warn on use of default value for an option

Contributing.md

@@ -205,7 +205,7 @@ and `ListFormatting` the key structure for configuration. You'll need to make a
 
 Rustfmt strives to be highly configurable. Often the first part of a patch is
 creating a configuration option for the feature you are implementing. All
-handling of configuration options is done in [src/config.rs](src/config.rs). Look for the
+handling of configuration options is done in [src/config/mod.rs](src/config/mod.rs). Look for the
 `create_config!` macro at the end of the file for all the options. The rest of
 the file defines a bunch of enums used for options, and the machinery to produce
 the config struct and parse a config file, etc. Checking an option is done by

src/config/config_type.rs

@@ -281,6 +281,7 @@ macro_rules! create_config {
                 match key {
                     $(
                         stringify!($i) => {
+                            self.$i.1 = true;
                             self.$i.2 = val.parse::<$ty>()
                                 .expect(&format!("Failed to parse override for {} (\"{}\") as a {}",
                                                  stringify!($i),
@@ -421,6 +422,16 @@ macro_rules! create_config {
             fn set_ignore(&mut self, dir: &Path) {
                 self.ignore.2.add_prefix(dir);
             }
+
+            /// Returns true if the config key was explicitely set and is the default value.
+            pub fn is_default(&self, key: &str) -> bool {
+                $(
+                    if let stringify!($i) = key {
+                        return self.$i.1 && self.$i.2 == $def;
+                    }
+                 )+
+                false
+            }
         }
 
         // Template for the default configuration

src/config/file_lines.rs

@@ -126,7 +126,7 @@ impl Range {
 /// It is represented as a multimap keyed on file names, with values a collection of
 /// non-overlapping ranges sorted by their start point. An inner `None` is interpreted to mean all
 /// lines in all files.
-#[derive(Clone, Debug, Default)]
+#[derive(Clone, Debug, Default, PartialEq)]
 pub struct FileLines(Option<HashMap<FileName, Vec<Range>>>);
 
 /// Normalizes the ranges so that the invariants for `FileLines` hold: ranges are non-overlapping,

src/config/options.rs

@@ -223,7 +223,7 @@ configuration_option_enum! { Verbosity:
     Quiet,
 }
 
-#[derive(Deserialize, Serialize, Clone, Debug)]
+#[derive(Deserialize, Serialize, Clone, Debug, PartialEq)]
 pub struct WidthHeuristics {
     // Maximum width of the args of a function call before falling back
     // to vertical formatting.
@@ -293,7 +293,7 @@ impl Default for EmitMode {
 }
 
 /// A set of directories, files and modules that rustfmt should ignore.
-#[derive(Default, Deserialize, Serialize, Clone, Debug)]
+#[derive(Default, Deserialize, Serialize, Clone, Debug, PartialEq)]
 pub struct IgnoreList(HashSet<PathBuf>);
 
 impl IgnoreList {

src/test/mod.rs

@@ -363,6 +363,9 @@ fn read_config(filename: &Path) -> Config {
     for (key, val) in &sig_comments {
         if key != "target" && key != "config" {
             config.override_value(key, val);
+            if config.is_default(key) {
+                warn!("Default value {} used explicitly for {}", val, key);
+            }
         }
     }