Add `keep-stage-std` to `x.py`

This keeps only the `std` artifacts compiled by the given stage, not the
compiler. This is useful when working on the latter stages of the
compiler in tandem with the standard library, since you don't have to
rebuild the *entire* compiler when the standard library changes.

3 files changed, 26 insertions, 2 deletions

diff --git a/src/bootstrap/compile.rs b/src/bootstrap/compile.rs
index 08907edef1d..02095c27335 100644
--- a/src/bootstrap/compile.rs
+++ b/src/bootstrap/compile.rs
@@ -59,7 +59,9 @@ impl Step for Std {
         let target = self.target;
         let compiler = self.compiler;
 
-        if builder.config.keep_stage.contains(&compiler.stage) {
+        if builder.config.keep_stage.contains(&compiler.stage)
+            || builder.config.keep_stage_std.contains(&compiler.stage)
+        {
             builder.info("Warning: Using a potentially old libstd. This may not behave well.");
             builder.ensure(StdLink { compiler, target_compiler: compiler, target });
             return;
@@ -472,6 +474,11 @@ impl Step for Rustc {
 
         if builder.config.keep_stage.contains(&compiler.stage) {
             builder.info("Warning: Using a potentially old librustc. This may not behave well.");
+            builder.info("Warning: Use `--keep-stage-std` if you want to rebuild the compiler when it changes");
+            builder.info(
+                "Warning: Please file a GitHub issue if `--keep-stage-std` doesn't work for you.",
+            );
+            builder.info("Warning: It may replace `--keep-stage` in the future");
             builder.ensure(RustcLink { compiler, target_compiler: compiler, target });
             return;
         }
diff --git a/src/bootstrap/config.rs b/src/bootstrap/config.rs
index c74501979f0..53fef7a838d 100644
--- a/src/bootstrap/config.rs
+++ b/src/bootstrap/config.rs
@@ -71,6 +71,7 @@ pub struct Config {
     pub on_fail: Option<String>,
     pub stage: u32,
     pub keep_stage: Vec<u32>,
+    pub keep_stage_std: Vec<u32>,
     pub src: PathBuf,
     pub jobs: Option<u32>,
     pub cmd: Subcommand,
@@ -539,6 +540,7 @@ impl Config {
         config.incremental = flags.incremental;
         config.dry_run = flags.dry_run;
         config.keep_stage = flags.keep_stage;
+        config.keep_stage_std = flags.keep_stage_std;
         config.bindir = "bin".into(); // default
         if let Some(value) = flags.deny_warnings {
             config.deny_warnings = value;
diff --git a/src/bootstrap/flags.rs b/src/bootstrap/flags.rs
index 842c84a3e5c..dad31fc77be 100644
--- a/src/bootstrap/flags.rs
+++ b/src/bootstrap/flags.rs
@@ -19,6 +19,7 @@ pub struct Flags {
     pub on_fail: Option<String>,
     pub stage: Option<u32>,
     pub keep_stage: Vec<u32>,
+    pub keep_stage_std: Vec<u32>,
 
     pub host: Option<Vec<TargetSelection>>,
     pub target: Option<Vec<TargetSelection>>,
@@ -144,6 +145,13 @@ To learn more about a subcommand, run `./x.py <subcommand> -h`",
             (pass multiple times to keep e.g., both stages 0 and 1)",
             "N",
         );
+        opts.optmulti(
+            "",
+            "keep-stage-std",
+            "stage(s) of the standard library to keep without recompiling \
+            (pass multiple times to keep e.g., both stages 0 and 1)",
+            "N",
+        );
         opts.optopt("", "src", "path to the root of the rust checkout", "DIR");
         let j_msg = format!(
             "number of jobs to run in parallel; \
@@ -510,7 +518,9 @@ Arguments:
                 println!("--stage not supported for x.py check, always treated as stage 0");
                 process::exit(1);
             }
-            if matches.opt_str("keep-stage").is_some() {
+            if matches.opt_str("keep-stage").is_some()
+                || matches.opt_str("keep-stage-std").is_some()
+            {
                 println!("--keep-stage not supported for x.py check, only one stage available");
                 process::exit(1);
             }
@@ -528,6 +538,11 @@ Arguments:
                 .into_iter()
                 .map(|j| j.parse().expect("`keep-stage` should be a number"))
                 .collect(),
+            keep_stage_std: matches
+                .opt_strs("keep-stage-std")
+                .into_iter()
+                .map(|j| j.parse().expect("`keep-stage-std` should be a number"))
+                .collect(),
             host: if matches.opt_present("host") {
                 Some(
                     split(&matches.opt_strs("host"))