Add a -Z query-threads compiler option

src/librustc/session/config.rs

@@ -1067,6 +1067,8 @@ fn parse_optimization_fuel(slot: &mut Option<(String, u64)>, v: Option<&str>) ->
         "prints the llvm optimization passes being run"),
     ast_json: bool = (false, parse_bool, [UNTRACKED],
         "print the AST as JSON and halt"),
+    query_threads: Option<usize> = (None, parse_opt_uint, [UNTRACKED],
+        "execute queries on a thread pool with N threads"),
     ast_json_noexpand: bool = (false, parse_bool, [UNTRACKED],
         "print the pre-expansion AST as JSON and halt"),
     ls: bool = (false, parse_bool, [UNTRACKED],
@@ -1663,6 +1665,10 @@ pub fn build_session_options_and_crate_config(matches: &getopts::Matches)
         }
     }
 
+    if debugging_opts.query_threads == Some(0) {
+        early_error(error_format, "Value for query threads must be a positive nonzero integer");
+    }
+
     if codegen_units == Some(0) {
         early_error(error_format, "Value for codegen units must be a positive nonzero integer");
     }

src/librustc/session/mod.rs

@@ -725,6 +725,12 @@ pub fn consider_optimizing<T: Fn() -> String>(&self, crate_name: &str, msg: T) -
         ret
     }
 
+    /// Returns the number of query threads that should be used for this
+    /// compilation
+    pub fn query_threads(&self) -> usize {
+        self.opts.debugging_opts.query_threads.unwrap_or(1)
+    }
+
     /// Returns the number of codegen units that should be used for this
     /// compilation
     pub fn codegen_units(&self) -> usize {