add pretty sting serialization

src/ser.rs

@@ -214,6 +214,22 @@ impl<'a> Serializer<'a> {
         }
     }
 
+    /// instantiate a "pretty" formatter
+    /// 
+    /// TODO: add better docs
+    pub fn pretty(dst: &'a mut String) -> Serializer<'a> {
+        Serializer {
+            dst: dst,
+            state: State::End,
+            settings: Settings {
+                array: Some(ArraySettings {
+                    indent: 4,
+                }),
+                pretty_string: true,
+            },
+        }
+    }
+
     fn display<T: fmt::Display>(&mut self,
                                 t: T,
                                 type_: &'static str) -> Result<(), Error> {
@@ -300,12 +316,27 @@ impl<'a> Serializer<'a> {
     }
 
     fn emit_str(&mut self, value: &str) -> Result<(), Error> {
+        let do_pretty = if self.settings.pretty_string {
+            value.contains('\n')
+        } else {
+            false
+        };
+        if do_pretty {
+            drop(write!(self.dst, "'''\n"));
+        } else {
             drop(write!(self.dst, "\""));
+        }
         for ch in value.chars() {
             match ch {
                 '\u{8}' => drop(write!(self.dst, "\\b")),
                 '\u{9}' => drop(write!(self.dst, "\\t")),
-                '\u{a}' => drop(write!(self.dst, "\\n")),
+                '\u{a}' => {
+                    if do_pretty {
+                        drop(write!(self.dst, "\n"));
+                    } else {
+                        drop(write!(self.dst, "\\n"));
+                    }
+                },
                 '\u{c}' => drop(write!(self.dst, "\\f")),
                 '\u{d}' => drop(write!(self.dst, "\\r")),
                 '\u{22}' => drop(write!(self.dst, "\\\"")),
@@ -316,7 +347,11 @@ impl<'a> Serializer<'a> {
                 ch => drop(write!(self.dst, "{}", ch)),
             }
         }
+        if do_pretty {
+            drop(write!(self.dst, "'''"));
+        } else {
             drop(write!(self.dst, "\""));
+        }
         Ok(())
     }
 

tests/pretty.rs

@@ -0,0 +1,20 @@
+extern crate toml;
+extern crate serde;
+
+use serde::ser::Serialize;
+
+const example: &str = "\
+[example]
+text = '''
+this is the first line
+this is the second line
+'''
+";
+
+#[test]
+fn test_pretty() {
+    let value: toml::Value = toml::from_str(example).unwrap();
+    let mut result = String::with_capacity(128);
+    value.serialize(&mut toml::Serializer::pretty(&mut result)).unwrap();
+    assert_eq!(example, &result);
+}