diff --git a/src/bin/main.rs b/src/bin/main.rs
index 1b1c1d412c83..03105a902208 100644
--- a/src/bin/main.rs
+++ b/src/bin/main.rs
@@ -26,7 +26,7 @@ use getopts::{Matches, Options};
 
 use rustfmt::{
     emit_post_matter, emit_pre_matter, format_and_emit_report, load_config, CliOptions, Config,
-    FileName, FmtResult, Input, Summary, Verbosity, WriteMode, WRITE_MODE_LIST,
+    FileName, FmtResult, Input, Summary, Verbosity, WriteMode,
 };
 
 fn main() {
@@ -111,7 +111,13 @@ fn make_opts() -> Options {
          found reverts to the input file path",
         "[Path for the configuration file]",
     );
-    opts.optopt("", "emit", "What data to emit and how", WRITE_MODE_LIST);
+    let is_nightly = is_nightly();
+    let emit_opts = if is_nightly {
+        "[files|stdout|coverage|checkstyle]"
+    } else {
+        "[files|stdout]"
+    };
+    opts.optopt("", "emit", "What data to emit and how", emit_opts);
     opts.optflagopt(
         "h",
         "help",
@@ -129,7 +135,7 @@ fn make_opts() -> Options {
     opts.optflag("q", "quiet", "Print less output");
     opts.optflag("V", "version", "Show version information");
 
-    if is_nightly() {
+    if is_nightly {
         opts.optflag(
             "",
             "unstable-features",
diff --git a/src/config/options.rs b/src/config/options.rs
index 15915bb99760..659152258605 100644
--- a/src/config/options.rs
+++ b/src/config/options.rs
@@ -14,7 +14,7 @@ use config::config_type::ConfigType;
 use config::file_lines::FileLines;
 use config::lists::*;
 use config::Config;
-use {FmtResult, WRITE_MODE_LIST};
+use FmtResult;
 
 use failure::err_msg;
 
@@ -174,14 +174,12 @@ configuration_option_enum! { ReportTactic:
 }
 
 configuration_option_enum! { WriteMode:
-    // Backs the original file up and overwrites the original.
-    Replace,
     // Overwrites original file without backup.
     Overwrite,
+    // Backs the original file up and overwrites the original.
+    Replace,
     // Writes the output to stdout.
     Display,
-    // Writes the diff to stdout.
-    Diff,
     // Displays how much of the input file was processed
     Coverage,
     // Unfancy stdout
@@ -196,6 +194,13 @@ configuration_option_enum! { WriteMode:
     None,
 }
 
+const STABLE_WRITE_MODES: [WriteMode; 4] = [
+    WriteMode::Replace,
+    WriteMode::Overwrite,
+    WriteMode::Display,
+    WriteMode::Check,
+];
+
 configuration_option_enum! { Color:
     // Always use color, whether it is a piped or terminal output
     Always,
@@ -331,7 +336,7 @@ pub struct CliOptions {
     pub quiet: bool,
     pub verbose: bool,
     pub config_path: Option<PathBuf>,
-    pub write_mode: Option<WriteMode>,
+    pub write_mode: WriteMode,
     pub check: bool,
     pub color: Option<Color>,
     pub file_lines: FileLines, // Default is all lines in all files.
@@ -355,7 +360,7 @@ impl CliOptions {
             options.unstable_features = matches.opt_present("unstable-features");
         }
 
-        if options.unstable_features {
+        if !options.unstable_features {
             if matches.opt_present("skip-children") {
                 options.skip_children = Some(true);
             }
@@ -375,16 +380,21 @@ impl CliOptions {
                 return Err(format_err!("Invalid to use `--emit` and `--check`"));
             }
             if let Ok(write_mode) = write_mode_from_emit_str(emit_str) {
-                if write_mode == WriteMode::Overwrite && matches.opt_present("backup") {
-                    options.write_mode = Some(WriteMode::Replace);
-                } else {
-                    options.write_mode = Some(write_mode);
-                }
+                options.write_mode = write_mode;
             } else {
+                return Err(format_err!("Invalid value for `--emit`"));
+            }
+        }
+
+        if options.write_mode == WriteMode::Overwrite && matches.opt_present("backup") {
+            options.write_mode = WriteMode::Replace;
+        }
+
+        if !rust_nightly {
+            if !STABLE_WRITE_MODES.contains(&options.write_mode) {
                 return Err(format_err!(
-                    "Invalid value for `--emit`: {}, expected one of {}",
-                    emit_str,
-                    WRITE_MODE_LIST
+                    "Invalid value for `--emit` - using an unstable \
+                     value without `--unstable-features`",
                 ));
             }
         }
@@ -417,8 +427,8 @@ impl CliOptions {
         }
         if self.check {
             config.set().write_mode(WriteMode::Check);
-        } else if let Some(write_mode) = self.write_mode {
-            config.set().write_mode(write_mode);
+        } else {
+            config.set().write_mode(self.write_mode);
         }
         if let Some(color) = self.color {
             config.set().color(color);
diff --git a/src/filemap.rs b/src/filemap.rs
index fc86beeede46..d61a3aa2a74e 100644
--- a/src/filemap.rs
+++ b/src/filemap.rs
@@ -156,19 +156,6 @@ where
             }
             write_system_newlines(out, text, config)?;
         }
-        WriteMode::Diff => {
-            let filename = filename_to_path();
-            if let Ok((ori, fmt)) = source_and_formatted_text(text, filename, config) {
-                let mismatch = make_diff(&ori, &fmt, 3);
-                let has_diff = !mismatch.is_empty();
-                print_diff(
-                    mismatch,
-                    |line_num| format!("Diff in {} at line {}:", filename.display(), line_num),
-                    config,
-                );
-                return Ok(has_diff);
-            }
-        }
         WriteMode::Modified => {
             let filename = filename_to_path();
             if let Ok((ori, fmt)) = source_and_formatted_text(text, filename, config) {
diff --git a/src/lib.rs b/src/lib.rs
index 5fb3e4b03e35..c50c45fbd36a 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -68,9 +68,6 @@ pub use config::{file_lines, load_config, Config, Verbosity, WriteMode};
 
 pub type FmtResult<T> = std::result::Result<T, failure::Error>;
 
-// FIXME: this is badly named since the user-facing name is `emit` not `write-mode`.
-pub const WRITE_MODE_LIST: &str = "[files|stdout|coverage|checkstyle]";
-
 #[macro_use]
 mod utils;