Add UV_CONSTRAINT environment variable to provide value for `--constraint` (#3162)

## Summary

This PR is adding `UV_CONSTRAINT` environment variable as analogous to
`PIP_CONSTRAINT` to allow providing constraint file via environment
variable. Implementing this will simplify adoption of uv in testing
procedure in projects that I'm involved (testing using tox).

This was my motivation for opening #1841 that is closed in favor of
#1789 which was closed without implementing this feature.

In this implementation, I have used space as a separator as analogous to
`pip`. This introduces an obvious problem if the path contains space.
Another option could be to use standard separator (`:` - UNIX like, `;`
- Windows). Which one did you prefer?

## Test Plan

It is my first contribution and first rust coding experience. It will be
nice if one could point how I should implement testing this.

README.md

@@ -502,6 +502,8 @@ uv accepts the following command-line arguments as environment variables:
   index URLs, rather than limiting its search to the first index URL that contains the package.
 - `UV_REQUIRE_HASHES`: Equivalent to the `--require-hashes` command-line argument. If set to `true`,
   uv will require that all dependencies have a hash specified in the requirements file.
+- 'UV_CONSTRAINT': Equivalent to the `--constraint` command-line argument. If set, uv will use this
+  file as the constraints file. Uses space-separated list of files.
 
 In each case, the corresponding command-line argument takes precedence over an environment variable.
 

crates/uv/src/cli.rs

@@ -224,6 +224,18 @@ fn parse_index_url(input: &str) -> Result<Maybe<IndexUrl>, String> {
     }
 }
 
+/// Parse a string into a [`PathBuf`], mapping the empty string to `None`.
+fn parse_file_path(input: &str) -> Result<Maybe<PathBuf>, String> {
+    if input.is_empty() {
+        Ok(Maybe::None)
+    } else {
+        match PathBuf::from_str(input) {
+            Ok(path) => Ok(Maybe::Some(path)),
+            Err(err) => Err(err.to_string()),
+        }
+    }
+}
+
 #[derive(Args)]
 #[allow(clippy::struct_excessive_bools)]
 pub(crate) struct PipCompileArgs {
@@ -240,8 +252,8 @@ pub(crate) struct PipCompileArgs {
     /// trigger the installation of that package.
     ///
     /// This is equivalent to pip's `--constraint` option.
-    #[arg(long, short)]
+    #[arg(long, short, env = "UV_CONSTRAINT", value_delimiter = ' ', value_parser = parse_file_path)]
-    pub(crate) constraint: Vec<PathBuf>,
+    pub(crate) constraint: Vec<Maybe<PathBuf>>,
 
     /// Override versions using the given requirements files.
     ///
@@ -869,8 +881,8 @@ pub(crate) struct PipInstallArgs {
     /// trigger the installation of that package.
     ///
     /// This is equivalent to pip's `--constraint` option.
-    #[arg(long, short)]
+    #[arg(long, short, env = "UV_CONSTRAINT", value_delimiter = ' ', value_parser = parse_file_path)]
-    pub(crate) constraint: Vec<PathBuf>,
+    pub(crate) constraint: Vec<Maybe<PathBuf>>,
 
     /// Override versions using the given requirements files.
     ///

crates/uv/src/settings.rs

@@ -154,7 +154,10 @@ impl PipCompileSettings {
         Self {
             // CLI-only settings.
             src_file,
-            constraint,
+            constraint: constraint
+                .into_iter()
+                .filter_map(Maybe::into_option)
+                .collect(),
             r#override,
             refresh: Refresh::from_args(refresh, refresh_package),
             upgrade: Upgrade::from_args(upgrade, upgrade_package),
@@ -393,7 +396,10 @@ impl PipInstallSettings {
             package,
             requirement,
             editable,
-            constraint,
+            constraint: constraint
+                .into_iter()
+                .filter_map(Maybe::into_option)
+                .collect(),
             r#override,
             upgrade: Upgrade::from_args(upgrade, upgrade_package),
             reinstall: Reinstall::from_args(reinstall, reinstall_package),