Change keep-stage to only affect the passed stage

The best way to build a stage 2 rustc is now probably
  ./x.py build --stage 2 src/rustc # once
  ./x.py build --stage 2 --keep-stage 1 src/rustc

src/bootstrap/compile.rs

@@ -67,9 +67,8 @@ impl Step for Std {
         let target = self.target;
         let compiler = self.compiler;
 
-        if let Some(keep_stage) = builder.config.keep_stage {
+        if builder.config.keep_stage.contains(&compiler.stage) {
-            if keep_stage <= compiler.stage {
+            builder.info("Warning: Using a potentially old libstd. This may not behave well.");
-                println!("Warning: Using a potentially old libstd. This may not behave well.");
             builder.ensure(StdLink {
                 compiler: compiler,
                 target_compiler: compiler,
@@ -77,7 +76,6 @@ impl Step for Std {
             });
             return;
         }
-        }
 
         builder.ensure(StartupObjects { compiler, target });
 
@@ -362,9 +360,10 @@ impl Step for Test {
         let target = self.target;
         let compiler = self.compiler;
 
-        if let Some(keep_stage) = builder.config.keep_stage {
+        builder.ensure(Std { compiler, target });
-            if keep_stage <= compiler.stage {
+
-                println!("Warning: Using a potentially old libtest. This may not behave well.");
+        if builder.config.keep_stage.contains(&compiler.stage) {
+            builder.info("Warning: Using a potentially old libtest. This may not behave well.");
             builder.ensure(TestLink {
                 compiler: compiler,
                 target_compiler: compiler,
@@ -372,9 +371,6 @@ impl Step for Test {
             });
             return;
         }
-        }
-
-        builder.ensure(Std { compiler, target });
 
         if builder.force_use_stage1(compiler, target) {
             builder.ensure(Test {
@@ -490,9 +486,10 @@ impl Step for Rustc {
         let compiler = self.compiler;
         let target = self.target;
 
-        if let Some(keep_stage) = builder.config.keep_stage {
+        builder.ensure(Test { compiler, target });
-            if keep_stage <= compiler.stage {
+
-                println!("Warning: Using a potentially old librustc. This may not behave well.");
+        if builder.config.keep_stage.contains(&compiler.stage) {
+            builder.info("Warning: Using a potentially old librustc. This may not behave well.");
             builder.ensure(RustcLink {
                 compiler: compiler,
                 target_compiler: compiler,
@@ -500,9 +497,6 @@ impl Step for Rustc {
             });
             return;
         }
-        }
-
-        builder.ensure(Test { compiler, target });
 
         if builder.force_use_stage1(compiler, target) {
             builder.ensure(Rustc {
@@ -660,15 +654,13 @@ impl Step for CodegenBackend {
 
         builder.ensure(Rustc { compiler, target });
 
-        if let Some(keep_stage) = builder.config.keep_stage {
+        if builder.config.keep_stage.contains(&compiler.stage) {
-            if keep_stage <= compiler.stage {
+            builder.info("Warning: Using a potentially old codegen backend. \
-                println!("Warning: Using a potentially old codegen backend. \
                 This may not behave well.");
             // Codegen backends are linked separately from this step today, so we don't do
             // anything here.
             return;
         }
-        }
 
         if builder.force_use_stage1(compiler, target) {
             builder.ensure(CodegenBackend {

src/bootstrap/config.rs

@@ -63,7 +63,7 @@ pub struct Config {
 
     pub on_fail: Option<String>,
     pub stage: Option<u32>,
-    pub keep_stage: Option<u32>,
+    pub keep_stage: Vec<u32>,
     pub src: PathBuf,
     pub jobs: Option<u32>,
     pub cmd: Subcommand,

src/bootstrap/flags.rs

@@ -31,7 +31,7 @@ pub struct Flags {
     pub verbose: usize, // number of -v args; each extra -v after the first is passed to Cargo
     pub on_fail: Option<String>,
     pub stage: Option<u32>,
-    pub keep_stage: Option<u32>,
+    pub keep_stage: Vec<u32>,
 
     pub host: Vec<Interned<String>>,
     pub target: Vec<Interned<String>>,
@@ -122,7 +122,7 @@ To learn more about a subcommand, run `./x.py <subcommand> -h`"
         opts.optopt("", "on-fail", "command to run on failure", "CMD");
         opts.optflag("", "dry-run", "dry run; don't build anything");
         opts.optopt("", "stage", "stage to build", "N");
-        opts.optopt("", "keep-stage", "stage to keep without recompiling", "N");
+        opts.optmulti("", "keep-stage", "stage(s) to keep without recompiling", "N");
         opts.optopt("", "src", "path to the root of the rust checkout", "DIR");
         opts.optopt("j", "jobs", "number of jobs to run in parallel", "JOBS");
         opts.optflag("h", "help", "print this help message");
@@ -402,7 +402,9 @@ Arguments:
             dry_run: matches.opt_present("dry-run"),
             on_fail: matches.opt_str("on-fail"),
             rustc_error_format: matches.opt_str("error-format"),
-            keep_stage: matches.opt_str("keep-stage").map(|j| j.parse().unwrap()),
+            keep_stage: matches.opt_strs("keep-stage")
+                .into_iter().map(|j| j.parse().unwrap())
+                .collect(),
             host: split(matches.opt_strs("host"))
                 .into_iter()
                 .map(|x| INTERNER.intern_string(x))