Support modules with different casing in build backend

Match the module name to its module directory with potentially different casing.

For example, a package may have the dist-info-normalized package name `pil_util`, but the importable module is named `PIL_util`.

We get the module either as dist-info-normalized package name, or explicitly from the user.
For dist-info-normalizing a package name, the rules are lowercasing, replacing `.` with `_` and
replace `-` with `_`. Since `.` and `-` are not allowed in module names, we can check whether a
directory name matches our expected module name by lowercasing it.

Fixes #12187

Author: konstin

crates/uv-build-backend/src/lib.rs

@@ -8,14 +8,15 @@ pub use source_dist::{build_source_dist, list_source_dist};
 pub use wheel::{build_editable, build_wheel, list_wheel, metadata};
 
 use crate::metadata::ValidationError;
+use itertools::Itertools;
 use std::fs::FileType;
 use std::io;
 use std::path::{Path, PathBuf};
 use thiserror::Error;
 use tracing::debug;
 use uv_fs::Simplified;
 use uv_globfilter::PortableGlobError;
-use uv_pypi_types::IdentifierParseError;
+use uv_pypi_types::{Identifier, IdentifierParseError};
 
 #[derive(Debug, Error)]
 pub enum Error {
@@ -54,8 +55,24 @@ pub enum Error {
     Zip(#[from] zip::result::ZipError),
     #[error("Failed to write RECORD file")]
     Csv(#[from] csv::Error),
-    #[error("Expected a Python module with an `__init__.py` at: `{}`", _0.user_display())]
-    MissingModule(PathBuf),
+    #[error(
+        "Expected a Python module for `{}` with an `__init__.py` at: `{}`",
+        module_name,
+        project_src.user_display()
+    )]
+    MissingModule {
+        module_name: Identifier,
+        project_src: PathBuf,
+    },
+    #[error(
+        "Expected a single Python module for `{}` with an `__init__.py`, found multiple:\n* `{}`",
+        module_name,
+        paths.iter().map(Simplified::user_display).join("`\n* `")
+    )]
+    MultipleModules {
+        module_name: Identifier,
+        paths: Vec<PathBuf>,
+    },
     #[error("Absolute module root is not allowed: `{}`", _0.display())]
     AbsoluteModuleRoot(PathBuf),
     #[error("Inconsistent metadata between prepare and build step: `{0}`")]

crates/uv-build-backend/src/wheel.rs

@@ -128,7 +128,7 @@ fn write_wheel(
     if settings.module_root.is_absolute() {
         return Err(Error::AbsoluteModuleRoot(settings.module_root.clone()));
     }
-    let strip_root = source_tree.join(settings.module_root);
+    let src_root = source_tree.join(settings.module_root);
 
     let module_name = if let Some(module_name) = settings.module_name {
         module_name
@@ -139,10 +139,8 @@ fn write_wheel(
     };
     debug!("Module name: `{:?}`", module_name);
 
-    let module_root = strip_root.join(module_name.as_ref());
-    if !module_root.join("__init__.py").is_file() {
-        return Err(Error::MissingModule(module_root));
-    }
+    let module_root = find_module_root(&src_root, module_name)?;
+
     let mut files_visited = 0;
     for entry in WalkDir::new(module_root)
         .into_iter()
@@ -169,7 +167,7 @@ fn write_wheel(
             .expect("walkdir starts with root");
         let wheel_path = entry
             .path()
-            .strip_prefix(&strip_root)
+            .strip_prefix(&src_root)
             .expect("walkdir starts with root");
         if exclude_matcher.is_match(match_path) {
             trace!("Excluding from module: `{}`", match_path.user_display());
@@ -243,6 +241,37 @@ fn write_wheel(
     Ok(())
 }
 
+/// Match the module name to its module directory with potentially different casing.
+///
+/// For example, a package may have the dist-info-normalized package name `pil_util`, but the
+/// importable module is named `PIL_util`.
+///
+/// We get the module either as dist-info-normalized package name, or explicitly from the user.
+/// For dist-info-normalizing a package name, the rules are lowercasing, replacing `.` with `_` and
+/// replace `-` with `_`. Since `.` and `-` are not allowed in module names, we can check whether a
+/// directory name matches our expected module name by lowercasing it.
+fn find_module_root(src_root: &Path, module_name: Identifier) -> Result<PathBuf, Error> {
+    let normalized = module_name.to_string();
+    let modules = fs_err::read_dir(src_root)?
+        .filter_ok(|entry| {
+            entry.file_name().to_string_lossy().to_lowercase() == normalized
+                && entry.path().join("__init__.py").is_file()
+        })
+        .map_ok(|entry| entry.path())
+        .collect::<Result<Vec<_>, _>>()?;
+    match modules.as_slice() {
+        [] => Err(Error::MissingModule {
+            module_name,
+            project_src: src_root.to_path_buf(),
+        }),
+        [module_root] => Ok(module_root.clone()),
+        multiple => Err(Error::MultipleModules {
+            module_name,
+            paths: multiple.to_vec(),
+        }),
+    }
+}
+
 /// Build a wheel from the source tree and place it in the output directory.
 pub fn build_editable(
     source_tree: &Path,
@@ -292,10 +321,9 @@ pub fn build_editable(
     };
     debug!("Module name: `{:?}`", module_name);
 
-    let module_root = src_root.join(module_name.as_ref());
-    if !module_root.join("__init__.py").is_file() {
-        return Err(Error::MissingModule(module_root));
-    }
+    // Check that a module root exists in the directory we're linking from the `.pth` file
+    find_module_root(&src_root, module_name)?;
+
     wheel_writer.write_bytes(
         &format!("{}.pth", pyproject_toml.name().as_dist_info_name()),
         src_root.as_os_str().as_encoded_bytes(),

crates/uv/tests/it/build_backend.rs

@@ -431,3 +431,105 @@ fn rename_module_editable_build() -> Result<()> {
 
     Ok(())
 }
+
+/// Check that the build succeeds even if the module name mismatches by case.
+#[test]
+fn build_module_name_normalization() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    let wheel_dir = context.temp_dir.path().join("dist");
+    fs_err::create_dir(&wheel_dir)?;
+
+    context
+        .temp_dir
+        .child("pyproject.toml")
+        .write_str(indoc! {r#"
+        [project]
+        name = "django-plugin"
+        version = "1.0.0"
+
+        [build-system]
+        requires = ["uv_build>=0.5,<0.7"]
+        build-backend = "uv_build"
+    "#})?;
+    fs_err::create_dir_all(context.temp_dir.join("src"))?;
+
+    // Error case 1: No matching module.
+    uv_snapshot!(context
+        .build_backend()
+        .arg("build-wheel")
+        .arg(&wheel_dir), @r"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    error: Expected a Python module for `django_plugin` with an `__init__.py` at: `src`
+    ");
+
+    // Use `Django_plugin` instead of `django_plugin`
+    context
+        .temp_dir
+        .child("src/Django_plugin/__init__.py")
+        .write_str(r#"print("Hi from bar")"#)?;
+
+    uv_snapshot!(context
+        .build_backend()
+        .arg("build-wheel")
+        .arg(&wheel_dir), @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    django_plugin-1.0.0-py3-none-any.whl
+
+    ----- stderr -----
+    ");
+
+    context
+        .pip_install()
+        .arg("--no-index")
+        .arg("--find-links")
+        .arg(&wheel_dir)
+        .arg("django-plugin")
+        .assert()
+        .success();
+
+    uv_snapshot!(Command::new(context.interpreter())
+        .arg("-c")
+        .arg("import Django_plugin")
+        // Python on windows
+        .env(EnvVars::PYTHONUTF8, "1"), @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    Hi from bar
+
+    ----- stderr -----
+    ");
+
+    // Error case 2: Multiple modules a matching name.
+    // Requires a case-sensitive filesystem.
+    #[cfg(unix)]
+    {
+        context
+            .temp_dir
+            .child("src/django_plugin/__init__.py")
+            .write_str(r#"print("Hi from bar")"#)?;
+
+        uv_snapshot!(context
+            .build_backend()
+            .arg("build-wheel")
+            .arg(&wheel_dir), @r"
+        success: false
+        exit_code: 2
+        ----- stdout -----
+
+        ----- stderr -----
+        error: Expected a single Python module for `django_plugin` with an `__init__.py`, found multiple:
+        * `src/Django_plugin`
+        * `src/django_plugin`
+        ");
+    }
+
+    Ok(())
+}