Add --target support to sync and install (#3257)

## Summary

The approach taken here is to model `--target` as an install scheme in
which all the directories are just subdirectories of the `--target`.
From there, everything else... just works? Like, upgrade, uninstalls,
editables, etc. all "just work".

Closes #1517.

crates/uv-interpreter/Cargo.toml

@@ -26,6 +26,7 @@ uv-warnings = { workspace = true }
 
 configparser = { workspace = true }
 fs-err = { workspace = true, features = ["tokio"] }
+itertools = { workspace = true }
 once_cell = { workspace = true }
 regex = { workspace = true }
 rmp-serde = { workspace = true }

crates/uv-interpreter/src/interpreter.rs

@@ -18,8 +18,8 @@ use uv_cache::{Cache, CacheBucket, CachedByTimestamp, Freshness, Timestamp};
 use uv_fs::{write_atomic_sync, PythonExt, Simplified};
 use uv_toolchain::PythonVersion;
 
-use crate::Error;
 use crate::Virtualenv;
+use crate::{Error, Target};
 
 /// A Python executable and its associated platform markers.
 #[derive(Debug, Clone)]
@@ -35,6 +35,7 @@ pub struct Interpreter {
     sys_executable: PathBuf,
     stdlib: PathBuf,
     tags: OnceCell<Tags>,
+    target: Option<Target>,
     gil_disabled: bool,
 }
 
@@ -62,6 +63,7 @@ impl Interpreter {
             sys_executable: info.sys_executable,
             stdlib: info.stdlib,
             tags: OnceCell::new(),
+            target: None,
         })
     }
 
@@ -91,6 +93,7 @@ impl Interpreter {
             sys_executable: PathBuf::from("/dev/null"),
             stdlib: PathBuf::from("/dev/null"),
             tags: OnceCell::new(),
+            target: None,
             gil_disabled: false,
         }
     }
@@ -106,6 +109,17 @@ impl Interpreter {
         }
     }
 
+    /// Return a new [`Interpreter`] to install into the given `--target` directory.
+    ///
+    /// Initializes the `--target` directory with the expected layout.
+    #[must_use]
+    pub fn with_target(self, target: Target) -> Self {
+        Self {
+            target: Some(target),
+            ..self
+        }
+    }
+
     /// Returns the path to the Python virtual environment.
     #[inline]
     pub fn platform(&self) -> &Platform {
@@ -135,9 +149,15 @@ impl Interpreter {
     ///
     /// See: <https://github.com/pypa/pip/blob/0ad4c94be74cc24874c6feb5bb3c2152c398a18e/src/pip/_internal/utils/virtualenv.py#L14>
     pub fn is_virtualenv(&self) -> bool {
+        // Maybe this should return `false` if it's a target?
         self.prefix != self.base_prefix
     }
 
+    /// Returns `true` if the environment is a `--target` environment.
+    pub fn is_target(&self) -> bool {
+        self.target.is_some()
+    }
+
     /// Returns `Some` if the environment is externally managed, optionally including an error
     /// message from the `EXTERNALLY-MANAGED` file.
     ///
@@ -148,6 +168,11 @@ impl Interpreter {
             return None;
         }
 
+        // If we're installing into a target directory, it's never externally managed.
+        if self.is_target() {
+            return None;
+        }
+
         let Ok(contents) = fs::read_to_string(self.stdlib.join("EXTERNALLY-MANAGED")) else {
             return None;
         };
@@ -303,28 +328,37 @@ impl Interpreter {
         self.gil_disabled
     }
 
+    /// Return the `--target` directory for this interpreter, if any.
+    pub fn target(&self) -> Option<&Target> {
+        self.target.as_ref()
+    }
+
     /// Return the [`Layout`] environment used to install wheels into this interpreter.
     pub fn layout(&self) -> Layout {
         Layout {
             python_version: self.python_tuple(),
             sys_executable: self.sys_executable().to_path_buf(),
             os_name: self.markers.os_name.clone(),
-            scheme: Scheme {
-                purelib: self.purelib().to_path_buf(),
-                platlib: self.platlib().to_path_buf(),
-                scripts: self.scripts().to_path_buf(),
-                data: self.data().to_path_buf(),
-                include: if self.is_virtualenv() {
-                    // If the interpreter is a venv, then the `include` directory has a different structure.
-                    // See: https://github.com/pypa/pip/blob/0ad4c94be74cc24874c6feb5bb3c2152c398a18e/src/pip/_internal/locations/_sysconfig.py#L172
-                    self.prefix.join("include").join("site").join(format!(
-                        "python{}.{}",
-                        self.python_major(),
-                        self.python_minor()
-                    ))
-                } else {
-                    self.include().to_path_buf()
-                },
+            scheme: if let Some(target) = self.target.as_ref() {
+                target.scheme()
+            } else {
+                Scheme {
+                    purelib: self.purelib().to_path_buf(),
+                    platlib: self.platlib().to_path_buf(),
+                    scripts: self.scripts().to_path_buf(),
+                    data: self.data().to_path_buf(),
+                    include: if self.is_virtualenv() {
+                        // If the interpreter is a venv, then the `include` directory has a different structure.
+                        // See: https://github.com/pypa/pip/blob/0ad4c94be74cc24874c6feb5bb3c2152c398a18e/src/pip/_internal/locations/_sysconfig.py#L172
+                        self.prefix.join("include").join("site").join(format!(
+                            "python{}.{}",
+                            self.python_major(),
+                            self.python_minor()
+                        ))
+                    } else {
+                        self.include().to_path_buf()
+                    },
+                }
             },
         }
     }

crates/uv-interpreter/src/lib.rs

@@ -19,12 +19,14 @@ pub use crate::find_python::{find_best_python, find_default_python, find_request
 pub use crate::interpreter::Interpreter;
 use crate::interpreter::InterpreterInfoError;
 pub use crate::python_environment::PythonEnvironment;
+pub use crate::target::Target;
 pub use crate::virtualenv::Virtualenv;
 
 mod cfg;
 mod find_python;
 mod interpreter;
 mod python_environment;
+mod target;
 mod virtualenv;
 
 #[derive(Debug, Error)]

crates/uv-interpreter/src/python_environment.rs

@@ -1,3 +1,4 @@
+use itertools::Either;
 use std::env;
 use std::path::{Path, PathBuf};
 
@@ -8,7 +9,7 @@ use uv_cache::Cache;
 use uv_fs::{LockedFile, Simplified};
 
 use crate::cfg::PyVenvConfiguration;
-use crate::{find_default_python, find_requested_python, Error, Interpreter};
+use crate::{find_default_python, find_requested_python, Error, Interpreter, Target};
 
 /// A Python environment, consisting of a Python [`Interpreter`] and its associated paths.
 #[derive(Debug, Clone)]
@@ -68,7 +69,16 @@ impl PythonEnvironment {
         }
     }
 
-    /// Returns the location of the Python interpreter.
+    /// Create a [`PythonEnvironment`] from an existing [`Interpreter`] and `--target` directory.
+    #[must_use]
+    pub fn with_target(self, target: Target) -> Self {
+        Self {
+            interpreter: self.interpreter.with_target(target),
+            ..self
+        }
+    }
+
+    /// Returns the root (i.e., `prefix`) of the Python interpreter.
     pub fn root(&self) -> &Path {
         &self.root
     }
@@ -97,15 +107,19 @@ impl PythonEnvironment {
     /// Some distributions also create symbolic links from `purelib` to `platlib`; in such cases, we
     /// still deduplicate the entries, returning a single path.
     pub fn site_packages(&self) -> impl Iterator<Item = &Path> {
-        let purelib = self.interpreter.purelib();
-        let platlib = self.interpreter.platlib();
-        std::iter::once(purelib).chain(
-            if purelib == platlib || is_same_file(purelib, platlib).unwrap_or(false) {
-                None
-            } else {
-                Some(platlib)
-            },
-        )
+        if let Some(target) = self.interpreter.target() {
+            Either::Left(std::iter::once(target.root()))
+        } else {
+            let purelib = self.interpreter.purelib();
+            let platlib = self.interpreter.platlib();
+            Either::Right(std::iter::once(purelib).chain(
+                if purelib == platlib || is_same_file(purelib, platlib).unwrap_or(false) {
+                    None
+                } else {
+                    Some(platlib)
+                },
+            ))
+        }
     }
 
     /// Returns the path to the `bin` directory inside a virtual environment.
@@ -115,7 +129,13 @@ impl PythonEnvironment {
 
     /// Grab a file lock for the virtual environment to prevent concurrent writes across processes.
     pub fn lock(&self) -> Result<LockedFile, std::io::Error> {
-        if self.interpreter.is_virtualenv() {
+        if let Some(target) = self.interpreter.target() {
+            // If we're installing into a `--target`, use a target-specific lock file.
+            LockedFile::acquire(
+                target.root().join(".lock"),
+                target.root().simplified_display(),
+            )
+        } else if self.interpreter.is_virtualenv() {
             // If the environment a virtualenv, use a virtualenv-specific lock file.
             LockedFile::acquire(self.root.join(".lock"), self.root.simplified_display())
         } else {

crates/uv-interpreter/src/target.rs

@@ -0,0 +1,40 @@
+use std::path::{Path, PathBuf};
+
+use pypi_types::Scheme;
+
+/// A `--target` directory into which packages can be installed, separate from a virtual environment
+/// or system Python interpreter.
+#[derive(Debug, Clone)]
+pub struct Target(PathBuf);
+
+impl Target {
+    /// Return the [`Scheme`] for the `--target` directory.
+    pub fn scheme(&self) -> Scheme {
+        Scheme {
+            purelib: self.0.clone(),
+            platlib: self.0.clone(),
+            scripts: self.0.join("bin"),
+            data: self.0.clone(),
+            include: self.0.join("include"),
+        }
+    }
+
+    /// Initialize the `--target` directory.
+    pub fn init(&self) -> std::io::Result<()> {
+        fs_err::create_dir_all(&self.0)?;
+        fs_err::create_dir_all(self.0.join("bin"))?;
+        fs_err::create_dir_all(self.0.join("include"))?;
+        Ok(())
+    }
+
+    /// Return the path to the `--target` directory.
+    pub fn root(&self) -> &Path {
+        &self.0
+    }
+}
+
+impl From<PathBuf> for Target {
+    fn from(path: PathBuf) -> Self {
+        Self(path)
+    }
+}

crates/uv-workspace/src/settings.rs

@@ -47,6 +47,7 @@ pub struct PipOptions {
     pub python: Option<String>,
     pub system: Option<bool>,
     pub break_system_packages: Option<bool>,
+    pub target: Option<PathBuf>,
     pub offline: Option<bool>,
     pub index_url: Option<IndexUrl>,
     pub extra_index_url: Option<Vec<IndexUrl>>,

crates/uv/src/cli.rs

@@ -768,6 +768,11 @@ pub(crate) struct PipSyncArgs {
     #[arg(long, overrides_with("break_system_packages"))]
     pub(crate) no_break_system_packages: bool,
 
+    /// Install packages into the specified directory, rather than into the virtual environment
+    /// or system Python interpreter.
+    #[arg(long)]
+    pub(crate) target: Option<PathBuf>,
+
     /// Use legacy `setuptools` behavior when building source distributions without a
     /// `pyproject.toml`.
     #[arg(long, overrides_with("no_legacy_setup_py"))]
@@ -1132,6 +1137,11 @@ pub(crate) struct PipInstallArgs {
     #[arg(long, overrides_with("break_system_packages"))]
     pub(crate) no_break_system_packages: bool,
 
+    /// Install packages into the specified directory, rather than into the virtual environment
+    /// or system Python interpreter.
+    #[arg(long)]
+    pub(crate) target: Option<PathBuf>,
+
     /// Use legacy `setuptools` behavior when building source distributions without a
     /// `pyproject.toml`.
     #[arg(long, overrides_with("no_legacy_setup_py"))]
@@ -1335,6 +1345,11 @@ pub(crate) struct PipUninstallArgs {
     #[arg(long, overrides_with("break_system_packages"))]
     pub(crate) no_break_system_packages: bool,
 
+    /// Uninstall packages from the specified directory, rather than from the virtual environment
+    /// or system Python interpreter.
+    #[arg(long)]
+    pub(crate) target: Option<PathBuf>,
+
     /// Run offline, i.e., without accessing the network.
     #[arg(long, overrides_with("no_offline"))]
     pub(crate) offline: bool,

crates/uv/src/commands/pip_install.rs

@@ -1,11 +1,9 @@
 use std::borrow::Cow;
 use std::fmt::Write;
-
 use std::path::Path;
 
 use anstream::eprint;
 use anyhow::{anyhow, Context, Result};
-
 use itertools::Itertools;
 use owo_colors::OwoColorize;
 use tempfile::tempdir_in;
@@ -33,7 +31,7 @@ use uv_configuration::{KeyringProviderType, TargetTriple};
 use uv_dispatch::BuildDispatch;
 use uv_fs::Simplified;
 use uv_installer::{BuiltEditable, Downloader, Plan, Planner, ResolvedEditable, SitePackages};
-use uv_interpreter::{Interpreter, PythonEnvironment};
+use uv_interpreter::{Interpreter, PythonEnvironment, Target};
 use uv_normalize::PackageName;
 use uv_requirements::{
     ExtrasSpecification, LookaheadResolver, NamedRequirementsResolver, RequirementsSource,
@@ -84,6 +82,7 @@ pub(crate) async fn pip_install(
     python: Option<String>,
     system: bool,
     break_system_packages: bool,
+    target: Option<Target>,
     native_tls: bool,
     cache: Cache,
     dry_run: bool,
@@ -134,6 +133,14 @@ pub(crate) async fn pip_install(
         venv.python_executable().user_display().cyan()
     );
 
+    // Apply any `--target` directory.
+    let venv = if let Some(target) = target {
+        target.init()?;
+        venv.with_target(target)
+    } else {
+        venv
+    };
+
     // If the environment is externally managed, abort.
     if let Some(externally_managed) = venv.interpreter().is_externally_managed() {
         if break_system_packages {