Implement Serialize on IgnoreList (#3565)

* Implement Serialize on IgnoreList

* Add a test for #3536

src/config/mod.rs

@@ -462,6 +462,79 @@ mod test {
         assert_eq!(s.contains("(unstable)"), true);
     }
 
+    #[test]
+    fn test_dump_default_config() {
+        const DEFAULT_CONFIG: &str = r#"max_width = 100
+hard_tabs = false
+tab_spaces = 4
+newline_style = "Auto"
+use_small_heuristics = "Default"
+indent_style = "Block"
+wrap_comments = false
+format_code_in_doc_comments = false
+comment_width = 80
+normalize_comments = false
+normalize_doc_attributes = false
+license_template_path = ""
+format_strings = false
+format_macro_matchers = false
+format_macro_bodies = true
+empty_item_single_line = true
+struct_lit_single_line = true
+fn_single_line = false
+where_single_line = false
+imports_indent = "Block"
+imports_layout = "Mixed"
+merge_imports = false
+reorder_imports = true
+reorder_modules = true
+reorder_impl_items = false
+type_punctuation_density = "Wide"
+space_before_colon = false
+space_after_colon = true
+spaces_around_ranges = false
+binop_separator = "Front"
+remove_nested_parens = true
+combine_control_expr = true
+overflow_delimited_expr = false
+struct_field_align_threshold = 0
+enum_discrim_align_threshold = 0
+match_arm_blocks = true
+force_multiline_blocks = false
+fn_args_density = "Tall"
+brace_style = "SameLineWhere"
+control_brace_style = "AlwaysSameLine"
+trailing_semicolon = true
+trailing_comma = "Vertical"
+match_block_trailing_comma = false
+blank_lines_upper_bound = 1
+blank_lines_lower_bound = 0
+edition = "2015"
+version = "One"
+inline_attribute_width = 0
+merge_derives = true
+use_try_shorthand = false
+use_field_init_shorthand = false
+force_explicit_abi = true
+condense_wildcard_suffixes = false
+color = "Auto"
+required_version = "1.2.2"
+unstable_features = false
+disable_all_formatting = false
+skip_children = false
+hide_parse_errors = false
+error_on_line_overflow = false
+error_on_unformatted = false
+report_todo = "Never"
+report_fixme = "Never"
+ignore = []
+emit_mode = "Files"
+make_backup = false
+"#;
+        let toml = Config::default().all_options().to_toml().unwrap();
+        assert_eq!(&toml, DEFAULT_CONFIG);
+    }
+
     // FIXME(#2183): these tests cannot be run in parallel because they use env vars.
     // #[test]
     // fn test_as_not_nightly_channel() {

src/config/options.rs

@@ -5,7 +5,8 @@ use std::path::{Path, PathBuf};
 use atty;
 use config_proc_macro::config_type;
 use serde::de::{SeqAccess, Visitor};
-use serde::{Deserialize, Deserializer, Serialize};
+use serde::ser::SerializeSeq;
+use serde::{Deserialize, Deserializer, Serialize, Serializer};
 
 use crate::config::lists::*;
 use crate::config::Config;
@@ -254,16 +255,27 @@ impl Default for EmitMode {
 }
 
 /// A set of directories, files and modules that rustfmt should ignore.
-#[derive(Default, Serialize, Clone, Debug, PartialEq)]
+#[derive(Default, Clone, Debug, PartialEq)]
 pub struct IgnoreList {
     /// A set of path specified in rustfmt.toml.
-    #[serde(flatten)]
     path_set: HashSet<PathBuf>,
     /// A path to rustfmt.toml.
-    #[serde(skip_serializing)]
     rustfmt_toml_path: PathBuf,
 }
 
+impl Serialize for IgnoreList {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        let mut seq = serializer.serialize_seq(Some(self.path_set.len()))?;
+        for e in &self.path_set {
+            seq.serialize_element(e)?;
+        }
+        seq.end()
+    }
+}
+
 impl<'de> Deserialize<'de> for IgnoreList {
     fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
     where