Perform ThinLTO on x86_64-pc-windows-msvc dist builder

src/bootstrap/src/core/build_steps/tool.rs

@@ -19,7 +19,7 @@ use crate::core::build_steps::{compile, llvm};
 use crate::core::builder;
 use crate::core::builder::{
     Builder, Cargo as CargoCommand, CommandLineStep, RunConfig, ShouldRun, Step, StepMetadata,
-    apply_pgo, cargo_profile_var,
+    apply_pgo,
 };
 use crate::core::config::{DebuginfoLevel, OverrideAllocator, RustcLto, TargetSelection};
 use crate::utils::exec::{BootstrapCommand, command};
@@ -123,14 +123,20 @@ impl Step for ToolBuild {
         if is_lto_stage(&self.build_compiler)
             && (self.mode == Mode::ToolRustcPrivate || self.path == "src/tools/cargo")
         {
-            let lto = match builder.config.rust_lto {
-                RustcLto::Off => Some("off"),
-                RustcLto::Thin => Some("thin"),
-                RustcLto::Fat => Some("fat"),
-                RustcLto::ThinLocal => None,
-            };
-            if let Some(lto) = lto {
-                cargo.env(cargo_profile_var("LTO", &builder.config, self.mode), lto);
+            // We don't use Cargo's LTO setting here to workaround https://github.com/rust-lang/cargo/issues/14575
+            match builder.config.rust_lto {
+                RustcLto::Thin => {
+                    cargo.rustflag("-Clto=thin");
+                    cargo.rustflag("-Cembed-bitcode=yes");
+                }
+                RustcLto::Fat => {
+                    cargo.rustflag("-Clto=fat");
+                    cargo.rustflag("-Cembed-bitcode=yes");
+                }
+                RustcLto::ThinLocal => { /* Do nothing, this is the default */ }
+                RustcLto::Off => {
+                    cargo.rustflag("-Clto=off");
+                }
             }
         }
 

src/bootstrap/src/core/builder/mod.rs

@@ -13,7 +13,7 @@ use clap::ValueEnum;
 #[cfg(feature = "tracing")]
 use tracing::instrument;
 
-pub use self::cargo::{Cargo, apply_pgo, cargo_profile_var};
+pub use self::cargo::{Cargo, apply_pgo};
 pub use crate::Compiler;
 use crate::core::build_steps::compile::{Std, StdLink};
 use crate::core::build_steps::tool::RustcPrivateCompilers;

src/ci/github-actions/jobs.yml

@@ -721,6 +721,7 @@ auto:
         --target=x86_64-pc-windows-msvc
         --enable-full-tools
         --enable-profiler
+        --set rust.lto=thin
         --set rust.codegen-units=1
       SCRIPT: python x.py build --set rust.debug=true opt-dist && PGO_HOST=x86_64-pc-windows-msvc ./build/x86_64-pc-windows-msvc/stage1-tools-bin/opt-dist windows-ci -- python x.py dist bootstrap --include-default-paths
       DIST_REQUIRE_ALL_TOOLS: 1