Allow manually opting in and out of Linux linker overrides

bootstrap.example.toml

@@ -758,12 +758,8 @@
 # Currently, the only standard options supported here are `"llvm"`, `"cranelift"` and `"gcc"`.
 #rust.codegen-backends = ["llvm"]
 
-# Indicates whether LLD will be compiled and made available in the sysroot for rustc to execute, and
+# Indicates whether LLD will be compiled and made available in the sysroot for rustc to execute,
-# whether to set it as rustc's default linker on `x86_64-unknown-linux-gnu`. This will also only be
+#rust.lld = false, except for targets that opt into LLD (see `target.default-linker-linux-override`)
-# when *not* building an external LLVM (so only when using `download-ci-llvm` or building LLVM from
-# the in-tree source): setting `llvm-config` in the `[target.x86_64-unknown-linux-gnu]` section will
-# make this default to false.
-#rust.lld = false in all cases, except on `x86_64-unknown-linux-gnu` as described above, where it is true
 
 # Indicates if we should override the linker used to link Rust crates during bootstrap to be LLD.
 # If set to `true` or `"external"`, a global `lld` binary that has to be in $PATH
@@ -1067,3 +1063,17 @@
 # Link the compiler and LLVM against `jemalloc` instead of the default libc allocator.
 # This overrides the global `rust.jemalloc` option. See that option for more info.
 #jemalloc = rust.jemalloc (bool)
+
+# The linker configuration that will *override* the default linker used for Linux
+# targets in the built compiler.
+#
+# The following values are supported:
+# - `off` => do not apply any override and use the default linker. This can be used to opt out of
+#   linker overrides set by bootstrap for specific targets (see below).
+# - `self-contained-lld-cc` => override the default linker to be self-contained LLD (`rust-lld`)
+#   that is invoked through `cc`.
+#
+# Currently, the following targets automatically opt into the self-contained LLD linker, unless you
+# pass `off`:
+# - x86_64-unknown-linux-gnu
+#default-linker-linux-override = "off" (for most targets)

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

@@ -26,6 +26,7 @@ use crate::core::builder;
 use crate::core::builder::{
     Builder, Cargo, Kind, RunConfig, ShouldRun, Step, StepMetadata, crate_description,
 };
+use crate::core::config::toml::target::DefaultLinuxLinkerOverride;
 use crate::core::config::{
     CompilerBuiltins, DebuginfoLevel, LlvmLibunwind, RustcLto, TargetSelection,
 };
@@ -1355,9 +1356,14 @@ pub fn rustc_cargo_env(builder: &Builder<'_>, cargo: &mut Cargo, target: TargetS
         cargo.env("CFG_DEFAULT_LINKER", s);
     }
 
-    // Enable rustc's env var for `rust-lld` when requested.
+    // Enable rustc's env var to use a linker override on Linux when requested.
-    if builder.config.lld_enabled {
+    if let Some(linker) = target_config.map(|c| c.default_linker_linux_override) {
-        cargo.env("CFG_DEFAULT_LINKER_SELF_CONTAINED_LLD_CC", "1");
+        match linker {
+            DefaultLinuxLinkerOverride::Off => {}
+            DefaultLinuxLinkerOverride::SelfContainedLldCc => {
+                cargo.env("CFG_DEFAULT_LINKER_SELF_CONTAINED_LLD_CC", "1");
+            }
+        }
     }
 
     if builder.config.rust_verify_llvm_ir {

src/bootstrap/src/core/builder/tests.rs

@@ -514,7 +514,9 @@ mod snapshot {
     };
     use crate::core::builder::{Builder, Kind, StepDescription, StepMetadata};
     use crate::core::config::TargetSelection;
-    use crate::core::config::toml::rust::with_lld_opt_in_targets;
+    use crate::core::config::toml::target::{
+        DefaultLinuxLinkerOverride, with_default_linux_linker_overrides,
+    };
     use crate::utils::cache::Cache;
     use crate::utils::helpers::get_host_target;
     use crate::utils::tests::{ConfigBuilder, TestCtx};
@@ -782,9 +784,11 @@ mod snapshot {
 
     #[test]
     fn build_compiler_lld_opt_in() {
-        with_lld_opt_in_targets(vec![host_target()], || {
+        with_default_linux_linker_overrides(
-            let ctx = TestCtx::new();
+            [(host_target(), DefaultLinuxLinkerOverride::SelfContainedLldCc)].into(),
-            insta::assert_snapshot!(
+            || {
+                let ctx = TestCtx::new();
+                insta::assert_snapshot!(
                 ctx.config("build")
                     .path("compiler")
                     .render_steps(), @r"
@@ -792,7 +796,26 @@ mod snapshot {
             [build] rustc 0 <host> -> rustc 1 <host>
             [build] rustc 0 <host> -> LldWrapper 1 <host>
             ");
-        });
+            },
+        );
+    }
+
+    #[test]
+    fn build_compiler_lld_opt_in_lld_disabled() {
+        with_default_linux_linker_overrides(
+            [(host_target(), DefaultLinuxLinkerOverride::SelfContainedLldCc)].into(),
+            || {
+                let ctx = TestCtx::new();
+                insta::assert_snapshot!(
+                ctx.config("build")
+                    .path("compiler")
+                    .args(&["--set", "rust.lld=false"])
+                    .render_steps(), @r"
+                [build] llvm <host>
+                [build] rustc 0 <host> -> rustc 1 <host>
+                ");
+            },
+        );
     }
 
     #[test]

src/bootstrap/src/core/config/config.rs

@@ -42,9 +42,11 @@ use crate::core::config::toml::install::Install;
 use crate::core::config::toml::llvm::Llvm;
 use crate::core::config::toml::rust::{
     BootstrapOverrideLld, Rust, RustOptimize, check_incompatible_options_for_ci_rustc,
-    default_lld_opt_in_targets, parse_codegen_backends,
+    parse_codegen_backends,
+};
+use crate::core::config::toml::target::{
+    DefaultLinuxLinkerOverride, Target, TomlTarget, default_linux_linker_overrides,
 };
-use crate::core::config::toml::target::{Target, TomlTarget};
 use crate::core::config::{
     CompilerBuiltins, DebuginfoLevel, DryRun, GccCiMode, LlvmLibunwind, Merge, ReplaceOpt,
     RustcLto, SplitDebuginfo, StringOrBool, threads_from_config,
@@ -829,6 +831,11 @@ impl Config {
                     .to_owned();
         }
 
+        let mut lld_enabled = rust_lld_enabled.unwrap_or(false);
+
+        // Linux targets for which the user explicitly overrode the used linker
+        let mut targets_with_user_linker_override = HashSet::new();
+
         if let Some(t) = toml.target {
             for (triple, cfg) in t {
                 let TomlTarget {
@@ -837,6 +844,7 @@ impl Config {
                     ar: target_ar,
                     ranlib: target_ranlib,
                     default_linker: target_default_linker,
+                    default_linker_linux: target_default_linker_linux_override,
                     linker: target_linker,
                     split_debuginfo: target_split_debuginfo,
                     llvm_config: target_llvm_config,
@@ -860,6 +868,33 @@ impl Config {
 
                 let mut target = Target::from_triple(&triple);
 
+                if target_default_linker_linux_override.is_some() {
+                    targets_with_user_linker_override.insert(triple.clone());
+                }
+
+                let default_linker_linux_override = match target_default_linker_linux_override {
+                    Some(DefaultLinuxLinkerOverride::SelfContainedLldCc) => {
+                        if rust_default_linker.is_some() {
+                            panic!(
+                                "cannot set both `default-linker` and `default-linker-linux` for target `{triple}`"
+                            );
+                        }
+                        if !triple.contains("linux-gnu") {
+                            panic!(
+                                "`default-linker-linux` can only be set for Linux GNU targets, not for `{triple}`"
+                            );
+                        }
+                        if !lld_enabled {
+                            panic!(
+                                "Trying to override the default Linux linker for `{triple}` to be self-contained LLD, but LLD is not being built. Enable it with rust.lld = true."
+                            );
+                        }
+                        DefaultLinuxLinkerOverride::SelfContainedLldCc
+                    }
+                    Some(DefaultLinuxLinkerOverride::Off) => DefaultLinuxLinkerOverride::Off,
+                    None => DefaultLinuxLinkerOverride::default(),
+                };
+
                 if let Some(ref s) = target_llvm_config {
                     if download_rustc_commit.is_some() && triple == *host_target.triple {
                         panic!(
@@ -893,6 +928,7 @@ impl Config {
                 target.linker = target_linker.map(PathBuf::from);
                 target.crt_static = target_crt_static;
                 target.default_linker = target_default_linker;
+                target.default_linker_linux_override = default_linker_linux_override;
                 target.musl_root = target_musl_root.map(PathBuf::from);
                 target.musl_libdir = target_musl_libdir.map(PathBuf::from);
                 target.wasi_root = target_wasi_root.map(PathBuf::from);
@@ -918,6 +954,38 @@ impl Config {
             }
         }
 
+        for (target, linker_override) in default_linux_linker_overrides() {
+            // If the user overrode the default Linux linker, do not apply bootstrap defaults
+            if targets_with_user_linker_override.contains(&target) {
+                continue;
+            }
+            let default_linux_linker_override = match linker_override {
+                DefaultLinuxLinkerOverride::Off => continue,
+                DefaultLinuxLinkerOverride::SelfContainedLldCc => {
+                    // If we automatically default to the self-contained LLD linker,
+                    // we also need to handle the rust.lld option.
+                    match rust_lld_enabled {
+                        // If LLD was not enabled explicitly, we enable it
+                        None => {
+                            lld_enabled = true;
+                            Some(DefaultLinuxLinkerOverride::SelfContainedLldCc)
+                        }
+                        // If it was enabled already, we don't need to do anything
+                        Some(true) => Some(DefaultLinuxLinkerOverride::SelfContainedLldCc),
+                        // If it was explicitly disabled, we do not apply the
+                        // linker override
+                        Some(false) => None,
+                    }
+                }
+            };
+            if let Some(linker_override) = default_linux_linker_override {
+                target_config
+                    .entry(TargetSelection::from_user(&target))
+                    .or_default()
+                    .default_linker_linux_override = linker_override;
+            }
+        }
+
         let llvm_from_ci = parse_download_ci_llvm(
             &dwn_ctx,
             &rust_info,
@@ -926,28 +994,6 @@ impl Config {
             llvm_assertions,
         );
 
-        // We make `x86_64-unknown-linux-gnu` use the self-contained linker by default, so we will
-        // build our internal lld and use it as the default linker, by setting the `rust.lld` config
-        // to true by default:
-        // - on the `x86_64-unknown-linux-gnu` target
-        // - when building our in-tree llvm (i.e. the target has not set an `llvm-config`), so that
-        //   we're also able to build the corresponding lld
-        // - or when using an external llvm that's downloaded from CI, which also contains our prebuilt
-        //   lld
-        // - otherwise, we'd be using an external llvm, and lld would not necessarily available and
-        //   thus, disabled
-        // - similarly, lld will not be built nor used by default when explicitly asked not to, e.g.
-        //   when the config sets `rust.lld = false`
-        let lld_enabled = if default_lld_opt_in_targets().contains(&host_target.triple.to_string())
-            && hosts == [host_target]
-        {
-            let no_llvm_config =
-                target_config.get(&host_target).is_none_or(|config| config.llvm_config.is_none());
-            rust_lld_enabled.unwrap_or(llvm_from_ci || no_llvm_config)
-        } else {
-            rust_lld_enabled.unwrap_or(false)
-        };
-
         if llvm_from_ci {
             let warn = |option: &str| {
                 println!(

src/bootstrap/src/core/config/toml/rust.rs

@@ -438,28 +438,3 @@ pub(crate) fn parse_codegen_backends(
     }
     found_backends
 }
-
-#[cfg(not(test))]
-pub fn default_lld_opt_in_targets() -> Vec<String> {
-    vec!["x86_64-unknown-linux-gnu".to_string()]
-}
-
-#[cfg(test)]
-thread_local! {
-    static TEST_LLD_OPT_IN_TARGETS: std::cell::RefCell<Option<Vec<String>>> = std::cell::RefCell::new(None);
-}
-
-#[cfg(test)]
-pub fn default_lld_opt_in_targets() -> Vec<String> {
-    TEST_LLD_OPT_IN_TARGETS.with(|cell| cell.borrow().clone()).unwrap_or_default()
-}
-
-#[cfg(test)]
-pub fn with_lld_opt_in_targets<R>(targets: Vec<String>, f: impl FnOnce() -> R) -> R {
-    TEST_LLD_OPT_IN_TARGETS.with(|cell| {
-        let prev = cell.replace(Some(targets));
-        let result = f();
-        cell.replace(prev);
-        result
-    })
-}

src/bootstrap/src/core/config/toml/target.rs

@@ -9,6 +9,9 @@
 //! * [`Target`]: This struct represents the processed and validated configuration for a
 //!   build target, which is is stored in the main `Config` structure.
 
+use std::collections::HashMap;
+
+use serde::de::Error;
 use serde::{Deserialize, Deserializer};
 
 use crate::core::config::{
@@ -24,6 +27,7 @@ define_config! {
         ar: Option<String> = "ar",
         ranlib: Option<String> = "ranlib",
         default_linker: Option<PathBuf> = "default-linker",
+        default_linker_linux: Option<DefaultLinuxLinkerOverride> = "default-linker-linux-override",
         linker: Option<String> = "linker",
         split_debuginfo: Option<String> = "split-debuginfo",
         llvm_config: Option<String> = "llvm-config",
@@ -60,6 +64,7 @@ pub struct Target {
     pub ar: Option<PathBuf>,
     pub ranlib: Option<PathBuf>,
     pub default_linker: Option<PathBuf>,
+    pub default_linker_linux_override: DefaultLinuxLinkerOverride,
     pub linker: Option<PathBuf>,
     pub split_debuginfo: Option<SplitDebuginfo>,
     pub sanitizers: Option<bool>,
@@ -89,3 +94,60 @@ impl Target {
         target
     }
 }
+
+/// Overrides the default linker used on a Linux linker.
+/// On Linux, the linker is usually invoked through `cc`, therefore this exists as a separate
+/// configuration from simply setting `default-linker`, which corresponds to `-Clinker`.
+#[derive(Debug, Default, Copy, Clone, PartialEq, Eq)]
+pub enum DefaultLinuxLinkerOverride {
+    /// Do not apply any override and use the default linker for the given target.
+    #[default]
+    Off,
+    /// Use the self-contained `rust-lld` linker, invoked through `cc`.
+    /// Corresponds to `-Clinker-features=+lld -Clink-self-contained=+linker`.
+    SelfContainedLldCc,
+}
+
+impl<'de> Deserialize<'de> for DefaultLinuxLinkerOverride {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        let name = String::deserialize(deserializer)?;
+        match name.as_str() {
+            "off" => Ok(Self::Off),
+            "self-contained-lld-cc" => Ok(Self::SelfContainedLldCc),
+            other => Err(D::Error::unknown_variant(other, &["off", "self-contained-lld-cc"])),
+        }
+    }
+}
+
+/// Set of linker overrides for selected Linux targets.
+#[cfg(not(test))]
+pub fn default_linux_linker_overrides() -> HashMap<String, DefaultLinuxLinkerOverride> {
+    [("x86_64-unknown-linux-gnu".to_string(), DefaultLinuxLinkerOverride::SelfContainedLldCc)]
+        .into()
+}
+
+#[cfg(test)]
+thread_local! {
+    static TEST_LINUX_LINKER_OVERRIDES: std::cell::RefCell<Option<HashMap<String, DefaultLinuxLinkerOverride>>> = std::cell::RefCell::new(None);
+}
+
+#[cfg(test)]
+pub fn default_linux_linker_overrides() -> HashMap<String, DefaultLinuxLinkerOverride> {
+    TEST_LINUX_LINKER_OVERRIDES.with(|cell| cell.borrow().clone()).unwrap_or_default()
+}
+
+#[cfg(test)]
+pub fn with_default_linux_linker_overrides<R>(
+    targets: HashMap<String, DefaultLinuxLinkerOverride>,
+    f: impl FnOnce() -> R,
+) -> R {
+    TEST_LINUX_LINKER_OVERRIDES.with(|cell| {
+        let prev = cell.replace(Some(targets));
+        let result = f();
+        cell.replace(prev);
+        result
+    })
+}