Use shorter paths in the OUT_DIR

As described in KDAB#1237, on Windows we're sometimes hitting the 260 character limit
for filepaths.

This is mostly due to a lot of nesting by Qt Creator, Corrosion and
Cargo who all include the target triple and other configuration information in the
build path.

However, we can at least reduce the character count a little bit on our
part.

For example, for qqmlengine.cxxqt.h, the path length is now reduced by
~40 characters from:

out/cxx-qt-build/target/crates/cxx-qt-lib/include/cxx-qt-lib-internals/src/qml/qqmlengine.cxxqt.h

to now:

out/cxxqtbuild/include/private/src/qml/qqmlengine.cxxqt.h

Author: LeonMatthesKDAB

crates/cxx-qt-build/src/dir.rs

@@ -21,6 +21,11 @@ pub(crate) const INCLUDE_VERB: &str = "create symlink";
 #[cfg(not(unix))]
 pub(crate) const INCLUDE_VERB: &str = "deep copy files";
 
+pub(crate) fn gen() -> PathBuf {
+    // Use a short name due to the Windows file path limit!
+    out().join("cxxqtgen")
+}
+
 // Clean a directory by removing it and recreating it.
 pub(crate) fn clean(path: impl AsRef<Path>) -> Result<()> {
     let result = std::fs::remove_dir_all(&path);
@@ -37,14 +42,47 @@ pub(crate) fn clean(path: impl AsRef<Path>) -> Result<()> {
 
 /// The target directory, namespaced by crate
 pub(crate) fn crate_target() -> PathBuf {
-    target().join("crates").join(crate_name())
+    let path = target();
+    if is_exporting() {
+        path.join("crates").join(crate_name())
+    } else {
+        // If we're not exporting, use a shortened path
+        // The paths for files in the OUT_DIR can get pretty long, especially if combined with
+        // Corrosion/CMake.
+        // This is an issue, as Windows has a maximum path length of 260 characters.
+        // The OUT_DIR is already namespaced by crate name, so we don't need to prefix again.
+        // See also: https://github.com/KDAB/cxx-qt/issues/1237
+        path
+    }
 }
 
 /// The target directory, namespaced by plugin
 pub(crate) fn module_target(module_uri: &str) -> PathBuf {
-    target()
-        .join("qml_modules")
-        .join(module_name_from_uri(module_uri))
+    module_export(module_uri).unwrap_or_else(|| {
+        out()
+            // Use a short name due to the Windows file path limit!
+            .join("cxxqtqml")
+            .join(module_name_from_uri(module_uri))
+    })
+}
+
+/// The export directory, namespaced by QML module
+///
+/// In conctrast to the crate_export directory, this is `Some` for downstream dependencies as well.
+/// This allows CMake to import QML modules from dependencies.
+///
+/// TODO: This may conflict if two dependencies are building QML modules with the same name!
+/// We should probably include a lockfile here to avoid this.
+pub(crate) fn module_export(module_uri: &str) -> Option<PathBuf> {
+    // In contrast to crate_export, we don't need to check for the specific crate here.
+    // QML modules should always be exported.
+    env::var("CXX_QT_EXPORT_DIR")
+        .ok()
+        .map(PathBuf::from)
+        .map(|dir| {
+            dir.join("qml_modules")
+                .join(module_name_from_uri(module_uri))
+        })
 }
 
 /// The target directory or another directory where we can write files that will be shared
@@ -54,7 +92,8 @@ pub(crate) fn target() -> PathBuf {
         return export;
     }
 
-    out().join("cxx-qt-build").join("target")
+    // Use a short name due to the Windows file path limit!
+    out().join("cxxqtbuild")
 }
 
 /// The export directory, if one was specified through the environment.

crates/cxx-qt-build/src/lib.rs

@@ -261,7 +261,7 @@ fn generate_cxxqt_cpp_files(
     header_dir: impl AsRef<Path>,
     include_prefix: &str,
 ) -> Vec<GeneratedCppFilePaths> {
-    let cxx_qt_dir = dir::out().join("cxx-qt-gen");
+    let cxx_qt_dir = dir::gen();
     std::fs::create_dir_all(&cxx_qt_dir).expect("Failed to create cxx-qt-gen directory!");
     std::fs::write(cxx_qt_dir.join("include-prefix.txt"), include_prefix).expect("");
 

crates/cxx-qt-lib-extras/build.rs

@@ -80,7 +80,7 @@ fn main() {
     });
     println!("cargo::rerun-if-changed=src/assertion_utils.h");
 
-    builder
-        .include_prefix("cxx-qt-lib-extras-internals")
-        .build();
+    // Use a short name due to the Windows file path limit!
+    // We don't re-export these headers anyway.
+    builder.include_prefix("private").build();
 }