project_settings: Fix default settings values for DiagnosticsSettings (#30686)

Follow-up to #30565

This PR fixes the default settings values for the `DiagnosticsSettings`.
The issue here was that due to the `#[derive(Default)]`, `button` would
be false by default, which unintentionally hid the diagnostics button by
default. The `#[serde(default = `default_true`)]` would only apply iff
the diagnostics key was already present in the user's settings. Thus, if
you have

```json
{
    "diagnostics": {...}
}
```

in your settings, the button would show (given it was not disabled).
However, if the key was not present, the button was not shown: Due to
the derived default for the entire struct, the value would be false.

This PR fixes this by implementing the default instead and moving the
`#[serde(default)]` up to the level of the struct.
I also did the same for the inline diagnostics settings, which already
had a default impl and thus only needed the serde default on the struct
instead of on all the struct fields.

Lastly, I simplified the title bar settings, since the serde attributes
previously had no effect anyway (deserialization happened in the
`TitlebarSettingsContent`, so these attributes had no effect) and we can
remove the `TitlebarSettingsContent` as well as the attributes if we
implement a proper default implementation instead.

Release Notes:

- Fixed the diagnostics status bar button being hidden by default.

crates/project/src/project_settings.rs

@@ -118,22 +118,19 @@ pub enum DirenvSettings {
     Direct,
 }
 
-#[derive(Clone, Debug, Default, Serialize, Deserialize, JsonSchema)]
+#[derive(Clone, Debug, Serialize, Deserialize, JsonSchema)]
+#[serde(default)]
 pub struct DiagnosticsSettings {
     /// Whether to show the project diagnostics button in the status bar.
-    #[serde(default = "default_true")]
     pub button: bool,
 
     /// Whether or not to include warning diagnostics.
-    #[serde(default = "default_true")]
     pub include_warnings: bool,
 
     /// Settings for showing inline diagnostics.
-    #[serde(default)]
     pub inline: InlineDiagnosticsSettings,
 
     /// Configuration, related to Rust language diagnostics.
-    #[serde(default)]
     pub cargo: Option<CargoDiagnosticsSettings>,
 }
 
@@ -146,33 +143,29 @@ impl DiagnosticsSettings {
 }
 
 #[derive(Clone, Copy, Debug, Serialize, Deserialize, JsonSchema)]
+#[serde(default)]
 pub struct InlineDiagnosticsSettings {
     /// Whether or not to show inline diagnostics
     ///
     /// Default: false
-    #[serde(default)]
     pub enabled: bool,
     /// Whether to only show the inline diagnostics after a delay after the
     /// last editor event.
     ///
     /// Default: 150
-    #[serde(default = "default_inline_diagnostics_debounce_ms")]
     pub update_debounce_ms: u64,
     /// The amount of padding between the end of the source line and the start
     /// of the inline diagnostic in units of columns.
     ///
     /// Default: 4
-    #[serde(default = "default_inline_diagnostics_padding")]
     pub padding: u32,
     /// The minimum column to display inline diagnostics. This setting can be
     /// used to horizontally align inline diagnostics at some position. Lines
     /// longer than this value will still push diagnostics further to the right.
     ///
     /// Default: 0
-    #[serde(default)]
     pub min_column: u32,
 
-    #[serde(default)]
     pub max_severity: Option<DiagnosticSeverity>,
 }
 
@@ -211,24 +204,27 @@ impl DiagnosticSeverity {
     }
 }
 
-impl Default for InlineDiagnosticsSettings {
+impl Default for DiagnosticsSettings {
     fn default() -> Self {
         Self {
-            enabled: false,
-            update_debounce_ms: default_inline_diagnostics_debounce_ms(),
-            padding: default_inline_diagnostics_padding(),
-            min_column: 0,
-            max_severity: None,
+            button: true,
+            include_warnings: true,
+            inline: Default::default(),
+            cargo: Default::default(),
         }
     }
 }
 
-fn default_inline_diagnostics_debounce_ms() -> u64 {
-    150
-}
-
-fn default_inline_diagnostics_padding() -> u32 {
-    4
+impl Default for InlineDiagnosticsSettings {
+    fn default() -> Self {
+        Self {
+            enabled: false,
+            update_debounce_ms: 150,
+            padding: 4,
+            min_column: 0,
+            max_severity: None,
+        }
+    }
 }
 
 #[derive(Copy, Clone, Debug, Default, Serialize, Deserialize, JsonSchema)]