Stop defaulting to $PATH searches when the binary can't be found and causing infinite recursion

Author: Manishearth

src/rustup/errors.rs

@@ -22,6 +22,10 @@ error_chain! {
             description("toolchain is not installed")
             display("toolchain '{}' is not installed", t)
         }
+        BinaryNotFound(t: String, bin: String) {
+            description("toolchain does not contain binary")
+            display("toolchain '{}' does not have the binary `{}`", t, bin)
+        }
         NeedMetadataUpgrade {
             description("rustup's metadata is out of date. run `rustup self upgrade-data`")
         }

src/rustup/toolchain.rs

@@ -301,13 +301,20 @@ impl<'a> Toolchain<'a> {
         };
 
         let bin_path = self.path.join("bin").join(&binary);
-        let mut cmd = Command::new(if utils::is_file(&bin_path) {
+        let path = if utils::is_file(&bin_path) {
             &bin_path
         } else {
-            // If the bin doesn't actually exist in the sysroot, let the OS try
-            // to resolve it globally for us
+            let recursion_count = env::var("RUST_RECURSION_COUNT").ok()
+                .and_then(|s| s.parse().ok()).unwrap_or(0);
+            if recursion_count > 4 {
+                return Err(ErrorKind::BinaryNotFound(self.name.clone(),
+                                                     binary.to_string_lossy()
+                                                           .into())
+                            .into())
+            }
             Path::new(&binary)
-        });
+        };
+        let mut cmd = Command::new(&path);
         self.set_env(&mut cmd);
         Ok(cmd)
     }