Indent guides (#11503)

Builds on top of existing work from #2249, but here's a showcase:


https://github.com/zed-industries/zed/assets/53836821/4b346965-6654-496c-b379-75425d9b493f

TODO:
- [x] handle line wrapping
- [x] implement handling in multibuffer (crashes currently)
- [x] add configuration option
- [x] new theme properties? What colors to use?
- [x] Possibly support indents with different colors or background
colors
- [x] investigate edge cases (e.g. indent guides and folds continue on
empty lines even if the next indent is different)
- [x] add more tests (also test `find_active_indent_index`)
- [x] docs (will do in a follow up PR)
- [x] benchmark performance impact

Release Notes:

- Added indent guides
([#5373](https://github.com/zed-industries/zed/issues/5373))

---------

Co-authored-by: Nate Butler <1714999+iamnbutler@users.noreply.github.com>
Co-authored-by: Remco <djsmits12@gmail.com>

crates/language/src/language_settings.rs

@@ -78,6 +78,8 @@ pub struct LanguageSettings {
     pub show_wrap_guides: bool,
     /// Character counts at which to show wrap guides in the editor.
     pub wrap_guides: Vec<usize>,
+    /// Indent guide related settings.
+    pub indent_guides: IndentGuideSettings,
     /// Whether or not to perform a buffer format before saving.
     pub format_on_save: FormatOnSave,
     /// Whether or not to remove any trailing whitespace from lines of a buffer
@@ -242,6 +244,9 @@ pub struct LanguageSettingsContent {
     /// Default: []
     #[serde(default)]
     pub wrap_guides: Option<Vec<usize>>,
+    /// Indent guide related settings.
+    #[serde(default)]
+    pub indent_guides: Option<IndentGuideSettings>,
     /// Whether or not to perform a buffer format before saving.
     ///
     /// Default: on
@@ -411,6 +416,59 @@ pub enum Formatter {
     CodeActions(HashMap<String, bool>),
 }
 
+/// The settings for indent guides.
+#[derive(Default, Debug, Copy, Clone, PartialEq, Eq, Serialize, Deserialize, JsonSchema)]
+pub struct IndentGuideSettings {
+    /// Whether to display indent guides in the editor.
+    ///
+    /// Default: true
+    #[serde(default = "default_true")]
+    pub enabled: bool,
+    /// The width of the indent guides in pixels, between 1 and 10.
+    ///
+    /// Default: 1
+    #[serde(default = "line_width")]
+    pub line_width: u32,
+    /// Determines how indent guides are colored.
+    ///
+    /// Default: Fixed
+    #[serde(default)]
+    pub coloring: IndentGuideColoring,
+    /// Determines how indent guide backgrounds are colored.
+    ///
+    /// Default: Disabled
+    #[serde(default)]
+    pub background_coloring: IndentGuideBackgroundColoring,
+}
+
+fn line_width() -> u32 {
+    1
+}
+
+/// Determines how indent guides are colored.
+#[derive(Default, Debug, Copy, Clone, PartialEq, Eq, Serialize, Deserialize, JsonSchema)]
+#[serde(rename_all = "snake_case")]
+pub enum IndentGuideColoring {
+    /// Do not render any lines for indent guides.
+    Disabled,
+    /// Use the same color for all indentation levels.
+    #[default]
+    Fixed,
+    /// Use a different color for each indentation level.
+    IndentAware,
+}
+
+/// Determines how indent guide backgrounds are colored.
+#[derive(Default, Debug, Copy, Clone, PartialEq, Eq, Serialize, Deserialize, JsonSchema)]
+#[serde(rename_all = "snake_case")]
+pub enum IndentGuideBackgroundColoring {
+    /// Do not render any background for indent guides.
+    #[default]
+    Disabled,
+    /// Use a different color for each indentation level.
+    IndentAware,
+}
+
 /// The settings for inlay hints.
 #[derive(Copy, Clone, Debug, Serialize, Deserialize, JsonSchema, PartialEq, Eq)]
 pub struct InlayHintSettings {
@@ -715,6 +773,7 @@ fn merge_settings(settings: &mut LanguageSettings, src: &LanguageSettingsContent
     );
     merge(&mut settings.show_wrap_guides, src.show_wrap_guides);
     merge(&mut settings.wrap_guides, src.wrap_guides.clone());
+    merge(&mut settings.indent_guides, src.indent_guides);
     merge(
         &mut settings.code_actions_on_format,
         src.code_actions_on_format.clone(),