Rollup merge of rust-lang#141970 - onur-ozkan:skip-stage1-std, r=Kobzol

implement new `x` flag: `--skip-std-check-if-no-download-rustc`

One of our developers (``@RalfJung)`` [reported](https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/Surprising.20stages.20for.20check.20build.20after.20stage.20reorg/with/521925606)[#t-infra/bootstrap > Surprising stages for check build after stage reorg](https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/Surprising.20stages.20for.20check.20build.20after.20stage.20reorg/with/521925606) that working on both the compiler and the library simultaneously with RA enabled is extremely difficult because checking library creates a heavy load on machines (by building stage1 compiler) on each modification. `--skip-std-check-if-no-download-rustc` flag is intended to reduce this heavy load on their IDE integration as much as possible.

Fixes: rust-lang#141955

Author: matthiaskrgr

src/bootstrap/src/core/build_steps/check.rs

@@ -77,6 +77,13 @@ impl Step for Std {
     }
 
     fn run(self, builder: &Builder<'_>) {
+        if !builder.download_rustc() && builder.config.skip_std_check_if_no_download_rustc {
+            eprintln!(
+                "WARNING: `--skip-std-check-if-no-download-rustc` flag was passed and `rust.download-rustc` is not available. Skipping."
+            );
+            return;
+        }
+
         builder.require_submodule("library/stdarch", None);
 
         let stage = self.custom_stage.unwrap_or(builder.top_stage);

src/bootstrap/src/core/config/config.rs

@@ -423,6 +423,11 @@ pub struct Config {
 
     /// Cache for determining path modifications
     pub path_modification_cache: Arc<Mutex<HashMap<Vec<&'static str>, PathFreshness>>>,
+
+    /// Skip checking the standard library if `rust.download-rustc` isn't available.
+    /// This is mostly for RA as building the stage1 compiler to check the library tree
+    /// on each code change might be too much for some computers.
+    pub skip_std_check_if_no_download_rustc: bool,
 }
 
 #[derive(Debug, Default, Clone, Copy, PartialEq, Eq)]
@@ -1507,6 +1512,7 @@ impl Config {
         config.enable_bolt_settings = flags.enable_bolt_settings;
         config.bypass_bootstrap_lock = flags.bypass_bootstrap_lock;
         config.is_running_on_ci = flags.ci.unwrap_or(CiEnv::is_ci());
+        config.skip_std_check_if_no_download_rustc = flags.skip_std_check_if_no_download_rustc;
 
         // Infer the rest of the configuration.
 

src/bootstrap/src/core/config/flags.rs

@@ -182,6 +182,11 @@ pub struct Flags {
     /// Make bootstrap to behave as it's running on the CI environment or not.
     #[arg(global = true, long, value_name = "bool")]
     pub ci: Option<bool>,
+    /// Skip checking the standard library if `rust.download-rustc` isn't available.
+    /// This is mostly for RA as building the stage1 compiler to check the library tree
+    /// on each code change might be too much for some computers.
+    #[arg(global = true, long)]
+    pub skip_std_check_if_no_download_rustc: bool,
 }
 
 impl Flags {

src/bootstrap/src/utils/change_tracker.rs

@@ -416,4 +416,9 @@ pub const CONFIG_CHANGE_HISTORY: &[ChangeInfo] = &[
         severity: ChangeSeverity::Warning,
         summary: "Stage0 library no longer matches the in-tree library, which means stage1 compiler now uses the beta library.",
     },
+    ChangeInfo {
+        change_id: 141970,
+        severity: ChangeSeverity::Info,
+        summary: "Added new bootstrap flag `--skip-std-check-if-no-download-rustc` that skips std checks when download-rustc is unavailable. Mainly intended for developers to reduce RA overhead.",
+    },
 ];

src/doc/rustc-dev-guide/src/building/suggested.md

@@ -59,6 +59,14 @@ always overrides the inner ones.
 
 ## Configuring `rust-analyzer` for `rustc`
 
+### Checking the "library" tree
+
+Checking the "library" tree requires a stage1 compiler, which can be a heavy process on some computers.
+For this reason, bootstrap has a flag called `--skip-std-check-if-no-download-rustc` that skips checking the
+"library" tree if `rust.download-rustc` isn't available. If you want to avoid putting a heavy load on your computer
+with `rust-analyzer`, you can add the `--skip-std-check-if-no-download-rustc` flag to your `./x check` command in
+the `rust-analyzer` configuration.
+
 ### Project-local rust-analyzer setup
 
 `rust-analyzer` can help you check and format your code whenever you save a