api: Flatten and simplify Servo preferences (#34966)

Flatten and simplify Servo's preferences code. In addition, have both
preferences and options passed in as arguments to `Servo::new()` and
make sure not to use the globally set preferences in `servoshell` (as
much as possible now).

Instead of a complex procedural macro to generate preferences, just
expose a very simple derive macro that adds string based getters and
setters.

- All command-line parsing is moved to servoshell.
- There is no longer the concept of a missing preference.
- Preferences no longer have to be part of the resources bundle because
  they now have reasonable default values.
- servoshell specific preferences are no longer part of the preferences
  exposed by the Servo API.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/config/macro/Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "servo_config_macro"
+version.workspace = true
+authors.workspace = true
+license.workspace = true
+edition.workspace = true
+publish.workspace = true
+rust-version.workspace = true
+
+[lib]
+name = "servo_config_macro"
+proc-macro = true
+path = "lib.rs"
+
+[dependencies]
+proc-macro2 = { workspace = true }
+quote = { workspace = true }
+syn = { workspace = true }
+synstructure = { workspace = true }

components/config/macro/lib.rs

@@ -0,0 +1,55 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+use proc_macro2::TokenStream;
+use quote::quote;
+use syn::{Data, Fields};
+use synstructure::decl_derive;
+
+decl_derive!([ServoPreferences] => servo_preferences_derive);
+
+/// A derive macro that adds string-based getter and setter for each field of this struct
+/// (enums and other types are not supported). Each field must be able to be convertable
+/// (with `into()`) into a `PrefValue`.
+fn servo_preferences_derive(input: synstructure::Structure) -> TokenStream {
+    let ast = input.ast();
+
+    let Data::Struct(ref data) = ast.data else {
+        unimplemented!();
+    };
+    let Fields::Named(ref named_fields) = data.fields else {
+        unimplemented!()
+    };
+
+    let mut get_match_cases = quote!();
+    for field in named_fields.named.iter() {
+        let name = field.ident.as_ref().unwrap();
+        get_match_cases.extend(quote!(stringify!(#name) => self.#name.clone().into(),))
+    }
+
+    let mut set_match_cases = quote!();
+    for field in named_fields.named.iter() {
+        let name = field.ident.as_ref().unwrap();
+        set_match_cases.extend(quote!(stringify!(#name) => self.#name = value.try_into().unwrap(),))
+    }
+
+    let structure_name = &ast.ident;
+    quote! {
+        impl #structure_name {
+            pub fn get_value(&self, name: &str) -> PrefValue {
+                match name {
+                    #get_match_cases
+                    _ => { panic!("Unknown preference: {:?}", name); }
+                }
+            }
+
+            pub fn set_value(&mut self, name: &str, value: PrefValue) {
+                match name {
+                    #set_match_cases
+                    _ => { panic!("Unknown preference: {:?}", name); }
+                }
+            }
+        }
+    }
+}