Add automatic tool management starting with rustup and cargo. (#24)

* Verifies that `rustup` is installed and gives instructions on how to
install it if not.
* Verifies that `cargo`  is installed and the correct version.

Author: xStrom

CHANGELOG.md

@@ -7,6 +7,7 @@
 * `init` command to set up custom per-project Prep configuration. ([#23] by [@xStrom])
 * `copyright` command to easily verify that all Rust source files have correct copyright headers. ([#22] by [@xStrom])
 * `--crates <main|aux|all>` option to the `clippy` command. ([#18] by [@xStrom])
+* Automatic Cargo installation via `rustup`. ([#24] by [@xStrom])
 * Ability to run from within a sub-directory of a workspace as opposed to just from the workspace root. ([#23] by [@xStrom])
 
 ### Changed
@@ -29,6 +30,7 @@
 [#18]: https://github.com/Nevermore/prep/pull/18
 [#22]: https://github.com/Nevermore/prep/pull/22
 [#23]: https://github.com/Nevermore/prep/pull/23
+[#24]: https://github.com/Nevermore/prep/pull/24
 
 [Unreleased]: https://github.com/Nevermore/prep/compare/v0.1.0...HEAD
 [0.1.0]: https://github.com/Nevermore/prep/compare/v0.0.0...v0.1.0

prep/src/cmd/clippy.rs

@@ -1,19 +1,18 @@
 // Copyright 2026 the Prep Authors
 // SPDX-License-Identifier: Apache-2.0 OR MIT
 
-use std::process::Command;
-
 use anyhow::{Context, ensure};
 
 use crate::cmd::CargoTargets;
 use crate::session::Session;
+use crate::tools::cargo;
 use crate::ui;
 
 /// Runs Clippy analysis on the given `targets`.
 ///
 /// In `strict` mode warnings are treated as errors.
 pub fn run(session: &Session, targets: CargoTargets, strict: bool) -> anyhow::Result<()> {
-    let mut cmd = Command::new("cargo");
+    let mut cmd = cargo::new("")?;
     let mut cmd = cmd
         .current_dir(session.root_dir())
         .arg("clippy")

prep/src/cmd/format.rs

@@ -1,16 +1,15 @@
 // Copyright 2026 the Prep Authors
 // SPDX-License-Identifier: Apache-2.0 OR MIT
 
-use std::process::Command;
-
 use anyhow::{Context, ensure};
 
 use crate::session::Session;
+use crate::tools::cargo;
 use crate::ui;
 
 /// Format the workspace
 pub fn run(session: &Session, check: bool) -> anyhow::Result<()> {
-    let mut cmd = Command::new("cargo");
+    let mut cmd = cargo::new("")?;
     let mut cmd = cmd.current_dir(session.root_dir()).arg("fmt").arg("--all");
     if check {
         cmd = cmd.arg("--check");

prep/src/main.rs

@@ -6,6 +6,7 @@
 mod cmd;
 mod config;
 mod session;
+mod tools;
 mod ui;
 
 use clap::{CommandFactory, FromArgMatches, Parser, Subcommand};

prep/src/session.rs

@@ -10,6 +10,7 @@ use anyhow::{Context, Result, bail};
 use cargo_metadata::MetadataCommand;
 
 use crate::config::Config;
+use crate::tools::cargo;
 
 const PREP_DIR: &str = ".prep";
 const CONFIG_FILE: &str = "prep.toml";
@@ -44,7 +45,9 @@ impl Session {
         let root_dir = match root_dir {
             Some(root_dir) => root_dir,
             None => {
+                let cmd = cargo::new("")?;
                 let metadata = MetadataCommand::new()
+                    .cargo_path(cmd.get_program())
                     .exec()
                     .context("failed to fetch Cargo metadata")?;
                 let workspace_dir = metadata.workspace_root.into_std_path_buf();

prep/src/tools/cargo.rs

@@ -0,0 +1,92 @@
+// Copyright 2026 the Prep Authors
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+use std::collections::HashMap;
+use std::io::ErrorKind;
+use std::process::Command;
+use std::sync::{LazyLock, RwLock};
+
+use anyhow::{Context, Result, bail, ensure};
+
+use crate::tools::rustup;
+use crate::ui;
+
+/// Cargo executable name.
+const BIN: &str = "cargo";
+
+/// Toolchain version -> Cargo path
+static PATHS: LazyLock<RwLock<HashMap<String, String>>> =
+    LazyLock::new(|| RwLock::new(HashMap::new()));
+
+/// Returns the cargo command.
+///
+/// Provide an empty `version` to pick the default cargo version.
+pub fn new(version: &str) -> Result<Command> {
+    let paths = PATHS.read().expect("cargo setup lock poisoned");
+    if let Some(path) = paths.get(version) {
+        return Ok(Command::new(path));
+    }
+    drop(paths);
+    let mut paths = PATHS.write().expect("cargo setup lock poisoned");
+    if let Some(path) = paths.get(version) {
+        return Ok(Command::new(path));
+    }
+    let path = set_up(version)?;
+    let cmd = Command::new(&path);
+    paths.insert(version.into(), path);
+    Ok(cmd)
+}
+
+/// Ensures that Cargo is installed and ready to use.
+pub fn set_up(version: &str) -> Result<String> {
+    // TODO: Call rustup toolchain install with correct components etc first
+
+    let mut cmd = rustup::new()?;
+    let mut cmd = cmd.arg("which").arg(BIN);
+    if !version.is_empty() {
+        cmd = cmd.args(["--toolchain", version]);
+    }
+
+    ui::print_cmd(cmd);
+
+    let output = cmd.output().context("failed to run rustup")?;
+    ensure!(output.status.success(), "rustup failed: {}", output.status);
+
+    let path = String::from_utf8(output.stdout).context("rustup output not valid UTF-8")?;
+    let path = path.trim();
+
+    if !verify(path, version)? {
+        bail!("cargo not found");
+    }
+
+    Ok(path.into())
+}
+
+/// Returns `true` if Cargo was found, `false` if no Cargo was found.
+///
+/// Other versions will return an error.
+pub fn verify(path: &str, version: &str) -> Result<bool> {
+    let mut cmd = Command::new(path);
+    let cmd = cmd.arg("-V");
+
+    ui::print_cmd(cmd);
+
+    let output = cmd.output();
+    if output
+        .as_ref()
+        .is_err_and(|e| e.kind() == ErrorKind::NotFound)
+    {
+        return Ok(false);
+    }
+    let output = output.context("failed to run cargo")?;
+    ensure!(output.status.success(), "cargo failed: {}", output.status);
+
+    let cmd_version = String::from_utf8(output.stdout).context("cargo output not valid UTF-8")?;
+
+    let expected = format!("cargo {version}");
+    if !cmd_version.starts_with(&expected) {
+        bail!("expected {expected}, got: {cmd_version}");
+    }
+
+    Ok(true)
+}

prep/src/tools/mod.rs

@@ -0,0 +1,5 @@
+// Copyright 2026 the Prep Authors
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+pub mod cargo;
+pub mod rustup;

prep/src/tools/rustup.rs

@@ -0,0 +1,77 @@
+// Copyright 2026 the Prep Authors
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+use std::io::ErrorKind;
+use std::process::Command;
+use std::sync::RwLock;
+
+use anyhow::{Context, Result, bail, ensure};
+
+use crate::ui;
+
+/// Rustup executable name.
+const BIN: &str = "rustup";
+
+/// Whether rustup is ready to use.
+static READY: RwLock<bool> = RwLock::new(false);
+
+/// Returns the rustup command.
+pub fn new() -> Result<Command> {
+    if !*READY.read().expect("rustup setup lock poisoned") {
+        let mut ready = READY.write().expect("rustup setup lock poisoned");
+        if !*ready {
+            set_up().context("failed to set up rustup")?;
+            *ready = true;
+        }
+    }
+    Ok(Command::new(BIN))
+}
+
+/// Ensures that rustup is installed and ready to use.
+pub fn set_up() -> Result<()> {
+    // Check if rustup is already available
+    let found = verify()?;
+    if !found {
+        ui::print_err(
+            "\
+			Prep requires rustup v1 to function.\n\
+			\n\
+			There is no automatic setup implemented for it, sorry.\n\
+			Please go to https://rustup.rs/ and install it manually.\n\
+			\n\
+			If you already have rustup installed then this error here is probably a bug.\n\
+			Please report it at https://github.com/Nevermore/prep\n\
+			",
+        );
+        bail!("rustup not found");
+    }
+    Ok(())
+}
+
+/// Returns `true` if rustup v1 was found, `false` if no rustup was found.
+///
+/// Other versions will return an error.
+pub fn verify() -> Result<bool> {
+    let mut cmd = Command::new(BIN);
+    let cmd = cmd.arg("-V");
+
+    ui::print_cmd(cmd);
+
+    let output = cmd.output();
+    if output
+        .as_ref()
+        .is_err_and(|e| e.kind() == ErrorKind::NotFound)
+    {
+        return Ok(false);
+    }
+    let output = output.context("failed to run rustup")?;
+    ensure!(output.status.success(), "rustup failed: {}", output.status);
+
+    let version = String::from_utf8(output.stdout).context("rustup output not valid UTF-8")?;
+
+    if !version.starts_with("rustup 1.") {
+        bail!("expected rustup v1, got: {version}");
+    }
+
+    Ok(true)
+}