Free intermediate translation results as soon as possible

This fixes the recently introduced peak memory usage regression by
freeing the intermediate results as soon as they're not required
anymore instead of keeping them around for the whole compilation
process.

Refs #8077

src/librustc/driver/driver.rs

@@ -409,12 +409,19 @@ pub fn stop_after_phase_5(sess: Session) -> bool {
 pub fn compile_input(sess: Session, cfg: ast::CrateConfig, input: &input,
                      outdir: &Option<Path>, output: &Option<Path>) {
     let outputs = build_output_filenames(input, outdir, output, [], sess);
-    let crate = phase_1_parse_input(sess, cfg.clone(), input);
-    if stop_after_phase_1(sess) { return; }
-    let expanded_crate = phase_2_configure_and_expand(sess, cfg, crate);
-    let analysis = phase_3_run_analysis_passes(sess, expanded_crate);
-    if stop_after_phase_3(sess) { return; }
-    let trans = phase_4_translate_to_llvm(sess, expanded_crate, &analysis, outputs);
+    // We need nested scopes here, because the intermediate results can keep
+    // large chunks of memory alive and we want to free them as soon as
+    // possible to keep the peak memory usage low
+    let trans = {
+        let expanded_crate = {
+            let crate = phase_1_parse_input(sess, cfg.clone(), input);
+            if stop_after_phase_1(sess) { return; }
+            phase_2_configure_and_expand(sess, cfg, crate)
+        };
+        let analysis = phase_3_run_analysis_passes(sess, expanded_crate);
+        if stop_after_phase_3(sess) { return; }
+        phase_4_translate_to_llvm(sess, expanded_crate, &analysis, outputs)
+    };
     phase_5_run_llvm_passes(sess, &trans, outputs);
     if stop_after_phase_5(sess) { return; }
     phase_6_link_output(sess, &trans, outputs);