Add optional runtime fallback directory (#8610)

book/src/install.md

@@ -257,12 +257,32 @@ following order:
 1. `runtime/` sibling directory to `$CARGO_MANIFEST_DIR` directory (this is intended for
   developing and testing helix only).
 2. `runtime/` subdirectory of OS-dependent helix user config directory.
-3. `$HELIX_RUNTIME`.
-4. `runtime/` subdirectory of path to Helix executable.
+3. `$HELIX_RUNTIME`
+4. Distribution-specific fallback directory (set at compile time—not run time—
+   with the `HELIX_DEFAULT_RUNTIME` environment variable)
+5. `runtime/` subdirectory of path to Helix executable.
 
 This order also sets the priority for selecting which file will be used if multiple runtime
 directories have files with the same name.
 
+#### Note to packagers
+
+If you are making a package of Helix for end users, to provide a good out of
+the box experience, you should set the `HELIX_DEFAULT_RUNTIME` environment
+variable at build time (before invoking `cargo build`) to a directory which
+will store the final runtime files after installation. For example, say you want
+to package the runtime into `/usr/lib/helix/runtime`. The rough steps a build
+script could follow are:
+
+1. `export HELIX_DEFAULT_RUNTIME=/usr/lib/helix/runtime`
+1. `cargo build --profile opt --locked --path helix-term`
+1. `cp -r runtime $BUILD_DIR/usr/lib/helix/`
+1. `cp target/opt/hx $BUILD_DIR/usr/bin/hx`
+
+This way the resulting `hx` binary will always look for its runtime directory in
+`/usr/lib/helix/runtime` if the user has no custom runtime in `~/.config/helix`
+or `HELIX_RUNTIME`.
+
 ### Validating the installation
 
 To make sure everything is set up as expected you should run the Helix health

helix-loader/src/lib.rs

@@ -60,7 +60,8 @@ pub fn initialize_log_file(specified_file: Option<PathBuf>) {
 /// 1. sibling directory to `CARGO_MANIFEST_DIR` (if environment variable is set)
 /// 2. subdirectory of user config directory (always included)
 /// 3. `HELIX_RUNTIME` (if environment variable is set)
-/// 4. subdirectory of path to helix executable (always included)
+/// 4. `HELIX_DEFAULT_RUNTIME` (if environment variable is set *at build time*)
+/// 5. subdirectory of path to helix executable (always included)
 ///
 /// Postcondition: returns at least two paths (they might not exist).
 fn prioritize_runtime_dirs() -> Vec<PathBuf> {
@@ -81,6 +82,14 @@ fn prioritize_runtime_dirs() -> Vec<PathBuf> {
         rt_dirs.push(dir.into());
     }
 
+    // If this variable is set during build time, it will always be included
+    // in the lookup list. This allows downstream packagers to set a fallback
+    // directory to a location that is conventional on their distro so that they
+    // need not resort to a wrapper script or a global environment variable.
+    if let Some(dir) = std::option_env!("HELIX_DEFAULT_RUNTIME") {
+        rt_dirs.push(dir.into());
+    }
+
     // fallback to location of the executable being run
     // canonicalize the path in case the executable is symlinked
     let exe_rt_dir = std::env::current_exe()