Accept single string for backend-path

crates/uv-build/src/lib.rs

@@ -14,9 +14,9 @@ use std::{env, iter};
 
 use fs_err as fs;
 use indoc::formatdoc;
-use itertools::Itertools;
+use itertools::{Either, Itertools};
 use once_cell::sync::Lazy;
-use pyproject_toml::{BuildSystem, Project};
+use pyproject_toml::Project;
 use regex::Regex;
 use serde::{Deserialize, Serialize};
 use tempfile::{tempdir_in, TempDir};
@@ -183,6 +183,37 @@ pub struct PyProjectToml {
     pub project: Option<Project>,
 }
 
+/// The `[build-system]` section of a pyproject.toml as specified in PEP 517.
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq)]
+#[serde(rename_all = "kebab-case")]
+pub struct BuildSystem {
+    /// PEP 508 dependencies required to execute the build system.
+    pub requires: Vec<Requirement>,
+    /// A string naming a Python object that will be used to perform the build.
+    pub build_backend: Option<String>,
+    /// Specify that their backend code is hosted in-tree, this key contains a list of directories.
+    pub backend_path: Option<BackendPath>,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq)]
+#[serde(untagged)]
+pub enum BackendPath {
+    /// A list of paths (matching the schema).
+    List(Vec<String>),
+    /// A single path, included for compatibility (see, e.g., `flit_core==2.3.0`, which uses
+    /// `backend-path = "."`).
+    String(String),
+}
+
+impl BackendPath {
+    fn iter(&self) -> impl Iterator<Item = &str> {
+        match self {
+            BackendPath::List(list) => Either::Left(list.iter().map(std::string::String::as_str)),
+            BackendPath::String(s) => Either::Right(iter::once(s.as_str())),
+        }
+    }
+}
+
 /// `[build-backend]` from pyproject.toml
 #[derive(Debug, Clone, PartialEq, Eq)]
 struct Pep517Backend {
@@ -194,7 +225,7 @@ struct Pep517Backend {
     /// `build-backend.requirements` in pyproject.toml
     requirements: Vec<Requirement>,
     /// <https://peps.python.org/pep-0517/#in-tree-build-backends>
-    backend_path: Option<Vec<String>>,
+    backend_path: Option<BackendPath>,
 }
 
 impl Pep517Backend {
@@ -207,9 +238,8 @@ impl Pep517Backend {
 
         let backend_path_encoded = self
             .backend_path
-            .clone()
-            .unwrap_or_default()
             .iter()
+            .flat_map(BackendPath::iter)
             .map(|path| {
                 // Turn into properly escaped python string
                 '"'.to_string()