Add support for cargo's --config arg (#209)

# Objective

Closes #201.

Allows users to quickly try out different configurations without
modifying `Cargo.toml`.

For us, it's most useful for finding the best default config for the web
profiles.

# Solution

Add support for `--config` argument for `bevy build` and `bevy run`,
analogue to the `cargo` counterpart.
This essentially allows you to modify the `Cargo.toml` without editing
the file, e.g. `--config "profile.web.debug=false"`.

We already exploit `cargo`'s `--config` arg to configure our default web
compilation profiles.
So we have to ensure that the user-provided args _overwrite_ our default
ones.

To do this, we change the default web profiles to be prepended to the
user-provided `--config` args, instead of converting them to `--config`
arguments directly.
Since `--config` is resolved left-to-right, the defaults will be
overwritten by the user.

# Testing

Try for example: 
```
bevy build --config "profile.web.debug=false" --config 'profile.web.inherits= "release"' web
```

Now instead of 
```
Finished `web` profile [unoptimized + debuginfo] target(s)
```

you should get
```
Finished `web` profile [optimized] target(s)
```

Debug info removed and optimizations enabled!
(Of course this can be done easier via `bevy build --release web`, but
like this it's easier to benchmark different compilation profiles to
choose the best default)

src/bin/main.rs

@@ -10,7 +10,7 @@ fn main() -> Result<()> {
             bevy_cli::template::generate_template(&new.name, &new.template, &new.branch)?;
         }
         Subcommands::Lint { args } => bevy_cli::lint::lint(args)?,
-        Subcommands::Build(args) => bevy_cli::build::build(&args)?,
+        Subcommands::Build(mut args) => bevy_cli::build::build(&mut args)?,
         Subcommands::Run(args) => bevy_cli::run::run(&args)?,
     }
 

src/build/mod.rs

@@ -12,7 +12,7 @@ use crate::{
 
 pub mod args;
 
-pub fn build(args: &BuildArgs) -> anyhow::Result<()> {
+pub fn build(args: &mut BuildArgs) -> anyhow::Result<()> {
     if args.is_web() {
         build_web(args)?;
     } else {
@@ -32,7 +32,7 @@ pub fn build(args: &BuildArgs) -> anyhow::Result<()> {
 /// - Optimizing the Wasm binary (in release mode)
 /// - Creating JavaScript bindings
 /// - Creating a bundled folder (if requested)
-pub fn build_web(args: &BuildArgs) -> anyhow::Result<WebBundle> {
+pub fn build_web(args: &mut BuildArgs) -> anyhow::Result<WebBundle> {
     let Some(BuildSubcommands::Web(web_args)) = &args.subcommand else {
         bail!("tried to build for the web without matching arguments");
     };
@@ -49,9 +49,13 @@ pub fn build_web(args: &BuildArgs) -> anyhow::Result<WebBundle> {
         args.profile(),
     )?;
 
-    let cargo_args = args
-        .cargo_args_builder()
-        .append(configure_default_web_profiles(&metadata)?);
+    let mut profile_args = configure_default_web_profiles(&metadata)?;
+    // `--config` args are resolved from left to right,
+    // so the default configuration needs to come before the user args
+    profile_args.append(&mut args.cargo_args.common_args.config);
+    args.cargo_args.common_args.config = profile_args;
+
+    let cargo_args = args.cargo_args_builder();
 
     println!("Compiling to WebAssembly...");
     cargo::build::command().args(cargo_args).ensure_status()?;

src/external_cli/arg_builder.rs

@@ -1,5 +1,6 @@
 /// A helper to make passing arguments to [`Command`](std::process::Command) more convenient.
 #[derive(Debug, Clone)]
+#[must_use]
 pub struct ArgBuilder(Vec<String>);
 
 impl ArgBuilder {
@@ -83,7 +84,7 @@ impl ArgBuilder {
         }
     }
 
-    /// Add an argument with multiple values.
+    /// Add an argument with multiple values, separated by commas.
     ///
     /// # Example
     ///
@@ -107,6 +108,33 @@ impl ArgBuilder {
         }
     }
 
+    /// Add an argument with multiple values, reusing the same argument name.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # use bevy_cli::external_cli::arg_builder::ArgBuilder;
+    /// let features = ["dev", "file_watcher"];
+    /// ArgBuilder::new().add_values_separately("--features", features);
+    /// ```
+    pub fn add_values_separately<N, V>(
+        mut self,
+        name: N,
+        value_list: impl IntoIterator<Item = V>,
+    ) -> Self
+    where
+        N: Into<String>,
+        V: Into<String>,
+    {
+        let arg: String = name.into();
+
+        for value in value_list {
+            self = self.add_with_value(&arg, value);
+        }
+
+        self
+    }
+
     /// Add all arguments from the other builder to this one.
     pub fn append(mut self, mut other: ArgBuilder) -> Self {
         self.0.append(&mut other.0);
@@ -211,6 +239,32 @@ mod tests {
         );
     }
 
+    #[test]
+    fn add_values_separately_adds_multiple_args() {
+        let args = ArgBuilder::new().add_values_separately(
+            "--config",
+            [r#"profile.web.inherits="dev""#, "profile.web.debug=false"],
+        );
+        assert_eq!(
+            args.into_iter().collect::<Vec<String>>(),
+            vec![
+                "--config",
+                r#"profile.web.inherits="dev""#,
+                "--config",
+                "profile.web.debug=false"
+            ]
+        );
+    }
+
+    #[test]
+    fn add_values_separately_empty_no_changes() {
+        let args = ArgBuilder::new().add_values_separately("--config", Vec::<String>::new());
+        assert_eq!(
+            args.into_iter().collect::<Vec<String>>(),
+            Vec::<String>::new()
+        );
+    }
+
     #[test]
     fn append_adds_args_after_self() {
         let args = ArgBuilder::new()

src/external_cli/cargo/build.rs

@@ -4,7 +4,7 @@ use clap::{ArgAction, Args};
 
 use crate::external_cli::arg_builder::ArgBuilder;
 
-use super::{program, CargoCompilationArgs, CargoFeatureArgs, CargoManifestArgs};
+use super::{program, CargoCommonArgs, CargoCompilationArgs, CargoFeatureArgs, CargoManifestArgs};
 
 /// Create a command to run `cargo build`.
 pub(crate) fn command() -> Command {
@@ -15,6 +15,8 @@ pub(crate) fn command() -> Command {
 
 #[derive(Debug, Args)]
 pub struct CargoBuildArgs {
+    #[clap(flatten)]
+    pub common_args: CargoCommonArgs,
     #[clap(flatten)]
     pub package_args: CargoPackageBuildArgs,
     #[clap(flatten)]
@@ -30,6 +32,7 @@ pub struct CargoBuildArgs {
 impl CargoBuildArgs {
     pub(crate) fn args_builder(&self, is_web: bool) -> ArgBuilder {
         ArgBuilder::new()
+            .append(self.common_args.args_builder())
             .append(self.package_args.args_builder())
             .append(self.target_args.args_builder())
             .append(self.feature_args.args_builder())

src/external_cli/cargo/mod.rs

@@ -145,3 +145,21 @@ impl CargoManifestArgs {
             .add_flag_if("--frozen", self.is_frozen)
     }
 }
+
+/// Common options available for `cargo` commands.
+#[derive(Debug, Args, Clone)]
+pub struct CargoCommonArgs {
+    /// Override a configuration value.
+    ///
+    /// The argument should be in TOML syntax of KEY=VALUE,
+    /// or provided as a path to an extra configuration file.
+    /// This flag may be specified multiple times.
+    #[clap(long = "config", value_name = "KEY=VALUE|PATH")]
+    pub config: Vec<String>,
+}
+
+impl CargoCommonArgs {
+    pub(crate) fn args_builder(&self) -> ArgBuilder {
+        ArgBuilder::new().add_values_separately("--config", self.config.iter())
+    }
+}

src/external_cli/cargo/run.rs

@@ -4,7 +4,7 @@ use clap::Args;
 
 use crate::external_cli::arg_builder::ArgBuilder;
 
-use super::{program, CargoCompilationArgs, CargoFeatureArgs, CargoManifestArgs};
+use super::{program, CargoCommonArgs, CargoCompilationArgs, CargoFeatureArgs, CargoManifestArgs};
 
 /// Create a command to run `cargo run`.
 pub(crate) fn command() -> Command {
@@ -15,6 +15,8 @@ pub(crate) fn command() -> Command {
 
 #[derive(Debug, Args, Clone)]
 pub struct CargoRunArgs {
+    #[clap(flatten)]
+    pub common_args: CargoCommonArgs,
     #[clap(flatten)]
     pub package_args: CargoPackageRunArgs,
     #[clap(flatten)]
@@ -30,6 +32,7 @@ pub struct CargoRunArgs {
 impl CargoRunArgs {
     pub(crate) fn args_builder(&self, is_web: bool) -> ArgBuilder {
         ArgBuilder::new()
+            .append(self.common_args.args_builder())
             .append(self.package_args.args_builder())
             .append(self.target_args.args_builder())
             .append(self.feature_args.args_builder())

src/run/args.rs

@@ -60,6 +60,7 @@ impl From<RunArgs> for BuildArgs {
         BuildArgs {
             skip_prompts: args.skip_prompts,
             cargo_args: CargoBuildArgs {
+                common_args: args.cargo_args.common_args,
                 compilation_args: args.cargo_args.compilation_args,
                 feature_args: args.cargo_args.feature_args,
                 manifest_args: args.cargo_args.manifest_args,

src/run/mod.rs

@@ -17,8 +17,8 @@ mod serve;
 
 pub fn run(args: &RunArgs) -> anyhow::Result<()> {
     if let Some(RunSubcommands::Web(web_args)) = &args.subcommand {
-        let build_args = args.clone().into();
-        let web_bundle = build_web(&build_args)?;
+        let mut build_args = args.clone().into();
+        let web_bundle = build_web(&mut build_args)?;
 
         let port = web_args.port;
         let url = format!("http://localhost:{port}");

src/web/profiles.rs

@@ -3,23 +3,23 @@ use std::{collections::HashMap, fs};
 use anyhow::Context as _;
 use toml_edit::DocumentMut;
 
-use crate::external_cli::{arg_builder::ArgBuilder, cargo::metadata::Metadata};
+use crate::external_cli::cargo::metadata::Metadata;
 
 /// Create `--config` args to configure the default profiles to use when compiling for the web.
-pub(crate) fn configure_default_web_profiles(metadata: &Metadata) -> anyhow::Result<ArgBuilder> {
+pub(crate) fn configure_default_web_profiles(metadata: &Metadata) -> anyhow::Result<Vec<String>> {
     let manifest = fs::read_to_string(metadata.workspace_root.join("Cargo.toml"))
         .context("failed to read workspace manifest")?
         .parse::<DocumentMut>()
         .context("failed to parse workspace manifest")?;
 
-    let mut args = ArgBuilder::new();
+    let mut args = Vec::new();
 
     if !is_profile_defined_in_manifest(&manifest, "web") {
-        args = args.append(configure_web_profile());
+        configure_web_profile(&mut args);
     }
 
     if !is_profile_defined_in_manifest(&manifest, "web-release") {
-        args = args.append(configure_web_release_profile());
+        configure_web_release_profile(&mut args);
     }
 
     Ok(args)
@@ -34,21 +34,21 @@ fn is_profile_defined_in_manifest(manifest: &DocumentMut, profile: &str) -> bool
 /// Configure the default profile for web debug builds.
 ///
 /// It is optimized for fast iteration speeds.
-fn configure_web_profile() -> ArgBuilder {
-    configure_profile("web", "dev", HashMap::new())
+fn configure_web_profile(args: &mut Vec<String>) {
+    configure_profile("web", "dev", HashMap::new(), args);
 }
 
 /// Configure the default profile for web release builds.
 ///
 /// It is optimized both for run time performance and loading times.
-fn configure_web_release_profile() -> ArgBuilder {
+fn configure_web_release_profile(args: &mut Vec<String>) {
     let config = HashMap::from_iter([
         // Optimize for size, greatly reducing loading times
         ("opt-level", "s"),
         // Remove debug information, reducing file size further
         ("strip", "debuginfo"),
     ]);
-    configure_profile("web-release", "release", config)
+    configure_profile("web-release", "release", config, args);
 }
 
 /// Create `--config` args for `cargo` to configure a new compilation profile.
@@ -61,17 +61,17 @@ fn configure_web_release_profile() -> ArgBuilder {
 /// # config
 /// key = "value"
 /// ```
-fn configure_profile(profile: &str, inherits: &str, config: HashMap<&str, &str>) -> ArgBuilder {
-    let mut args = ArgBuilder::new().add_with_value(
-        "--config",
-        format!(r#"profile.{profile}.inherits="{inherits}""#),
-    );
+fn configure_profile(
+    profile: &str,
+    inherits: &str,
+    config: HashMap<&str, &str>,
+    args: &mut Vec<String>,
+) {
+    args.push(format!(r#"profile.{profile}.inherits="{inherits}""#));
 
     for (key, value) in config {
-        args = args.add_with_value("--config", format!(r#"profile.{profile}.{key}="{value}""#));
+        args.push(format!(r#"profile.{profile}.{key}="{value}""#));
     }
-
-    args
 }
 
 #[cfg(test)]