Support Implementing Deserialize::deserialize_in_place

[commonsensesoftware]

Background

Configuration binding (e.g. Configuration::bind) calls through Deserialize::deserialize_in_place provided by the serde crate. The default implementation of Deserialize::deserialize_in_place simply calls Deserialize::deserialize and completely replaces the provided struct.

Goal

The ideal scenario would be to implement, or provide a callable implementation for, Deserialize::deserialize_in_place that a consumer can leverage. This would allow partially updating the supplied struct without requiring the presence of all configuration keys for each struct field.

It's not clear whether this is possible. The solution may require one or more macros.

[NicolasMelot]

I sent a pull request for an implementation that takes input data from a local data structure as discussed in issue #9. Instead of the Deserialize::deserialize_in_place path suggested here, I went for finding out the common data representation serde uses and using it to serialize a local data structure into it, then use it to create a ConfigurationProvider. I did not thoroughly test it yet, but it works with the silly example shown below:

Cargo.toml:

[dependencies]
more-config = { path = "../more-rs-config/src", version = "2.1.0", features = [ "cmd","mem","json","binder","struct" ] }
serde = { version = "*", features = ["derive"] }
serde_json = { version = "1.0" }
serde-intermediate = "1.6"

src/main.rs:

use serde::{Serialize, Deserialize};

#[derive(Default, Deserialize, Serialize, Clone)]
#[serde(rename_all(deserialize = "PascalCase"))]
#[serde(rename_all(serialize = "PascalCase"))]
struct Client {
    region: String,
    url: String,
}

#[derive(Default, Deserialize, Serialize, Clone)]
#[serde(rename_all(deserialize = "PascalCase"))]
#[serde(rename_all(serialize = "PascalCase"))]
struct AppOptions {
    text: String,
    demo: bool,
    clients: Vec<Client>,
}

use config::{*, ext::*};

fn main() {
    let default = AppOptions {
        text: String::from("Default text from struct"),
        demo: true,
        clients: Vec::new()
    };  

    let config = DefaultConfigurationBuilder::new()
        .add_struct(default.clone())
        .add_json_file("base.json")
        .build()
        .unwrap();
    let options: AppOptions = config.reify();

    let app: AppOptions = tmerge(default.clone(), options).unwrap_or(default);
        
    println!("{}", serde_json::to_string_pretty(&app).unwrap_or(String::from("No data")));
}

base.json:

{
  "clients": [{
    "region": "us-west",
    "url": "https://tempuri.org"
  }]  
}

If I remove the line .add_struct(...) in main.rs, then the program produces an error because of the lack of field text in base.json:

user@host:~/sandbox/demo/demo cargo run
   Compiling more-config v2.1.0 (/home/user/sandbox/demo/more-rs-config/src)
   Compiling demo v0.1.0 (/home/user/sandbox/demo/demo)
warning: unused variable: `default`
  --> src/main.rs:29:9
   |
29 |     let default = AppOptions {
   |         ^^^^^^^ help: if this is intentional, prefix it with an underscore: `_default`
   |
   = note: `#[warn(unused_variables)]` on by default

warning: `demo` (bin "demo") generated 1 warning (run `cargo fix --bin "demo"` to apply 1 suggestion)
    Finished dev [unoptimized + debuginfo] target(s) in 1.14s
     Running `target/debug/demo`
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: MissingValue("Text")', /home/user/sandbox/demo/more-rs-config/src/binder.rs:42:32
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
user@host:~/sandbox/demo/demo$ 

But if I restore the line, then the data is loaded correctly first from the struct, and any additional data in base.json overrides the initial configuration value:

user@host:~/sandbox/demo/demo$ cargo run
   Compiling more-config v2.1.0 (/home/user/sandbox/demo/more-rs-config/src)
   Compiling demo v0.1.0 (/home/user/sandbox/demo/demo)
    Finished dev [unoptimized + debuginfo] target(s) in 1.28s
     Running `target/debug/demo`
{
  "Text": "Default text from struct",
  "Demo": true,
  "Clients": [
    {
      "Region": "us-west",
      "Url": "https://tempuri.org"
    }
  ]
}
user@host:~/sandbox/demo/demo$

I need to write a few tests to make sure silly use cases are handled satisfactorily. Feel free to suggest some here. Until then, please have a look at the code and see if that is an acceptable solution to the issue.

[NicolasMelot]

I pushed another commit with unit tests, some doc for the new provider and a few updates to disallow pushing primitive data, (e.g. integers or booleans). Now one can push vectors, tuples and maps of any type, although string keys are much preferred. Please give feedback.