Auto merge of #107241 - clubby789:bootstrap-lto-off, r=simulacrum

Add `rust.lto=off` to bootstrap and set as compiler/library default

Closes #107202

The issue mentions `embed-bitcode=on`, but here c8e6a9e8b6/src/bootstrap/compile.rs (L379-L381)
it appears that this is always set for std stage 1+, so I'm unsure if changes are needed here.

`@rustbot` label +A-bootstrap

config.toml.example

@@ -662,7 +662,8 @@ changelog-seen = 2
 
 # Select LTO mode that will be used for compiling rustc. By default, thin local LTO
 # (LTO within a single crate) is used (like for any Rust crate). You can also select
-# "thin" or "fat" to apply Thin/Fat LTO to the `rustc_driver` dylib.
+# "thin" or "fat" to apply Thin/Fat LTO to the `rustc_driver` dylib, or "off" to disable
+# LTO entirely.
 #lto = "thin-local"
 
 # =============================================================================

src/bootstrap/compile.rs

@@ -379,6 +379,9 @@ pub fn std_cargo(builder: &Builder<'_>, target: TargetSelection, stage: u32, car
     if stage >= 1 {
         cargo.rustflag("-Cembed-bitcode=yes");
     }
+    if builder.config.rust_lto == RustcLto::Off {
+        cargo.rustflag("-Clto=off");
+    }
 
     // By default, rustc does not include unwind tables unless they are required
     // for a particular target. They are not required by RISC-V targets, but
@@ -722,6 +725,13 @@ impl Step for Rustc {
                     cargo.rustflag("-Cembed-bitcode=yes");
                 }
                 RustcLto::ThinLocal => { /* Do nothing, this is the default */ }
+                RustcLto::Off => {
+                    cargo.rustflag("-Clto=off");
+                }
+            }
+        } else {
+            if builder.config.rust_lto == RustcLto::Off {
+                cargo.rustflag("-Clto=off");
             }
         }
 

src/bootstrap/config.rs

@@ -333,8 +333,9 @@ impl SplitDebuginfo {
 }
 
 /// LTO mode used for compiling rustc itself.
-#[derive(Default, Clone)]
+#[derive(Default, Clone, PartialEq)]
 pub enum RustcLto {
+    Off,
     #[default]
     ThinLocal,
     Thin,
@@ -349,6 +350,7 @@ impl std::str::FromStr for RustcLto {
             "thin-local" => Ok(RustcLto::ThinLocal),
             "thin" => Ok(RustcLto::Thin),
             "fat" => Ok(RustcLto::Fat),
+            "off" => Ok(RustcLto::Off),
             _ => Err(format!("Invalid value for rustc LTO: {}", s)),
         }
     }

src/bootstrap/defaults/config.compiler.toml

@@ -12,6 +12,8 @@ debug-logging = true
 incremental = true
 # Print backtrace on internal compiler errors during bootstrap
 backtrace-on-ice = true
+# Make the compiler and standard library faster to build, at the expense of a ~20% runtime slowdown.
+lto = "off"
 
 [llvm]
 # Will download LLVM from CI if available on your platform.

src/bootstrap/defaults/config.library.toml

@@ -8,6 +8,8 @@ bench-stage = 0
 [rust]
 # This greatly increases the speed of rebuilds, especially when there are only minor changes. However, it makes the initial build slightly slower.
 incremental = true
+# Make the compiler and standard library faster to build, at the expense of a ~20% runtime slowdown.
+lto = "off"
 
 [llvm]
 # Will download LLVM from CI if available on your platform.