Ensure indentation is consistent

src/lexer.coffee

@@ -40,6 +40,7 @@ exports.Lexer = class Lexer
     @indebt     = 0              # The over-indentation at the current level.
     @outdebt    = 0              # The under-outdentation at the current level.
     @indents    = []             # The stack of all current indentation levels.
+    @indentLiteral = ''          # The indentation
     @ends       = []             # The stack for pairing up tokens.
     @tokens     = []             # Stream of parsed tokens in the form `['TYPE', value, location data]`.
     @seenFor    = no             # Used to recognize FORIN and FOROF tokens.
@@ -347,6 +348,12 @@ exports.Lexer = class Lexer
     size = indent.length - 1 - indent.lastIndexOf '\n'
     noNewlines = @unfinished()
 
+    newIndentLiteral = if size > 0 then indent[-size..] else ''
+    minLiteralLength = Math.min newIndentLiteral.length, @indentLiteral.length
+    if newIndentLiteral[...minLiteralLength] isnt @indentLiteral[...minLiteralLength]
+      @error 'indentation mismatch', offset: indent.length
+      return indent.length
+
     if size - @indebt is @indent
       if noNewlines then @suppressNewlines() else @newlineToken 0
       return indent.length
@@ -358,13 +365,15 @@ exports.Lexer = class Lexer
         return indent.length
       unless @tokens.length
         @baseIndent = @indent = size
+        @indentLiteral = newIndentLiteral
         return indent.length
       diff = size - @indent + @outdebt
       @token 'INDENT', diff, indent.length - size, size
       @indents.push diff
       @ends.push {tag: 'OUTDENT'}
       @outdebt = @indebt = 0
       @indent = size
+      @indentLiteral = newIndentLiteral
     else if size < @baseIndent
       @error 'missing indentation', offset: indent.length
     else
@@ -401,6 +410,7 @@ exports.Lexer = class Lexer
 
     @token 'TERMINATOR', '\n', outdentLength, 0 unless @tag() is 'TERMINATOR' or noNewlines
     @indent = decreasedIndent
+    @indentLiteral = @indentLiteral[...decreasedIndent]
     this
 
   # Matches and consumes non-meaningful whitespace. Tag the previous token

test/compilation.coffee

@@ -76,7 +76,7 @@ test "#2516: Unicode spaces should not be part of identifiers", ->
   eq 5, {c: 5}[ 'c' ]
 
   # A line where every space in non-breaking
-  eq 1 + 1, 2  
+  eq 1 + 1, 2  
 
 test "don't accidentally stringify keywords", ->
   ok (-> this == 'this')() is false

test/formatting.coffee

@@ -254,3 +254,33 @@ test "#1275: allow indentation before closing brackets", ->
     a = 1
    )
   eq 1, a
+
+test "don’t allow mixing of spaces and tabs for indentation", ->
+  try
+    CoffeeScript.compile '''
+      new Layer
+       x: 0
+      	y: 1
+    '''
+    ok no
+  catch e
+    eq 'indentation mismatch', e.message
+
+test "indentation can be a mix of spaces and tabs, if each line is the same", ->
+  doesNotThrow ->
+    CoffeeScript.compile '''
+      new Layer
+      	 x: 0
+      	 y: 1
+    '''
+
+test "each code block that starts at indentation 0 can use a different style", ->
+    doesNotThrow ->
+      CoffeeScript.compile '''
+        new Layer
+         x: 0
+         y: 1
+        new Layer
+        	x: 0
+        	y: 1
+      '''