctions/sketch.go

@@ -25,6 +25,22 @@ import (
 	"github.com/arduino/arduino-check/project/sketch"
 )
 
+// IncorrectSketchSrcFolderNameCase checks for incorrect case of src subfolder name in recursive format libraries.
+func IncorrectSketchSrcFolderNameCase() (result checkresult.Type, output string) {
+	directoryListing, err := checkdata.ProjectPath().ReadDir()
+	if err != nil {
+		panic(err)
+	}
+	directoryListing.FilterDirs()
+
+	path, found := containsIncorrectPathBaseCase(directoryListing, "src")
+	if found {
+		return checkresult.Fail, path.String()
+	}
+
+	return checkresult.Pass, ""
+}
+
 // ProhibitedCharactersInSketchFileName checks for prohibited characters in the sketch file names.
 func ProhibitedCharactersInSketchFileName() (result checkresult.Type, output string) {
 	directoryListing, _ := checkdata.ProjectPath().ReadDir()

Diff for: check/checkfunctions/sketch_test.go

@@ -60,6 +60,15 @@ func checkSketchCheckFunction(checkFunction Type, testTables []sketchCheckFuncti
 	}
 }
 
+func TestIncorrectSketchSrcFolderNameCase(t *testing.T) {
+	testTables := []sketchCheckFunctionTestTable{
+		{"Incorrect case", "IncorrectSrcFolderNameCase", checkresult.Fail, ""},
+		{"Correct case", "Valid", checkresult.Pass, ""},
+	}
+
+	checkSketchCheckFunction(IncorrectSketchSrcFolderNameCase, testTables, t)
+}
+
 func TestProhibitedCharactersInSketchFileName(t *testing.T) {
 	testTables := []sketchCheckFunctionTestTable{
 		{"Has prohibited characters", "ProhibitedCharactersInFileName", checkresult.Fail, "^Prohibited CharactersInFileName.h$"},

Diff for: check/checkfunctions/testdata/sketches/IncorrectSrcFolderNameCase/IncorrectSrcFolderNameCase.ino

@@ -0,0 +1,2 @@
+void setup() {}
+void loop() {}

Diff for: check/checkfunctions/testdata/sketches/IncorrectSrcFolderNameCase/SRC/src.cpp

@@ -0,0 +1,2 @@
+void setup() {}
+void loop() {}