Add \a escape sequence (#5864)

spec/compiler/lexer/lexer_spec.cr

@@ -231,6 +231,7 @@ describe "Lexer" do
   it_lexes_number :i8, ["0i8", "0"]
 
   it_lexes_char "'a'", 'a'
+  it_lexes_char "'\\a'", 7.chr
   it_lexes_char "'\\b'", 8.chr
   it_lexes_char "'\\n'", '\n'
   it_lexes_char "'\\t'", '\t'

spec/compiler/lexer/lexer_string_spec.cr

@@ -227,7 +227,7 @@ describe "Lexer string" do
 
     tester.string_should_start_correctly
     tester.next_token_should_be(:NEWLINE)
-    tester.next_string_token_should_be("abc")
+    tester.next_string_token_should_be("\abc")
     tester.string_should_have_an_interpolation_of("foo")
     tester.string_should_end_correctly
   end

spec/compiler/parser/parser_spec.cr

@@ -82,7 +82,7 @@ describe "Parser" do
   it_parses %(:"\\\\foo"), "\\foo".symbol
   it_parses %(:"\\\"foo"), "\"foo".symbol
   it_parses %(:"\\\"foo\\\""), "\"foo\"".symbol
-  it_parses %(:"\\b\\n\\r\\t\\v\\f\\e"), "\b\n\r\t\v\f\e".symbol
+  it_parses %(:"\\a\\b\\n\\r\\t\\v\\f\\e"), "\a\b\n\r\t\v\f\e".symbol
   it_parses %(:"\\u{61}"), "a".symbol
 
   it_parses "[1, 2]", ([1.int32, 2.int32] of ASTNode).array

spec/std/char_spec.cr

@@ -133,6 +133,7 @@ describe "Char" do
   end
 
   it "escapes" do
+    7.chr.ord.should eq(7) # TODO: use \a
     '\b'.ord.should eq(8)
     '\t'.ord.should eq(9)
     '\n'.ord.should eq(10)

spec/std/string_spec.cr

@@ -1448,6 +1448,7 @@ describe "String" do
     "a".dump.should eq %("a")
     "\\".dump.should eq %("\\\\")
     "\"".dump.should eq %("\\\"")
+    # TODO: "\a".dump.should eq %("\\a")
     "\b".dump.should eq %("\\b")
     "\e".dump.should eq %("\\e")
     "\f".dump.should eq %("\\f")
@@ -1475,6 +1476,7 @@ describe "String" do
     "a".inspect.should eq %("a")
     "\\".inspect.should eq %("\\\\")
     "\"".inspect.should eq %("\\\"")
+    # TODO: "\a".inspect.should eq %("\\a")
     "\b".inspect.should eq %("\\b")
     "\e".inspect.should eq %("\\e")
     "\f".inspect.should eq %("\\f")
@@ -1653,6 +1655,7 @@ describe "String" do
   end
 
   it "escapes chars" do
+    # TODO: "\a"[0].should eq('\a')
     "\b"[0].should eq('\b')
     "\t"[0].should eq('\t')
     "\n"[0].should eq('\n')

src/compiler/crystal/syntax/lexer.cr

@@ -490,6 +490,8 @@ module Crystal
             case char
             when '\\'
               case char = next_char
+              when 'a'
+                io << "\u{7}"
               when 'b'
                 io << "\u{8}"
               when 'n'
@@ -608,6 +610,8 @@ module Crystal
             @token.value = '\\'
           when '\''
             @token.value = '\''
+          when 'a'
+            @token.value = 7.chr # TODO: use \a
           when 'b'
             @token.value = '\b'
           when 'e'
@@ -1790,6 +1794,8 @@ module Crystal
             end
           else
             case char = next_char
+            when 'a'
+              string_token_escape_value "\u{7}"
             when 'b'
               string_token_escape_value "\u{8}"
             when 'n'

src/string.cr

@@ -3937,15 +3937,16 @@ class String
     while reader.has_next?
       current_char = reader.current_char
       case current_char
-      when '"'  then io << "\\\""
-      when '\\' then io << "\\\\"
-      when '\b' then io << "\\b"
-      when '\e' then io << "\\e"
-      when '\f' then io << "\\f"
-      when '\n' then io << "\\n"
-      when '\r' then io << "\\r"
-      when '\t' then io << "\\t"
-      when '\v' then io << "\\v"
+      when '"'     then io << "\\\""
+      when '\\'    then io << "\\\\"
+      when (7.chr) then io << "\\a" # TODO: use \a
+      when '\b'    then io << "\\b"
+      when '\e'    then io << "\\e"
+      when '\f'    then io << "\\f"
+      when '\n'    then io << "\\n"
+      when '\r'    then io << "\\r"
+      when '\t'    then io << "\\t"
+      when '\v'    then io << "\\v"
       when '#'
         current_char = reader.next_char
         if current_char == '{'