Apply LTO config to rustdoc

Before, the LTO configuration from `config.toml` was not applied to `rustdoc`.

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

@@ -4,8 +4,10 @@ use std::{env, fs};
 use crate::core::build_steps::toolstate::ToolState;
 use crate::core::build_steps::{compile, llvm};
 use crate::core::builder;
-use crate::core::builder::{Builder, Cargo as CargoCommand, RunConfig, ShouldRun, Step};
-use crate::core::config::{DebuginfoLevel, TargetSelection};
+use crate::core::builder::{
+    Builder, Cargo as CargoCommand, RunConfig, ShouldRun, Step, cargo_profile_var,
+};
+use crate::core::config::{DebuginfoLevel, RustcLto, TargetSelection};
 use crate::utils::channel::GitInfo;
 use crate::utils::exec::{BootstrapCommand, command};
 use crate::utils::helpers::{add_dylib_path, exe, t};
@@ -645,7 +647,7 @@ impl Step for Rustdoc {
         }
 
         // NOTE: Never modify the rustflags here, it breaks the build cache for other tools!
-        let cargo = prepare_tool_cargo(
+        let mut cargo = prepare_tool_cargo(
             builder,
             build_compiler,
             Mode::ToolRustc,
@@ -656,6 +658,17 @@ impl Step for Rustdoc {
             features.as_slice(),
         );
 
+        // rustdoc is performance sensitive, so apply LTO to it.
+        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), lto);
+        }
+
         let _guard = builder.msg_tool(
             Kind::Build,
             Mode::ToolRustc,

src/bootstrap/src/core/builder/cargo.rs

@@ -10,7 +10,7 @@ use crate::core::config::flags::Color;
 use crate::utils::build_stamp;
 use crate::utils::helpers::{self, LldThreads, check_cfg_arg, linker_args, linker_flags};
 use crate::{
-    BootstrapCommand, CLang, Compiler, DocTests, DryRun, EXTRA_CHECK_CFGS, GitRepo, Mode,
+    BootstrapCommand, CLang, Compiler, Config, DocTests, DryRun, EXTRA_CHECK_CFGS, GitRepo, Mode,
     TargetSelection, command, prepare_behaviour_dump_dir, t,
 };
 
@@ -473,10 +473,7 @@ impl Builder<'_> {
             build_stamp::clear_if_dirty(self, &my_out, &rustdoc);
         }
 
-        let profile_var = |name: &str| {
-            let profile = if self.config.rust_optimize.is_release() { "RELEASE" } else { "DEV" };
-            format!("CARGO_PROFILE_{}_{}", profile, name)
-        };
+        let profile_var = |name: &str| cargo_profile_var(name, &self.config);
 
         // See comment in rustc_llvm/build.rs for why this is necessary, largely llvm-config
         // needs to not accidentally link to libLLVM in stage0/lib.
@@ -1221,3 +1218,8 @@ impl Builder<'_> {
         }
     }
 }
+
+pub fn cargo_profile_var(name: &str, config: &Config) -> String {
+    let profile = if config.rust_optimize.is_release() { "RELEASE" } else { "DEV" };
+    format!("CARGO_PROFILE_{}_{}", profile, name)
+}

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

@@ -11,7 +11,7 @@ use std::{env, fs};
 
 use clap::ValueEnum;
 
-pub use self::cargo::Cargo;
+pub use self::cargo::{Cargo, cargo_profile_var};
 pub use crate::Compiler;
 use crate::core::build_steps::{
     check, clean, clippy, compile, dist, doc, gcc, install, llvm, run, setup, test, tool, vendor,

src/bootstrap/src/utils/change_tracker.rs

@@ -340,4 +340,9 @@ pub const CONFIG_CHANGE_HISTORY: &[ChangeInfo] = &[
         severity: ChangeSeverity::Info,
         summary: "Change the compiler profile to default to rust.debug-assertions = true",
     },
+    ChangeInfo {
+        change_id: 135832,
+        severity: ChangeSeverity::Info,
+        summary: "Rustdoc now respects the value of rust.lto.",
+    },
 ];