Fix empty lines handling on code blocks (#135)

Fixes #132.
Fixes #133.

src/parser/lexer.mll

@@ -74,7 +74,8 @@ let trim_leading_whitespace : string -> string = fun s ->
   let count_leading_whitespace : string -> int = fun line ->
     let rec count_leading_whitespace' : int -> int = fun index ->
       if index >= String.length line then
-        index
+        (* Ignore empty and whitespace-only lines. *)
+        max_int
       else
         match line.[index] with
         | ' ' | '\t' -> count_leading_whitespace' (index + 1)
@@ -92,10 +93,13 @@ let trim_leading_whitespace : string -> string = fun s ->
     |> List.fold_left min max_int
   in
   let remove_whitespace : string -> string = fun line ->
-    String.sub
+    if String.length line < least_amount_of_whitespace then
       line
-      least_amount_of_whitespace
-      (String.length line - least_amount_of_whitespace)
+    else
+      String.sub
+        line
+        least_amount_of_whitespace
+        (String.length line - least_amount_of_whitespace)
   in
   lines
   |> List.map remove_whitespace

test/parser/expect/code-block/leading-whitespace-with-empty-line.txt

@@ -0,0 +1,4 @@
+((output (ok (((f.ml (1 0) (3 6)) (code_block  "foo\
+                                              \n\
+                                              \nbar")))))
+ (warnings ()))

test/parser/expect/code-block/leading-whitespace-with-whitespace-line-long.txt

@@ -0,0 +1,4 @@
+((output (ok (((f.ml (1 0) (3 6)) (code_block  "foo\
+                                              \n  \
+                                              \nbar")))))
+ (warnings ()))

test/parser/expect/code-block/leading-whitespace-with-whitespace-line-short.txt

@@ -0,0 +1,4 @@
+((output (ok (((f.ml (1 0) (3 7)) (code_block  "foo\
+                                              \n \
+                                              \nbar")))))
+ (warnings ()))

test/parser/test.ml

@@ -275,6 +275,9 @@ let tests : test_suite list = [
     t "leading-whitespace-two-cr-lf" "{[ foo\r\n bar]}";
     t "leading-whitespace-two-different-indent" "{[ foo\n   bar]}";
     t "leading-whitespace-two-different-indent-rev" "{[   foo\n bar]}";
+    t "leading-whitespace-with-empty-line" "{[ foo\n\n bar]}";
+    t "leading-whitespace-with-whitespace-line-short" "{[  foo\n \n  bar]}";
+    t "leading-whitespace-with-whitespace-line-long" "{[ foo\n   \n bar]}";
     t "leading-tab" "{[\tfoo]}";
     t "leading-tab-two" "{[\tfoo\n\tbar]}";
     t "leading-tab-two-different-indent" "{[\tfoo\n\t\tbar]}";