Add output file option to compile (#93)

`pip-compile` has the same option. I need this esp. since piping doesn't
work as we write to stdout.

crates/puffin-cli/src/commands/compile.rs

@@ -1,4 +1,6 @@
 use std::fmt::Write;
+use std::fs::File;
+use std::io::{stdout, BufWriter};
 use std::path::Path;
 
 use anyhow::Result;
@@ -18,6 +20,7 @@ use crate::printer::Printer;
 /// Resolve a set of requirements into a set of pinned versions.
 pub(crate) async fn compile(
     src: &Path,
+    output_file: Option<&Path>,
     cache: Option<&Path>,
     mut printer: Printer,
 ) -> Result<ExitStatus> {
@@ -80,12 +83,11 @@ pub(crate) async fn compile(
         .dimmed()
     )?;
 
-    for (name, package) in resolution.iter() {
-        #[allow(clippy::print_stdout)]
-        {
-            println!("{}=={}", name, package.version());
-        }
-    }
+    if let Some(output_file) = output_file {
+        resolution.write_requirement_format(&mut BufWriter::new(File::create(output_file)?))?;
+    } else {
+        resolution.write_requirement_format(&mut stdout().lock())?;
+    };
 
     Ok(ExitStatus::Success)
 }

crates/puffin-cli/src/main.rs

@@ -1,12 +1,11 @@
 use std::path::PathBuf;
 use std::process::ExitCode;
 
+use crate::commands::ExitStatus;
 use clap::{Args, Parser, Subcommand};
 use colored::Colorize;
 use directories::ProjectDirs;
 
-use crate::commands::ExitStatus;
-
 mod commands;
 mod logging;
 mod printer;
@@ -49,6 +48,9 @@ enum Commands {
 
 #[derive(Args)]
 struct CompileArgs {
+    /// Output `requirements.txt` file
+    #[clap(short, long)]
+    output_file: Option<PathBuf>,
     /// Path to the `requirements.txt` file to compile.
     src: PathBuf,
 }
@@ -104,6 +106,7 @@ async fn main() -> ExitCode {
         Commands::Compile(args) => {
             commands::compile(
                 &args.src,
+                args.output_file.as_deref(),
                 dirs.as_ref()
                     .map(ProjectDirs::cache_dir)
                     .filter(|_| !cli.no_cache),

crates/puffin-resolver/src/resolution.rs

@@ -1,4 +1,6 @@
 use std::collections::BTreeMap;
+use std::io;
+use std::io::Write;
 
 use pep440_rs::Version;
 use puffin_client::File;
@@ -33,6 +35,14 @@ impl Resolution {
     pub fn is_empty(&self) -> bool {
         self.0.is_empty()
     }
+
+    /// Write the resolution in the `{name}=={version}` format of requirements.txt that pip uses.
+    pub fn write_requirement_format(&self, writer: &mut impl Write) -> io::Result<()> {
+        for (name, package) in self.iter() {
+            writeln!(writer, "{}=={}", name, package.version())?;
+        }
+        Ok(())
+    }
 }
 
 impl std::fmt::Display for Resolution {