use ci-rustc-sysroot for sysroot when download_rustc

Author: chenyukang

src/bootstrap/compile.rs

@@ -1102,6 +1102,8 @@ impl Step for Sysroot {
         let host_dir = builder.out.join(&compiler.host.triple);
         let sysroot = if compiler.stage == 0 {
             host_dir.join("stage0-sysroot")
+        } else if builder.download_rustc() {
+            host_dir.join("ci-rustc-sysroot")
         } else {
             host_dir.join(format!("stage{}", compiler.stage))
         };
@@ -1115,7 +1117,7 @@ impl Step for Sysroot {
                 "Cross-compiling is not yet supported with `download-rustc`",
             );
 
-            // #102002, cleanup stage1 and stage0-sysroot folders when using download-rustc
+            // #102002, cleanup stage1 and stage0-sysroot folders when using download-rustc so people don't use old versions of the toolchain by accident.
             let _ = fs::remove_dir_all(host_dir.join("stage1"));
             let _ = fs::remove_dir_all(host_dir.join("stage0-sysroot"));
 

src/bootstrap/tool.rs

@@ -512,10 +512,8 @@ impl Step for Rustdoc {
         // Similar to `compile::Assemble`, build with the previous stage's compiler. Otherwise
         // we'd have stageN/bin/rustc and stageN/bin/rustdoc be effectively different stage
         // compilers, which isn't what we want. Rustdoc should be linked in the same way as the
-        // rustc compiler it's paired with, so it must be built with the previous stage compiler,
-        // if download_rustc is true, we will use the same target stage.
-        let target_stage = target_compiler.stage - if builder.download_rustc() { 0 } else { 1 };
-        let build_compiler = builder.compiler(target_stage, builder.config.build);
+        // rustc compiler it's paired with, so it must be built with the previous stage compiler.
+        let build_compiler = builder.compiler(target_compiler.stage - 1, builder.config.build);
 
         // When using `download-rustc` and a stage0 build_compiler, copying rustc doesn't actually
         // build stage0 libstd (because the libstd in sysroot has the wrong ABI). Explicitly build