Rustelo/info/leptos_serve_fix.md

Leptos Serve Multiple Binary Targets Fix

Problem

When running cargo leptos serve, the following error occurred:

Error:
   0: at `/Users/jesusperezlorenzo/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/cargo-leptos-0.2.35/src/lib.rs:43:76`
   1: at `/Users/jesusperezlorenzo/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/cargo-leptos-0.2.35/src/config/mod.rs:58:84`
   2: Several bin targets found for member "server", please specify which one to use with: [[workspace.metadata.leptos]] bin-target = "name"

Root Cause

The server crate had multiple binary targets:

1. server (from src/main.rs) - the main application server
2. config_tool (from src/bin/config_tool.rs) - configuration management utility

Cargo-leptos couldn't determine which binary target to use for the leptos serve command.

Solution

1. Added bin-target specification to workspace configuration

In template/Cargo.toml, added the bin-target specification:

[[workspace.metadata.leptos]]
# The name used by wasm-bindgen/cargo-leptos for the JS/WASM bundle. Defaults to the crate name
output-name = "website"
# Specify which binary target to use (fixes multiple bin targets error)
bin-target = "server"
# ... rest of configuration

2. Added explicit binary target definitions

In template/server/Cargo.toml, added explicit binary targets:

# Binary targets
[[bin]]
name = "server"
path = "src/main.rs"

[[bin]]
name = "config_tool"
path = "src/bin/config_tool.rs"

3. Fixed tokio LocalSet runtime issue

Added proper LocalSet configuration to handle leptos local tasks:

In template/server/src/main.rs:

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    // Create a LocalSet to handle leptos local tasks
    let local = tokio::task::LocalSet::new();
    local.run_until(run_server()).await
}

async fn run_server() -> Result<(), Box<dyn std::error::Error>> {
    // All server logic moved here
    // ...
}

4. Fixed WASM compatibility issue

Added the js feature to uuid dependencies for WASM target compatibility:

In template/server/Cargo.toml:

uuid = { version = "1.17", features = ["v4", "serde", "js"], optional = true }

In template/shared/Cargo.toml:

uuid = { version = "1.17", features = ["v4", "serde", "js"] }

Verification

After applying the fixes:

1. ✅ cargo leptos serve no longer shows the "Several bin targets found" error
2. ✅ No more "spawn_local called from outside of a task::LocalSet" runtime panics
3. ✅ cargo leptos build completes successfully
4. ✅ WASM compilation works without uuid randomness errors
5. ✅ All project tests continue to pass

Usage

Now you can use cargo-leptos commands without issues:

# Start development server
cargo leptos serve

# Start with custom configuration
cargo leptos serve -- -c config.dev.toml

# Build the project
cargo leptos build

# Build for production
cargo leptos build --release

# Watch for changes
cargo leptos watch

Files Modified

1. template/Cargo.toml - Added bin-target = "server" to leptos metadata
2. template/server/src/main.rs - Added LocalSet configuration for leptos runtime
3. template/server/Cargo.toml - Added explicit binary targets and fixed uuid features
4. template/shared/Cargo.toml - Fixed uuid features for WASM compatibility
5. template/README.md - Added leptos serve documentation
6. template/docs/LEPTOS_SERVE.md - Created comprehensive leptos documentation

Key Configuration

The critical fix is in the workspace Cargo.toml:

[[workspace.metadata.leptos]]
bin-target = "server"  # This line fixes the multiple targets error
bin-package = "server"
lib-package = "client"

This tells cargo-leptos to use the server binary target (from src/main.rs) instead of the config_tool binary when running leptos commands.

Additional Benefits

The fixes provide:

• Clearer project structure documentation
• Better IDE support for binary targets
• More predictable build behavior
• Proper leptos runtime context for local task spawning
• Enhanced error handling and graceful shutdown

Testing

To verify the fix works:

# Should work without errors
cargo leptos serve --help

# Should build successfully
cargo leptos build

# Should start development server
cargo leptos serve -- -c config.dev.toml

Related Documentation

• Leptos Serve Documentation
• Cargo Leptos Documentation
• Leptos Framework Documentation