--- a/.hgignore	Sun Jan 07 15:21:16 2018 -0500
+++ b/.hgignore	Wed Jan 10 08:53:22 2018 -0800
@@ -56,6 +56,8 @@
 locale/*/LC_MESSAGES/hg.mo
 hgext/__index__.py
 
+rust/target/
+
 # Generated wheels
 wheelhouse/
 

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/.cargo/config	Wed Jan 10 08:53:22 2018 -0800
@@ -0,0 +1,7 @@
+# Rust builds with a modern MSVC and uses a newer CRT.
+# Python 2.7 has a shared library dependency on an older CRT (msvcr90.dll).
+# We statically link the modern CRT to avoid multiple msvcr*.dll libraries
+# being loaded and Python possibly picking up symbols from the newer runtime
+# (which would be loaded first).
+[target.'cfg(target_os = "windows")']
+rustflags = ["-Ctarget-feature=+crt-static"]

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/Cargo.toml	Wed Jan 10 08:53:22 2018 -0800
@@ -0,0 +1,2 @@
+[workspace]
+members = ["hgcli"]

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/README.rst	Wed Jan 10 08:53:22 2018 -0800
@@ -0,0 +1,78 @@
+===================
+Mercurial Rust Code
+===================
+
+This directory contains various Rust code for the Mercurial project.
+
+The top-level ``Cargo.toml`` file defines a workspace containing
+all primary Mercurial crates.
+
+Building
+========
+
+To build the Rust components::
+
+   $ cargo build
+
+If you prefer a non-debug / release configuration::
+
+   $ cargo build --release
+
+Features
+--------
+
+The following Cargo features are available:
+
+localdev (default)
+   Produce files that work with an in-source-tree build.
+
+   In this mode, the build finds and uses a ``python2.7`` binary from
+   ``PATH``. The ``hg`` binary assumes it runs from ``rust/target/<target>hg``
+   and it finds Mercurial files at ``dirname($0)/../../../``.
+
+Build Mechanism
+---------------
+
+The produced ``hg`` binary is *bound* to a CPython installation. The
+binary links against and loads a CPython library that is discovered
+at build time (by a ``build.rs`` Cargo build script). The Python
+standard library defined by this CPython installation is also used.
+
+Finding the appropriate CPython installation to use is done by
+the ``python27-sys`` crate's ``build.rs``. Its search order is::
+
+1. ``PYTHON_SYS_EXECUTABLE`` environment variable.
+2. ``python`` executable on ``PATH``
+3. ``python2`` executable on ``PATH``
+4. ``python2.7`` executable on ``PATH``
+
+Additional verification of the found Python will be performed by our
+``build.rs`` to ensure it meets Mercurial's requirements.
+
+Details about the build-time configured Python are built into the
+produced ``hg`` binary. This means that a built ``hg`` binary is only
+suitable for a specific, well-defined role. These roles are controlled
+by Cargo features (see above).
+
+Running
+=======
+
+The ``hgcli`` crate produces an ``hg`` binary. You can run this binary
+via ``cargo run``::
+
+   $ cargo run --manifest-path hgcli/Cargo.toml
+
+Or directly::
+
+   $ target/debug/hg
+   $ target/release/hg
+
+You can also run the test harness with this binary::
+
+   $ ./run-tests.py --with-hg ../rust/target/debug/hg
+
+.. note::
+
+   Integration with the test harness is still preliminary. Remember to
+   ``cargo build`` after changes because the test harness doesn't yet
+   automatically build Rust code.

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/hgcli/Cargo.lock	Wed Jan 10 08:53:22 2018 -0800
@@ -0,0 +1,127 @@
+[[package]]
+name = "aho-corasick"
+version = "0.5.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "memchr 0.1.11 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "cpython"
+version = "0.1.0"
+source = "git+https://github.com/indygreg/rust-cpython.git?rev=c90d65cf84abfffce7ef54476bbfed56017a2f52#c90d65cf84abfffce7ef54476bbfed56017a2f52"
+dependencies = [
+ "libc 0.2.34 (registry+https://github.com/rust-lang/crates.io-index)",
+ "num-traits 0.1.41 (registry+https://github.com/rust-lang/crates.io-index)",
+ "python27-sys 0.1.2 (git+https://github.com/indygreg/rust-cpython.git?rev=c90d65cf84abfffce7ef54476bbfed56017a2f52)",
+]
+
+[[package]]
+name = "hgcli"
+version = "0.1.0"
+dependencies = [
+ "cpython 0.1.0 (git+https://github.com/indygreg/rust-cpython.git?rev=c90d65cf84abfffce7ef54476bbfed56017a2f52)",
+ "libc 0.2.34 (registry+https://github.com/rust-lang/crates.io-index)",
+ "python27-sys 0.1.2 (git+https://github.com/indygreg/rust-cpython.git?rev=c90d65cf84abfffce7ef54476bbfed56017a2f52)",
+]
+
+[[package]]
+name = "kernel32-sys"
+version = "0.2.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "winapi 0.2.8 (registry+https://github.com/rust-lang/crates.io-index)",
+ "winapi-build 0.1.1 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "libc"
+version = "0.2.34"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
+name = "memchr"
+version = "0.1.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "libc 0.2.34 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "num-traits"
+version = "0.1.41"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
+name = "python27-sys"
+version = "0.1.2"
+source = "git+https://github.com/indygreg/rust-cpython.git?rev=c90d65cf84abfffce7ef54476bbfed56017a2f52#c90d65cf84abfffce7ef54476bbfed56017a2f52"
+dependencies = [
+ "libc 0.2.34 (registry+https://github.com/rust-lang/crates.io-index)",
+ "regex 0.1.80 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "regex"
+version = "0.1.80"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "aho-corasick 0.5.3 (registry+https://github.com/rust-lang/crates.io-index)",
+ "memchr 0.1.11 (registry+https://github.com/rust-lang/crates.io-index)",
+ "regex-syntax 0.3.9 (registry+https://github.com/rust-lang/crates.io-index)",
+ "thread_local 0.2.7 (registry+https://github.com/rust-lang/crates.io-index)",
+ "utf8-ranges 0.1.3 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "regex-syntax"
+version = "0.3.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
+name = "thread-id"
+version = "2.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "kernel32-sys 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)",
+ "libc 0.2.34 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "thread_local"
+version = "0.2.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "thread-id 2.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "utf8-ranges"
+version = "0.1.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
+name = "winapi"
+version = "0.2.8"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
+name = "winapi-build"
+version = "0.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[metadata]
+"checksum aho-corasick 0.5.3 (registry+https://github.com/rust-lang/crates.io-index)" = "ca972c2ea5f742bfce5687b9aef75506a764f61d37f8f649047846a9686ddb66"
+"checksum cpython 0.1.0 (git+https://github.com/indygreg/rust-cpython.git?rev=c90d65cf84abfffce7ef54476bbfed56017a2f52)" = "<none>"
+"checksum kernel32-sys 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)" = "7507624b29483431c0ba2d82aece8ca6cdba9382bff4ddd0f7490560c056098d"
+"checksum libc 0.2.34 (registry+https://github.com/rust-lang/crates.io-index)" = "36fbc8a8929c632868295d0178dd8f63fc423fd7537ad0738372bd010b3ac9b0"
+"checksum memchr 0.1.11 (registry+https://github.com/rust-lang/crates.io-index)" = "d8b629fb514376c675b98c1421e80b151d3817ac42d7c667717d282761418d20"
+"checksum num-traits 0.1.41 (registry+https://github.com/rust-lang/crates.io-index)" = "cacfcab5eb48250ee7d0c7896b51a2c5eec99c1feea5f32025635f5ae4b00070"
+"checksum python27-sys 0.1.2 (git+https://github.com/indygreg/rust-cpython.git?rev=c90d65cf84abfffce7ef54476bbfed56017a2f52)" = "<none>"
+"checksum regex 0.1.80 (registry+https://github.com/rust-lang/crates.io-index)" = "4fd4ace6a8cf7860714a2c2280d6c1f7e6a413486c13298bbc86fd3da019402f"
+"checksum regex-syntax 0.3.9 (registry+https://github.com/rust-lang/crates.io-index)" = "f9ec002c35e86791825ed294b50008eea9ddfc8def4420124fbc6b08db834957"
+"checksum thread-id 2.0.0 (registry+https://github.com/rust-lang/crates.io-index)" = "a9539db560102d1cef46b8b78ce737ff0bb64e7e18d35b2a5688f7d097d0ff03"
+"checksum thread_local 0.2.7 (registry+https://github.com/rust-lang/crates.io-index)" = "8576dbbfcaef9641452d5cf0df9b0e7eeab7694956dd33bb61515fb8f18cfdd5"
+"checksum utf8-ranges 0.1.3 (registry+https://github.com/rust-lang/crates.io-index)" = "a1ca13c08c41c9c3e04224ed9ff80461d97e121589ff27c753a16cb10830ae0f"
+"checksum winapi 0.2.8 (registry+https://github.com/rust-lang/crates.io-index)" = "167dc9d6949a9b857f3451275e911c3f44255842c1f7a76f33c55103a909087a"
+"checksum winapi-build 0.1.1 (registry+https://github.com/rust-lang/crates.io-index)" = "2d315eee3b34aca4797b2da6b13ed88266e6d612562a0c46390af8299fc699bc"

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/hgcli/Cargo.toml	Wed Jan 10 08:53:22 2018 -0800
@@ -0,0 +1,35 @@
+[package]
+name = "hgcli"
+version = "0.1.0"
+authors = ["Gregory Szorc <gregory.szorc@gmail.com>"]
+license = "GPL-2.0"
+
+build = "build.rs"
+
+[[bin]]
+name = "hg"
+path = "src/main.rs"
+
+[features]
+# localdev: detect Python in PATH and use files from source checkout.
+default = ["localdev"]
+localdev = []
+
+[dependencies]
+libc = "0.2.34"
+
+# We currently use a custom build of cpython and python27-sys with the
+# following changes:
+# * GILGuard call of prepare_freethreaded_python() is removed.
+# TODO switch to official release when our changes are incorporated.
+[dependencies.cpython]
+version = "0.1"
+default-features = false
+features = ["python27-sys"]
+git = "https://github.com/indygreg/rust-cpython.git"
+rev = "c90d65cf84abfffce7ef54476bbfed56017a2f52"
+
+[dependencies.python27-sys]
+version = "0.1.2"
+git = "https://github.com/indygreg/rust-cpython.git"
+rev = "c90d65cf84abfffce7ef54476bbfed56017a2f52"

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/hgcli/build.rs	Wed Jan 10 08:53:22 2018 -0800
@@ -0,0 +1,128 @@
+// build.rs -- Configure build environment for `hgcli` Rust package.
+//
+// Copyright 2017 Gregory Szorc <gregory.szorc@gmail.com>
+//
+// This software may be used and distributed according to the terms of the
+// GNU General Public License version 2 or any later version.
+
+use std::collections::HashMap;
+use std::env;
+use std::path::Path;
+#[cfg(target_os = "windows")]
+use std::path::PathBuf;
+
+use std::process::Command;
+
+struct PythonConfig {
+    python: String,
+    config: HashMap<String, String>,
+}
+
+fn get_python_config() -> PythonConfig {
+    // The python27-sys crate exports a Cargo variable defining the full
+    // path to the interpreter being used.
+    let python = env::var("DEP_PYTHON27_PYTHON_INTERPRETER").expect(
+        "Missing DEP_PYTHON27_PYTHON_INTERPRETER; bad python27-sys crate?",
+    );
+
+    if !Path::new(&python).exists() {
+        panic!(
+            "Python interpreter {} does not exist; this should never happen",
+            python
+        );
+    }
+
+    // This is a bit hacky but it gets the job done.
+    let separator = "SEPARATOR STRING";
+
+    let script = "import sysconfig; \
+c = sysconfig.get_config_vars(); \
+print('SEPARATOR STRING'.join('%s=%s' % i for i in c.items()))";
+
+    let mut command = Command::new(&python);
+    command.arg("-c").arg(script);
+
+    let out = command.output().unwrap();
+
+    if !out.status.success() {
+        panic!(
+            "python script failed: {}",
+            String::from_utf8_lossy(&out.stderr)
+        );
+    }
+
+    let stdout = String::from_utf8_lossy(&out.stdout);
+    let mut m = HashMap::new();
+
+    for entry in stdout.split(separator) {
+        let mut parts = entry.splitn(2, "=");
+        let key = parts.next().unwrap();
+        let value = parts.next().unwrap();
+        m.insert(String::from(key), String::from(value));
+    }
+
+    PythonConfig {
+        python: python,
+        config: m,
+    }
+}
+
+#[cfg(not(target_os = "windows"))]
+fn have_shared(config: &PythonConfig) -> bool {
+    match config.config.get("Py_ENABLE_SHARED") {
+        Some(value) => value == "1",
+        None => false,
+    }
+}
+
+#[cfg(target_os = "windows")]
+fn have_shared(config: &PythonConfig) -> bool {
+    // python27.dll should exist next to python2.7.exe.
+    let mut dll = PathBuf::from(&config.python);
+    dll.pop();
+    dll.push("python27.dll");
+
+    return dll.exists();
+}
+
+const REQUIRED_CONFIG_FLAGS: [&'static str; 2] = ["Py_USING_UNICODE", "WITH_THREAD"];
+
+fn main() {
+    let config = get_python_config();
+
+    println!("Using Python: {}", config.python);
+    println!("cargo:rustc-env=PYTHON_INTERPRETER={}", config.python);
+
+    let prefix = config.config.get("prefix").unwrap();
+
+    println!("Prefix: {}", prefix);
+
+    // TODO Windows builds don't expose these config flags. Figure out another
+    // way.
+    #[cfg(not(target_os = "windows"))]
+    for key in REQUIRED_CONFIG_FLAGS.iter() {
+        let result = match config.config.get(*key) {
+            Some(value) => value == "1",
+            None => false,
+        };
+
+        if !result {
+            panic!("Detected Python requires feature {}", key);
+        }
+    }
+
+    // We need a Python shared library.
+    if !have_shared(&config) {
+        panic!("Detected Python lacks a shared library, which is required");
+    }
+
+    let ucs4 = match config.config.get("Py_UNICODE_SIZE") {
+        Some(value) => value == "4",
+        None => false,
+    };
+
+    if !ucs4 {
+        #[cfg(not(target_os = "windows"))]
+        panic!("Detected Python doesn't support UCS-4 code points");
+    }
+}

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/hgcli/src/main.rs	Wed Jan 10 08:53:22 2018 -0800
@@ -0,0 +1,222 @@
+// main.rs -- Main routines for `hg` program
+//
+// Copyright 2017 Gregory Szorc <gregory.szorc@gmail.com>
+//
+// This software may be used and distributed according to the terms of the
+// GNU General Public License version 2 or any later version.
+
+extern crate libc;
+extern crate cpython;
+extern crate python27_sys;
+
+use cpython::{NoArgs, ObjectProtocol, PyModule, PyResult, Python};
+use libc::{c_char, c_int};
+
+use std::env;
+use std::path::PathBuf;
+use std::ffi::CString;
+#[cfg(target_family = "unix")]
+use std::os::unix::ffi::OsStringExt;
+
+#[derive(Debug)]
+struct Environment {
+    _exe: PathBuf,
+    python_exe: PathBuf,
+    python_home: PathBuf,
+    mercurial_modules: PathBuf,
+}
+
+/// Run Mercurial locally from a source distribution or checkout.
+///
+/// hg is <srcdir>/rust/target/<target>/hg
+/// Python interpreter is detected by build script.
+/// Python home is relative to Python interpreter.
+/// Mercurial files are relative to hg binary, which is relative to source root.
+#[cfg(feature = "localdev")]
+fn get_environment() -> Environment {
+    let exe = env::current_exe().unwrap();
+
+    let mut mercurial_modules = exe.clone();
+    mercurial_modules.pop(); // /rust/target/<target>
+    mercurial_modules.pop(); // /rust/target
+    mercurial_modules.pop(); // /rust
+    mercurial_modules.pop(); // /
+
+    let python_exe: &'static str = env!("PYTHON_INTERPRETER");
+    let python_exe = PathBuf::from(python_exe);
+
+    let mut python_home = python_exe.clone();
+    python_home.pop();
+
+    // On Windows, python2.7.exe exists at the root directory of the Python
+    // install. Everywhere else, the Python install root is one level up.
+    if !python_exe.ends_with("python2.7.exe") {
+        python_home.pop();
+    }
+
+    Environment {
+        _exe: exe.clone(),
+        python_exe: python_exe,
+        python_home: python_home,
+        mercurial_modules: mercurial_modules.to_path_buf(),
+    }
+}
+
+// On UNIX, argv starts as an array of char*. So it is easy to convert
+// to C strings.
+#[cfg(target_family = "unix")]
+fn args_to_cstrings() -> Vec<CString> {
+    env::args_os()
+        .map(|a| CString::new(a.into_vec()).unwrap())
+        .collect()
+}
+
+// TODO Windows support is incomplete. We should either use env::args_os()
+// (or call into GetCommandLineW() + CommandLinetoArgvW()), convert these to
+// PyUnicode instances, and pass these into Python/Mercurial outside the
+// standard PySys_SetArgvEx() mechanism. This will allow us to preserve the
+// raw bytes (since PySys_SetArgvEx() is based on char* and can drop wchar
+// data.
+//
+// For now, we use env::args(). This will choke on invalid UTF-8 arguments.
+// But it is better than nothing.
+#[cfg(target_family = "windows")]
+fn args_to_cstrings() -> Vec<CString> {
+    env::args().map(|a| CString::new(a).unwrap()).collect()
+}
+
+fn set_python_home(env: &Environment) {
+    let raw = CString::new(env.python_home.to_str().unwrap())
+        .unwrap()
+        .into_raw();
+    unsafe {
+        python27_sys::Py_SetPythonHome(raw);
+    }
+}
+
+fn update_encoding(_py: Python, _sys_mod: &PyModule) {
+    // Call sys.setdefaultencoding("undefined") if HGUNICODEPEDANTRY is set.
+    let pedantry = env::var("HGUNICODEPEDANTRY").is_ok();
+
+    if pedantry {
+        // site.py removes the sys.setdefaultencoding attribute. So we need
+        // to reload the module to get a handle on it. This is a lesser
+        // used feature and we'll support this later.
+        // TODO support this
+        panic!("HGUNICODEPEDANTRY is not yet supported");
+    }
+}
+
+fn update_modules_path(env: &Environment, py: Python, sys_mod: &PyModule) {
+    let sys_path = sys_mod.get(py, "path").unwrap();
+    sys_path
+        .call_method(py, "insert", (0, env.mercurial_modules.to_str()), None)
+        .expect("failed to update sys.path to location of Mercurial modules");
+}
+
+fn run() -> Result<(), i32> {
+    let env = get_environment();
+
+    //println!("{:?}", env);
+
+    // Tell Python where it is installed.
+    set_python_home(&env);
+
+    // Set program name. The backing memory needs to live for the duration of the
+    // interpreter.
+    //
+    // Yes, we use the path to the Python interpreter not argv[0] here. The
+    // reason is because Python uses the given path to find the location of
+    // Python files. Apparently we could define our own ``Py_GetPath()``
+    // implementation. But this may require statically linking Python, which is
+    // not desirable.
+    let program_name = CString::new(env.python_exe.to_str().unwrap())
+        .unwrap()
+        .as_ptr();
+    unsafe {
+        python27_sys::Py_SetProgramName(program_name as *mut i8);
+    }
+
+    unsafe {
+        python27_sys::Py_Initialize();
+    }
+
+    // https://docs.python.org/2/c-api/init.html#c.PySys_SetArgvEx has important
+    // usage information about PySys_SetArgvEx:
+    //
+    // * It says the first argument should be the script that is being executed.
+    //   If not a script, it can be empty. We are definitely not a script.
+    //   However, parts of Mercurial do look at sys.argv[0]. So we need to set
+    //   something here.
+    //
+    // * When embedding Python, we should use ``PySys_SetArgvEx()`` and set
+    //   ``updatepath=0`` for security reasons. Essentially, Python's default
+    //   logic will treat an empty argv[0] in a manner that could result in
+    //   sys.path picking up directories it shouldn't and this could lead to
+    //   loading untrusted modules.
+
+    // env::args() will panic if it sees a non-UTF-8 byte sequence. And
+    // Mercurial supports arbitrary encodings of input data. So we need to
+    // use OS-specific mechanisms to get the raw bytes without UTF-8
+    // interference.
+    let args = args_to_cstrings();
+    let argv: Vec<*const c_char> = args.iter().map(|a| a.as_ptr()).collect();
+
+    unsafe {
+        python27_sys::PySys_SetArgvEx(args.len() as c_int, argv.as_ptr() as *mut *mut i8, 0);
+    }
+
+    let result;
+    {
+        // These need to be dropped before we call Py_Finalize(). Hence the
+        // block.
+        let gil = Python::acquire_gil();
+        let py = gil.python();
+
+        // Mercurial code could call sys.exit(), which will call exit()
+        // itself. So this may not return.
+        // TODO this may cause issues on Windows due to the CRT mismatch.
+        // Investigate if we can intercept sys.exit() or SystemExit() to
+        // ensure we handle process exit.
+        result = match run_py(&env, py) {
+            // Print unhandled exceptions and exit code 255, as this is what
+            // `python` does.
+            Err(err) => {
+                err.print(py);
+                Err(255)
+            }
+            Ok(()) => Ok(()),
+        };
+    }
+
+    unsafe {
+        python27_sys::Py_Finalize();
+    }
+
+    result
+}
+
+fn run_py(env: &Environment, py: Python) -> PyResult<()> {
+    let sys_mod = py.import("sys").unwrap();
+
+    update_encoding(py, &sys_mod);
+    update_modules_path(&env, py, &sys_mod);
+
+    // TODO consider a better error message on failure to import.
+    let demand_mod = py.import("hgdemandimport")?;
+    demand_mod.call(py, "enable", NoArgs, None)?;
+
+    let dispatch_mod = py.import("mercurial.dispatch")?;
+    dispatch_mod.call(py, "run", NoArgs, None)?;
+
+    Ok(())
+}
+
+fn main() {
+    let exit_code = match run() {
+        Err(err) => err,
+        Ok(()) => 0,
+    };
+
+    std::process::exit(exit_code);
+}

--- a/tests/run-tests.py	Sun Jan 07 15:21:16 2018 -0500
+++ b/tests/run-tests.py	Wed Jan 10 08:53:22 2018 -0800
@@ -2435,12 +2435,27 @@
             self._tmpbindir = os.path.join(self._hgtmp, b'install', b'bin')
             os.makedirs(self._tmpbindir)
 
-            # This looks redundant with how Python initializes sys.path from
-            # the location of the script being executed.  Needed because the
-            # "hg" specified by --with-hg is not the only Python script
-            # executed in the test suite that needs to import 'mercurial'
-            # ... which means it's not really redundant at all.
-            self._pythondir = self._bindir
+            normbin = os.path.normpath(os.path.abspath(whg))
+            normbin = normbin.replace(os.sep.encode('ascii'), b'/')
+
+            # Other Python scripts in the test harness need to
+            # `import mercurial`. If `hg` is a Python script, we assume
+            # the Mercurial modules are relative to its path and tell the tests
+            # to load Python modules from its directory.
+            with open(whg, 'rb') as fh:
+                initial = fh.read(1024)
+
+            if re.match(b'#!.*python', initial):
+                self._pythondir = self._bindir
+            # If it looks like our in-repo Rust binary, use the source root.
+            # This is a bit hacky. But rhg is still not supported outside the
+            # source directory. So until it is, do the simple thing.
+            elif re.search(b'|/rust/target/[^/]+/hg', normbin):
+                self._pythondir = os.path.dirname(self._testdir)
+            # Fall back to the legacy behavior.
+            else:
+                self._pythondir = self._bindir
+
         else:
             self._installdir = os.path.join(self._hgtmp, b"install")
             self._bindir = os.path.join(self._installdir, b"bin")