2 files changed, 78 insertions, 0 deletions

diff --git a/crates/toolchain/Cargo.toml b/crates/toolchain/Cargo.toml
new file mode 100644
index 000000000..4856668f8
--- /dev/null
+++ b/crates/toolchain/Cargo.toml
@@ -0,0 +1,12 @@
+[package]
+name = "toolchain"
+version = "0.0.0"
+license = "MIT OR Apache-2.0"
+authors = ["rust-analyzer developers"]
+edition = "2018"
+
+[lib]
+doctest = false
+
+[dependencies]
+home = "0.5.3"
diff --git a/crates/toolchain/src/lib.rs b/crates/toolchain/src/lib.rs
new file mode 100644
index 000000000..3b6886f5b
--- /dev/null
+++ b/crates/toolchain/src/lib.rs
@@ -0,0 +1,66 @@
+//! Discovery of `cargo` & `rustc` executables.
+use std::{env, iter, path::PathBuf};
+
+pub fn cargo() -> PathBuf {
+    get_path_for_executable("cargo")
+}
+
+pub fn rustc() -> PathBuf {
+    get_path_for_executable("rustc")
+}
+
+pub fn rustup() -> PathBuf {
+    get_path_for_executable("rustup")
+}
+
+pub fn rustfmt() -> PathBuf {
+    get_path_for_executable("rustfmt")
+}
+
+/// Return a `PathBuf` to use for the given executable.
+///
+/// E.g., `get_path_for_executable("cargo")` may return just `cargo` if that
+/// gives a valid Cargo executable; or it may return a full path to a valid
+/// Cargo.
+fn get_path_for_executable(executable_name: &'static str) -> PathBuf {
+    // The current implementation checks three places for an executable to use:
+    // 1) Appropriate environment variable (erroring if this is set but not a usable executable)
+    //      example: for cargo, this checks $CARGO environment variable; for rustc, $RUSTC; etc
+    // 2) `<executable_name>`
+    //      example: for cargo, this tries just `cargo`, which will succeed if `cargo` is on the $PATH
+    // 3) `~/.cargo/bin/<executable_name>`
+    //      example: for cargo, this tries ~/.cargo/bin/cargo
+    //      It seems that this is a reasonable place to try for cargo, rustc, and rustup
+    let env_var = executable_name.to_ascii_uppercase();
+    if let Some(path) = env::var_os(&env_var) {
+        return path.into();
+    }
+
+    if lookup_in_path(executable_name) {
+        return executable_name.into();
+    }
+
+    if let Some(mut path) = home::home_dir() {
+        path.push(".cargo");
+        path.push("bin");
+        path.push(executable_name);
+        if let Some(path) = probe(path) {
+            return path;
+        }
+    }
+
+    executable_name.into()
+}
+
+fn lookup_in_path(exec: &str) -> bool {
+    let paths = env::var_os("PATH").unwrap_or_default();
+    env::split_paths(&paths).map(|path| path.join(exec)).find_map(probe).is_some()
+}
+
+fn probe(path: PathBuf) -> Option<PathBuf> {
+    let with_extension = match env::consts::EXE_EXTENSION {
+        "" => None,
+        it => Some(path.with_extension(it)),
+    };
+    iter::once(path).chain(with_extension).find(|it| it.is_file())
+}