Enable zstd for debug compression.

Set LLVM_ENABLE_ZSTD alongside LLVM_ENABLE_ZLIB so that --compress-debug-sections=zstd is an option.
Use static linking to avoid a new runtime dependency. Add an llvm.libzstd bootstrap option for LLVM
with zstd. Set it off by default except for the dist builder. Handle llvm-config --system-libs output
that contains static libraries.

compiler/rustc_llvm/build.rs

@@ -259,6 +259,7 @@ fn main() {
     cmd.args(&components);
 
     for lib in output(&mut cmd).split_whitespace() {
+        let mut is_static = false;
         let name = if let Some(stripped) = lib.strip_prefix("-l") {
             stripped
         } else if let Some(stripped) = lib.strip_prefix('-') {
@@ -266,8 +267,24 @@ fn main() {
         } else if Path::new(lib).exists() {
             // On MSVC llvm-config will print the full name to libraries, but
             // we're only interested in the name part
-            let name = Path::new(lib).file_name().unwrap().to_str().unwrap();
+            // On Unix when we get a static library llvm-config will print the
+            // full name and we *are* interested in the path, but we need to
+            // handle it separately. For example, when statically linking to
+            // libzstd llvm-config will output something like
+            //   -lrt -ldl -lm -lz /usr/local/lib/libzstd.a -lxml2
+            // and we transform the zstd part into
+            //   cargo:rustc-link-search-native=/usr/local/lib
+            //   cargo:rustc-link-lib=static=zstd
+            let path = Path::new(lib);
+            if lib.ends_with(".a") {
+                is_static = true;
+                println!("cargo:rustc-link-search=native={}", path.parent().unwrap().display());
+                let name = path.file_stem().unwrap().to_str().unwrap();
+                name.trim_start_matches("lib")
+            } else {
+                let name = path.file_name().unwrap().to_str().unwrap();
                 name.trim_end_matches(".lib")
+            }
         } else if lib.ends_with(".lib") {
             // Some MSVC libraries just come up with `.lib` tacked on, so chop
             // that off
@@ -285,7 +302,13 @@ fn main() {
             continue;
         }
 
-        let kind = if name.starts_with("LLVM") { llvm_kind } else { "dylib" };
+        let kind = if name.starts_with("LLVM") {
+            llvm_kind
+        } else if is_static {
+            "static"
+        } else {
+            "dylib"
+        };
         println!("cargo:rustc-link-lib={kind}={name}");
     }
 

config.example.toml

@@ -87,6 +87,9 @@
 # library provided by LLVM.
 #static-libstdcpp = false
 
+# Enable LLVM to use zstd for compression.
+#libzstd = false
+
 # Whether to use Ninja to build LLVM. This runs much faster than make.
 #ninja = true
 

src/bootstrap/download-ci-llvm-stamp

@@ -1,4 +1,4 @@
 Change this file to make users of the `download-ci-llvm` configuration download
 a new version of LLVM from CI, even if the LLVM submodule hasn’t changed.
 
-Last change is for: https://github.com/rust-lang/rust/pull/126298
+Last change is for: https://github.com/rust-lang/rust/pull/125642

src/bootstrap/src/core/build_steps/llvm.rs

@@ -368,9 +368,7 @@ impl Step for Llvm {
             cfg.define("LLVM_PROFDATA_FILE", path);
         }
 
-        // Disable zstd to avoid a dependency on libzstd.so.
+        // Libraries for ELF section compression.
-        cfg.define("LLVM_ENABLE_ZSTD", "OFF");
-
         if !target.is_windows() {
             cfg.define("LLVM_ENABLE_ZLIB", "ON");
         } else {
@@ -824,6 +822,14 @@ fn configure_llvm(builder: &Builder<'_>, target: TargetSelection, cfg: &mut cmak
         }
     }
 
+    // Libraries for ELF section compression.
+    if builder.config.llvm_libzstd {
+        cfg.define("LLVM_ENABLE_ZSTD", "FORCE_ON");
+        cfg.define("LLVM_USE_STATIC_ZSTD", "TRUE");
+    } else {
+        cfg.define("LLVM_ENABLE_ZSTD", "OFF");
+    }
+
     if let Some(ref linker) = builder.config.llvm_use_linker {
         cfg.define("LLVM_USE_LINKER", linker);
     }

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

@@ -218,6 +218,7 @@ pub struct Config {
     pub llvm_thin_lto: bool,
     pub llvm_release_debuginfo: bool,
     pub llvm_static_stdcpp: bool,
+    pub llvm_libzstd: bool,
     /// `None` if `llvm_from_ci` is true and we haven't yet downloaded llvm.
     #[cfg(not(test))]
     llvm_link_shared: Cell<Option<bool>>,
@@ -878,6 +879,7 @@ define_config! {
         plugins: Option<bool> = "plugins",
         ccache: Option<StringOrBool> = "ccache",
         static_libstdcpp: Option<bool> = "static-libstdcpp",
+        libzstd: Option<bool> = "libzstd",
         ninja: Option<bool> = "ninja",
         targets: Option<String> = "targets",
         experimental_targets: Option<String> = "experimental-targets",
@@ -1153,6 +1155,7 @@ impl Config {
             llvm_optimize: true,
             ninja_in_file: true,
             llvm_static_stdcpp: false,
+            llvm_libzstd: false,
             backtrace: true,
             rust_optimize: RustOptimize::Bool(true),
             rust_optimize_tests: true,
@@ -1787,6 +1790,7 @@ impl Config {
                 plugins,
                 ccache,
                 static_libstdcpp,
+                libzstd,
                 ninja,
                 targets,
                 experimental_targets,
@@ -1821,6 +1825,7 @@ impl Config {
             set(&mut config.llvm_thin_lto, thin_lto);
             set(&mut config.llvm_release_debuginfo, release_debuginfo);
             set(&mut config.llvm_static_stdcpp, static_libstdcpp);
+            set(&mut config.llvm_libzstd, libzstd);
             if let Some(v) = link_shared {
                 config.llvm_link_shared.set(Some(v));
             }
@@ -1871,6 +1876,7 @@ impl Config {
                 check_ci_llvm!(optimize_toml);
                 check_ci_llvm!(thin_lto);
                 check_ci_llvm!(release_debuginfo);
+                check_ci_llvm!(libzstd);
                 check_ci_llvm!(targets);
                 check_ci_llvm!(experimental_targets);
                 check_ci_llvm!(clang_cl);

src/bootstrap/src/utils/change_tracker.rs

@@ -220,4 +220,9 @@ pub const CONFIG_CHANGE_HISTORY: &[ChangeInfo] = &[
         severity: ChangeSeverity::Warning,
         summary: "For tarball sources, default value for `rust.channel` will be taken from `src/ci/channel` file.",
     },
+    ChangeInfo {
+        change_id: 125642,
+        severity: ChangeSeverity::Info,
+        summary: "New option `llvm.libzstd` to control whether llvm is built with zstd support.",
+    },
 ];

src/ci/docker/host-x86_64/dist-x86_64-linux/Dockerfile

@@ -79,6 +79,7 @@ ENV RUST_CONFIGURE_ARGS \
       --set target.x86_64-unknown-linux-gnu.ranlib=/rustroot/bin/llvm-ranlib \
       --set llvm.thin-lto=true \
       --set llvm.ninja=false \
+      --set llvm.libzstd=true \
       --set rust.jemalloc \
       --set rust.use-lld=true \
       --set rust.lto=thin \