Stop defaulting to $PATH searches when the binary can't be found and …

…causing infinite recursion

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             description("rustup's metadata is out of date.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 +301,13 @@ impl<'a> Toolchain<'a> {
         };
 
         let bin_path = self.path.join("bin").join(&binary);
-        let mut cmd = Command::new(if utils::is_file(&bin_path) {
-            &bin_path
+        let mut cmd = if utils::is_file(&bin_path) {
+            Command::new(&bin_path)
         } else {
-            // If the bin doesn't actually exist in the sysroot, let the OS try
-            // to resolve it globally for us
-            Path::new(&binary)
-        });
+            return Err(ErrorKind::BinaryNotFound(self.name.clone(),
+                                                 binary.to_string_lossy().into())
+                        .into())
+        };
         self.set_env(&mut cmd);
         Ok(cmd)
     }