Project Layout

A Flint project is simple by design.

my_project/
  flint.toml       ← manifest
  src/
    main.fl        ← entry point
    drivers/
      display.fl   ← local module
      sensor.fl    ← local module
  build/           ← generated by flint build (git-ignore this)

The Manifest (flint.toml)

flint.toml is TOML and lives at the project root. The minimal required fields are:

name = "my_project"
version = "0.1.0"
edition = "2026"
target = "rp2040"
board = "pico"

Optional Manifest Fields

# Allow recursive function calls on MCU targets.
# Disabled by default on MCU targets to catch stack-risk bugs.
allow-recursion = true

Source Files

All source lives under src/. Files use the .fl extension. Each file is exactly one module.

The entry point is always src/main.fl. Modules are imported with use using their path relative to src/:

// src/main.fl
use drivers/display
use drivers/sensor

fn main() -> never {
    display.init()
    let temp = sensor.read_celsius()
    display.show_temp(temp)
    loop {}
}

// src/drivers/display.fl
pub fn init() -> () {
    // ...
}

pub fn show_temp(temp_c: i32) -> () {
    // ...
}

The file src/drivers/display.fl maps to the module path drivers/display. Its exported functions are accessed as display.init(), display.show_temp(...).

Module Rules

• One file, one module. No module declarations in source.
• Import paths use /, not . or ::.
• The last path segment is the default module name: use drivers/display imports as display.
• Use as to rename: use drivers/display as screen.
• No wildcard imports (use foo/* is not supported).
• No relative imports (./foo or ../foo are not supported).

Build Output

flint build writes artifacts into build/ inside your project:

build/
  my_project.uf2
  my_project.elf
  my_project.bin

The build/ directory is generated. Add it to your .gitignore.

PIO Files

RP2040 PIO programs live anywhere under src/ with a .pio extension:

src/
  main.fl
  drivers/
    blink.pio

Reference them at compile time through micro/pio:

use micro/pio
let program = pio.program_file("drivers/blink.pio")?

The compiler assembles the PIO program during the build.

Manifestless Builds

You can build a single file or directory without a flint.toml, but you must specify the target explicitly:

flint build src/main.fl --target rp2040
flint build src/ --target rp2040

Manifestless builds are useful for quick experiments. For real projects, always use a manifest.