ra_project_model: look for Cargo in more places

See #3118

Author: cdisselkoen

Cargo.lock

@@ -22,3 +22,5 @@ serde = { version = "1.0.106", features = ["derive"] }
 serde_json = "1.0.48"
 
 anyhow = "1.0.26"
+
+dirs = "2.0"

crates/ra_project_model/Cargo.toml

@@ -8,7 +8,7 @@ use std::{
     process::Command,
 };
 
-use anyhow::{Context, Result};
+use anyhow::{Context, Error, Result};
 use cargo_metadata::{BuildScript, CargoOpt, Message, MetadataCommand, PackageId};
 use ra_arena::{Arena, Idx};
 use ra_db::Edition;
@@ -145,12 +145,8 @@ impl CargoWorkspace {
         cargo_toml: &Path,
         cargo_features: &CargoConfig,
     ) -> Result<CargoWorkspace> {
-        let _ = Command::new(cargo_binary())
-            .arg("--version")
-            .output()
-            .context("failed to run `cargo --version`, is `cargo` in PATH?")?;
-
         let mut meta = MetadataCommand::new();
+        meta.cargo_path(cargo_binary()?);
         meta.manifest_path(cargo_toml);
         if cargo_features.all_features {
             meta.features(CargoOpt::AllFeatures);
@@ -288,7 +284,7 @@ pub fn load_extern_resources(
     cargo_toml: &Path,
     cargo_features: &CargoConfig,
 ) -> Result<ExternResources> {
-    let mut cmd = Command::new(cargo_binary());
+    let mut cmd = Command::new(cargo_binary()?);
     cmd.args(&["check", "--message-format=json", "--manifest-path"]).arg(cargo_toml);
     if cargo_features.all_features {
         cmd.arg("--all-features");
@@ -337,6 +333,51 @@ fn is_dylib(path: &Path) -> bool {
     }
 }
 
-fn cargo_binary() -> String {
-    env::var("CARGO").unwrap_or_else(|_| "cargo".to_string())
+/// Return a `String` to use for executable `cargo`.
+///
+/// E.g., this may just be `cargo` if that gives a valid Cargo executable; or it
+/// may be a full path to a valid Cargo.
+fn cargo_binary() -> Result<String> {
+    // The current implementation checks three places for a `cargo` to use:
+    // 1) $CARGO environment variable (erroring if this is set but not a usable Cargo)
+    // 2) `cargo`
+    // 3) `~/.cargo/bin/cargo`
+    if let Ok(path) = env::var("CARGO") {
+        if is_valid_cargo(&path) {
+            Ok(path)
+        } else {
+            Err(Error::msg("`CARGO` environment variable points to something that's not a valid Cargo executable"))
+        }
+    } else {
+        let final_path: Option<String> = if is_valid_cargo("cargo") {
+            Some("cargo".to_owned())
+        } else {
+            if let Some(mut path) = dirs::home_dir() {
+                path.push(".cargo");
+                path.push("bin");
+                path.push("cargo");
+                if is_valid_cargo(&path) {
+                    Some(path.into_os_string().into_string().expect("Invalid Unicode in path"))
+                } else {
+                    None
+                }
+            } else {
+                None
+            }
+        };
+        final_path.ok_or(
+            // This error message may also be caused by $PATH or $CARGO not being set correctly for VSCode,
+            // even if they are set correctly in a terminal.
+            // On macOS in particular, launching VSCode from terminal with `code <dirname>` causes VSCode
+            // to inherit environment variables including $PATH and $CARGO from that terminal; but
+            // launching VSCode from Dock does not inherit environment variables from a terminal.
+            // For more discussion, see #3118.
+            Error::msg("Failed to find `cargo` executable. Make sure `cargo` is in `$PATH`, or set `$CARGO` to point to a valid Cargo executable.")
+        )
+    }
+}
+
+/// Does the given `Path` point to a usable `Cargo`?
+fn is_valid_cargo(p: impl AsRef<Path>) -> bool {
+    Command::new(p.as_ref()).arg("--version").output().is_ok()
 }