Add maximumLength schema enforcer for strings

spec/config-spec.coffee

@@ -1380,6 +1380,16 @@ describe "Config", ->
           expect(atom.config.set('foo.bar.aString', nope: 'nope')).toBe false
           expect(atom.config.get('foo.bar.aString')).toBe 'ok'
 
+        describe 'when the schema has a "maximumLength" key', ->
+          it "trims the string to be no longer than the specified maximum", ->
+            schema =
+              type: 'string'
+              default: 'ok'
+              maximumLength: 3
+            atom.config.setSchema('foo.bar.aString', schema)
+            atom.config.set('foo.bar.aString', 'abcdefg')
+            expect(atom.config.get('foo.bar.aString')).toBe 'abc'
+
       describe 'when the value has an "object" type', ->
         beforeEach ->
           schema =

src/config.coffee

@@ -1060,6 +1060,12 @@ Config.addSchemaEnforcers
         throw new Error("Validation failed at #{keyPath}, #{JSON.stringify(value)} must be a string")
       value
 
+    validateMaximumLength: (keyPath, value, schema) ->
+      if typeof schema.maximumLength is 'number' and value.length > schema.maximumLength
+        value.slice(0, schema.maximumLength)
+      else
+        value
+
   'null':
     # null sort of isnt supported. It will just unset in this case
     coerce: (keyPath, value, schema) ->