Stop using Cargo's OUT_DIR

(Note: this commit should technically have preceded the "Fix config paths
for disjoint build-dirs and in-tree installs" one, to make that one easier
to follow, but I wasn't 100% sure if this commit is right.)

From https://doc.rust-lang.org/cargo/reference/environment-variables.html

> OUT_DIR — If the package has a build script, this is set to the folder
> where the build script should place its output. See below for more
> information. (Only set during compilation.)

so OUT_DIR is something like "target/debug/build/fish-41da27d587f48978".
Whenever build.rs is re-run, we get a new one.

I don't think we need this flexibility anywhere.  It wouldn't protect
concurrent "cargo test" from interfering with each other - that's handled
by a file lock taken by Cargo.

Use "target/" instead (or CMAKE_BINARY_DIR if set).
Namespace the files better, so we don't create weird paths like

	target/test/complete_test/...
	target/fish_root/

build.rs

@@ -25,14 +25,8 @@ fn main() {
         .unwrap_or(canonicalize(MANIFEST_DIR).join("target"));
 
     // FISH_BUILD_DIR is set by CMake, if we are using it.
-    // OUT_DIR is set by Cargo when the build script is running (not compiling)
-    let default_build_dir = env::var("OUT_DIR").unwrap();
-    let build_dir = option_env!("FISH_BUILD_DIR").unwrap_or(&default_build_dir);
-    let build_dir = canonicalize_str(build_dir);
-    rsconf::set_env_value("FISH_BUILD_DIR", &build_dir);
-
     rsconf::set_env_value(
-        "FISH_BUILD_OUTPUT_DIR",
+        "FISH_BUILD_DIR",
         option_env!("FISH_BUILD_DIR").unwrap_or(cargo_target_dir.to_str().unwrap()),
     );
 

src/env/config_paths.rs

@@ -32,7 +32,7 @@
         // TODO: we should determine program_name from argv0 somewhere in this file
 
         // Detect if we're running right out of the CMAKE build directory
-        if exec_path.starts_with(env!("FISH_BUILD_OUTPUT_DIR")) {
+        if exec_path.starts_with(env!("FISH_BUILD_DIR")) {
             let manifest_dir = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
             FLOG!(
                 config,

src/path.rs

@@ -600,7 +600,7 @@ fn make_base_directory(xdg_var: &wstr, non_xdg_homepath: &wstr) -> BaseDirectory
         use std::path::PathBuf;
 
         let mut build_dir = PathBuf::from(env!("FISH_BUILD_DIR"));
-        build_dir.push("fish_root");
+        build_dir.push("fish-test-home");
 
         let err = match std::fs::create_dir_all(&build_dir) {
             Ok(_) => 0,

src/tests/mod.rs

@@ -42,6 +42,7 @@ pub mod prelude {
     use std::cell::RefCell;
     use std::env::set_current_dir;
     use std::ffi::CString;
+    use std::path::PathBuf;
 
     /// A wrapper around a Parser with some test helpers.
     pub struct TestParser {
@@ -89,8 +90,10 @@ pub fn test_init() -> impl ScopeGuarding<Target = ()> {
         DONE.get_or_init(|| {
             // If we are building with `cargo build` and have build w/ `cmake`, FISH_BUILD_DIR might
             // not yet exist.
-            std::fs::create_dir_all(env!("FISH_BUILD_DIR")).unwrap();
-            set_current_dir(env!("FISH_BUILD_DIR")).unwrap();
+            let mut test_dir = PathBuf::from(env!("FISH_BUILD_DIR"));
+            test_dir.push("fish-test");
+            std::fs::create_dir_all(&test_dir).unwrap();
+            set_current_dir(&test_dir).unwrap();
             {
                 let s = CString::new("").unwrap();
                 unsafe {