Remove usage of compiler_for from the compile::Rustc step

Author: Kobzol

src/bootstrap/src/core/build_steps/compile.rs

@@ -680,6 +680,14 @@ pub fn std_cargo(builder: &Builder<'_>, target: TargetSelection, cargo: &mut Car
     cargo.rustdocflag("-Zcrate-attr=warn(rust_2018_idioms)");
 }
 
+/// Link all libstd rlibs/dylibs into a sysroot of `target_compiler`.
+///
+/// Links those artifacts generated by `compiler` to the `stage` compiler's
+/// sysroot for the specified `host` and `target`.
+///
+/// Note that this assumes that `compiler` has already generated the libstd
+/// libraries for `target`, and this method will find them in the relevant
+/// output directory.
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub struct StdLink {
     pub compiler: Compiler,
@@ -710,14 +718,6 @@ impl Step for StdLink {
         run.never()
     }
 
-    /// Link all libstd rlibs/dylibs into the sysroot location.
-    ///
-    /// Links those artifacts generated by `compiler` to the `stage` compiler's
-    /// sysroot for the specified `host` and `target`.
-    ///
-    /// Note that this assumes that `compiler` has already generated the libstd
-    /// libraries for `target`, and this method will find them in the relevant
-    /// output directory.
     #[cfg_attr(
         feature = "tracing",
         instrument(
@@ -988,14 +988,8 @@ impl Rustc {
 }
 
 impl Step for Rustc {
-    /// We return the stage of the "actual" compiler (not the uplifted one).
-    ///
-    /// By "actual" we refer to the uplifting logic where we may not compile the requested stage;
-    /// instead, we uplift it from the previous stages. Which can lead to bootstrap failures in
-    /// specific situations where we request stage X from other steps. However we may end up
-    /// uplifting it from stage Y, causing the other stage to fail when attempting to link with
-    /// stage X which was never actually built.
-    type Output = u32;
+    type Output = ();
+
     const ONLY_HOSTS: bool = true;
     const DEFAULT: bool = false;
 
@@ -1043,7 +1037,7 @@ impl Step for Rustc {
             fields(previous_compiler = ?self.build_compiler, target = ?self.target),
         ),
     )]
-    fn run(self, builder: &Builder<'_>) -> u32 {
+    fn run(self, builder: &Builder<'_>) {
         let build_compiler = self.build_compiler;
         let target = self.target;
 
@@ -1059,7 +1053,7 @@ impl Step for Rustc {
                 &sysroot,
                 builder.config.ci_rustc_dev_contents(),
             );
-            return build_compiler.stage;
+            return;
         }
 
         // Build a standard library for `target` using the `build_compiler`.
@@ -1073,31 +1067,33 @@ impl Step for Rustc {
             builder.info("WARNING: Use `--keep-stage-std` if you want to rebuild the compiler when it changes");
             builder.ensure(RustcLink::from_rustc(self, build_compiler));
 
-            return build_compiler.stage;
+            return;
         }
 
-        let compiler_to_use =
-            builder.compiler_for(build_compiler.stage, build_compiler.host, target);
-        if compiler_to_use != build_compiler {
-            builder.ensure(Rustc::new(compiler_to_use, target));
-            let msg = if compiler_to_use.host == target {
-                format!(
-                    "Uplifting rustc (stage{} -> stage{})",
-                    compiler_to_use.stage,
-                    build_compiler.stage + 1
-                )
+        // The stage of the compiler that we're building
+        let stage = build_compiler.stage + 1;
+
+        // If we are building a stage3+ compiler, and full bootstrap is disabled, and we have a
+        // previous rustc available, we will uplift a compiler from a previous stage.
+        if build_compiler.stage >= 2
+            && !builder.config.full_bootstrap
+            && (target == builder.host_target || builder.hosts.contains(&target))
+        {
+            // If we're cross-compiling, the earliest rustc that we could have is stage 2.
+            // If we're not cross-compiling, then we should have rustc stage 1.
+            let stage_to_uplift = if target == builder.host_target { 1 } else { 2 };
+            let rustc_to_uplift = builder.compiler(stage_to_uplift, target);
+            let msg = if rustc_to_uplift.host == target {
+                format!("Uplifting rustc (stage{} -> stage{stage})", rustc_to_uplift.stage,)
             } else {
                 format!(
-                    "Uplifting rustc (stage{}:{} -> stage{}:{})",
-                    compiler_to_use.stage,
-                    compiler_to_use.host,
-                    build_compiler.stage + 1,
-                    target
+                    "Uplifting rustc (stage{}:{} -> stage{stage}:{target})",
+                    rustc_to_uplift.stage, rustc_to_uplift.host,
                 )
             };
             builder.info(&msg);
-            builder.ensure(RustcLink::from_rustc(self, compiler_to_use));
-            return compiler_to_use.stage;
+            builder.ensure(RustcLink::from_rustc(self, rustc_to_uplift));
+            return;
         }
 
         // Build a standard library for the current host target using the `build_compiler`.
@@ -1174,8 +1170,6 @@ impl Step for Rustc {
             self,
             builder.compiler(build_compiler.stage, builder.config.host_target),
         ));
-
-        build_compiler.stage
     }
 
     fn metadata(&self) -> Option<StepMetadata> {
@@ -1984,12 +1978,18 @@ impl Step for Sysroot {
     }
 }
 
+/// Prepare a compiler sysroot.
+///
+/// The sysroot may contain various things useful for running the compiler, like linkers and
+/// linker wrappers (LLD, LLVM bitcode linker, etc.).
+///
+/// This will assemble a compiler in `build/$target/stage$stage`.
 #[derive(Debug, PartialOrd, Ord, Clone, PartialEq, Eq, Hash)]
 pub struct Assemble {
     /// The compiler which we will produce in this step. Assemble itself will
     /// take care of ensuring that the necessary prerequisites to do so exist,
-    /// that is, this target can be a stage2 compiler and Assemble will build
-    /// previous stages for you.
+    /// that is, this can be e.g. a stage2 compiler and Assemble will build
+    /// the previous stages for you.
     pub target_compiler: Compiler,
 }
 
@@ -2007,11 +2007,6 @@ impl Step for Assemble {
         });
     }
 
-    /// Prepare a new compiler from the artifacts in `stage`
-    ///
-    /// This will assemble a compiler in `build/$host/stage$stage`. The compiler
-    /// must have been previously produced by the `stage - 1` builder.build
-    /// compiler.
     #[cfg_attr(
         feature = "tracing",
         instrument(
@@ -2149,7 +2144,7 @@ impl Step for Assemble {
             target_compiler.stage - 1,
             builder.config.host_target,
         );
-        let mut build_compiler =
+        let build_compiler =
             builder.compiler(target_compiler.stage - 1, builder.config.host_target);
 
         // Build enzyme
@@ -2173,24 +2168,13 @@ impl Step for Assemble {
         }
 
         // Build the libraries for this compiler to link to (i.e., the libraries
-        // it uses at runtime). NOTE: Crates the target compiler compiles don't
-        // link to these. (FIXME: Is that correct? It seems to be correct most
-        // of the time but I think we do link to these for stage2/bin compilers
-        // when not performing a full bootstrap).
+        // it uses at runtime).
         debug!(
             ?build_compiler,
             "target_compiler.host" = ?target_compiler.host,
             "building compiler libraries to link to"
         );
-        let actual_stage = builder.ensure(Rustc::new(build_compiler, target_compiler.host));
-        // Current build_compiler.stage might be uplifted instead of being built; so update it
-        // to not fail while linking the artifacts.
-        debug!(
-            "(old) build_compiler.stage" = build_compiler.stage,
-            "(adjusted) build_compiler.stage" = actual_stage,
-            "temporarily adjusting `build_compiler.stage` to account for uplifted libraries"
-        );
-        build_compiler.stage = actual_stage;
+        builder.ensure(Rustc::new(build_compiler, target_compiler.host));
 
         let mut codegen_backend_stamps = vec![];
         {