Support raw-dylib link kind on ELF

raw-dylib is a link kind that allows rustc to link against a library
without having any library files present.
This currently only exists on Windows. rustc will take all the symbols
from raw-dylib link blocks and put them in an import library, where they
can then be resolved by the linker.

While import libraries don't exist on ELF, it would still be convenient
to have this same functionality. Not having the libraries present at
build-time can be convenient for several reasons, especially
cross-compilation. With raw-dylib, code linking against a library can be
cross-compiled without needing to have these libraries available on the
build machine. If the libc crate makes use of this, it would allow
cross-compilation without having any libc available on the build
machine. This is not yet possible with this implementation, at least
against libc's like glibc that use symbol versioning.
The raw-dylib kind could be extended with support for symbol versioning
in the future.

This implementation is very experimental and I have not tested it very
well. I have tested it for a toy example and the lz4-sys crate, where it
was able to successfully link a binary despite not having a
corresponding library at build-time.

compiler/rustc_codegen_ssa/src/back/metadata.rs

@@ -9,8 +9,7 @@ use itertools::Itertools;
 use object::write::{self, StandardSegment, Symbol, SymbolSection};
 use object::{
     Architecture, BinaryFormat, Endianness, FileFlags, Object, ObjectSection, ObjectSymbol,
-    SectionFlags, SectionKind, SubArchitecture, SymbolFlags, SymbolKind, SymbolScope, elf, pe,
-    xcoff,
+    SectionFlags, SectionKind, SymbolFlags, SymbolKind, SymbolScope, elf, pe, xcoff,
 };
 use rustc_abi::Endian;
 use rustc_data_structures::memmap::Mmap;
@@ -206,61 +205,12 @@ pub(crate) fn create_object_file(sess: &Session) -> Option<write::Object<'static
         Endian::Little => Endianness::Little,
         Endian::Big => Endianness::Big,
     };
-    let (architecture, sub_architecture) = match &sess.target.arch[..] {
-        "arm" => (Architecture::Arm, None),
-        "aarch64" => (
-            if sess.target.pointer_width == 32 {
-                Architecture::Aarch64_Ilp32
-            } else {
-                Architecture::Aarch64
-            },
-            None,
-        ),
-        "x86" => (Architecture::I386, None),
-        "s390x" => (Architecture::S390x, None),
-        "mips" | "mips32r6" => (Architecture::Mips, None),
-        "mips64" | "mips64r6" => (Architecture::Mips64, None),
-        "x86_64" => (
-            if sess.target.pointer_width == 32 {
-                Architecture::X86_64_X32
-            } else {
-                Architecture::X86_64
-            },
-            None,
-        ),
-        "powerpc" => (Architecture::PowerPc, None),
-        "powerpc64" => (Architecture::PowerPc64, None),
-        "riscv32" => (Architecture::Riscv32, None),
-        "riscv64" => (Architecture::Riscv64, None),
-        "sparc" => {
-            if sess.unstable_target_features.contains(&sym::v8plus) {
-                // Target uses V8+, aka EM_SPARC32PLUS, aka 64-bit V9 but in 32-bit mode
-                (Architecture::Sparc32Plus, None)
-            } else {
-                // Target uses V7 or V8, aka EM_SPARC
-                (Architecture::Sparc, None)
-            }
-        }
-        "sparc64" => (Architecture::Sparc64, None),
-        "avr" => (Architecture::Avr, None),
-        "msp430" => (Architecture::Msp430, None),
-        "hexagon" => (Architecture::Hexagon, None),
-        "bpf" => (Architecture::Bpf, None),
-        "loongarch64" => (Architecture::LoongArch64, None),
-        "csky" => (Architecture::Csky, None),
-        "arm64ec" => (Architecture::Aarch64, Some(SubArchitecture::Arm64EC)),
-        // Unsupported architecture.
-        _ => return None,
-    };
-    let binary_format = if sess.target.is_like_osx {
-        BinaryFormat::MachO
-    } else if sess.target.is_like_windows {
-        BinaryFormat::Coff
-    } else if sess.target.is_like_aix {
-        BinaryFormat::Xcoff
-    } else {
-        BinaryFormat::Elf
+    let Some((architecture, sub_architecture)) =
+        sess.target.object_architecture(&sess.unstable_target_features)
+    else {
+        return None;
     };
+    let binary_format = sess.target.binary_format();
 
     let mut file = write::Object::new(binary_format, architecture, endianness);
     file.set_sub_architecture(sub_architecture);
@@ -300,7 +250,26 @@ pub(crate) fn create_object_file(sess: &Session) -> Option<write::Object<'static
 
         file.set_mangling(original_mangling);
     }
-    let e_flags = match architecture {
+    let e_flags = elf_e_flags(architecture, sess);
+    // adapted from LLVM's `MCELFObjectTargetWriter::getOSABI`
+    let os_abi = elf_os_abi(sess);
+    let abi_version = 0;
+    add_gnu_property_note(&mut file, architecture, binary_format, endianness);
+    file.flags = FileFlags::Elf { os_abi, abi_version, e_flags };
+    Some(file)
+}
+
+pub(super) fn elf_os_abi(sess: &Session) -> u8 {
+    match sess.target.options.os.as_ref() {
+        "hermit" => elf::ELFOSABI_STANDALONE,
+        "freebsd" => elf::ELFOSABI_FREEBSD,
+        "solaris" => elf::ELFOSABI_SOLARIS,
+        _ => elf::ELFOSABI_NONE,
+    }
+}
+
+pub(super) fn elf_e_flags(architecture: Architecture, sess: &Session) -> u32 {
+    match architecture {
         Architecture::Mips => {
             let arch = match sess.target.options.cpu.as_ref() {
                 "mips1" => elf::EF_MIPS_ARCH_1,
@@ -391,18 +360,7 @@ pub(crate) fn create_object_file(sess: &Session) -> Option<write::Object<'static
             e_flags
         }
         _ => 0,
-    };
-    // adapted from LLVM's `MCELFObjectTargetWriter::getOSABI`
-    let os_abi = match sess.target.options.os.as_ref() {
-        "hermit" => elf::ELFOSABI_STANDALONE,
-        "freebsd" => elf::ELFOSABI_FREEBSD,
-        "solaris" => elf::ELFOSABI_SOLARIS,
-        _ => elf::ELFOSABI_NONE,
-    };
-    let abi_version = 0;
-    add_gnu_property_note(&mut file, architecture, binary_format, endianness);
-    file.flags = FileFlags::Elf { os_abi, abi_version, e_flags };
-    Some(file)
+    }
 }
 
 /// Mach-O files contain information about:

compiler/rustc_feature/src/unstable.rs

@@ -592,6 +592,8 @@ declare_features! (
     (unstable, precise_capturing_in_traits, "1.83.0", Some(130044)),
     /// Allows macro attributes on expressions, statements and non-inline modules.
     (unstable, proc_macro_hygiene, "1.30.0", Some(54727)),
+    /// Allows the use of raw-dylibs on ELF platforms
+    (incomplete, raw_dylib_elf, "CURRENT_RUSTC_VERSION", Some(135694)),
     /// Makes `&` and `&mut` patterns eat only one layer of references in Rust 2024.
     (incomplete, ref_pat_eat_one_layer_2024, "1.79.0", Some(123076)),
     /// Makes `&` and `&mut` patterns eat only one layer of references in Rust 2024—structural variant

compiler/rustc_metadata/messages.ftl

@@ -233,6 +233,9 @@ metadata_prev_alloc_error_handler =
 metadata_prev_global_alloc =
     previous global allocator defined here
 
+metadata_raw_dylib_elf_unstable =
+    link kind `raw-dylib` is unstable on ELF platforms
+
 metadata_raw_dylib_no_nul =
     link name must not contain NUL characters if link kind is `raw-dylib`
 

compiler/rustc_metadata/src/native_libs.rs

@@ -17,7 +17,7 @@ use rustc_session::search_paths::PathKind;
 use rustc_session::utils::NativeLibKind;
 use rustc_span::def_id::{DefId, LOCAL_CRATE};
 use rustc_span::{Symbol, sym};
-use rustc_target::spec::LinkSelfContainedComponents;
+use rustc_target::spec::{BinaryFormat, LinkSelfContainedComponents};
 
 use crate::{errors, fluent_generated};
 
@@ -263,9 +263,26 @@ impl<'tcx> Collector<'tcx> {
                                 NativeLibKind::Framework { as_needed: None }
                             }
                             "raw-dylib" => {
-                                if !sess.target.is_like_windows {
+                                if sess.target.is_like_windows {
+                                    // raw-dylib is stable and working on Windows
+                                } else if sess.target.binary_format() == BinaryFormat::Elf
+                                    && features.raw_dylib_elf()
+                                {
+                                    // raw-dylib is unstable on ELF, but the user opted in
+                                } else if sess.target.binary_format() == BinaryFormat::Elf
+                                    && sess.is_nightly_build()
+                                {
+                                    feature_err(
+                                        sess,
+                                        sym::raw_dylib_elf,
+                                        span,
+                                        fluent_generated::metadata_raw_dylib_elf_unstable,
+                                    )
+                                    .emit();
+                                } else {
                                     sess.dcx().emit_err(errors::RawDylibOnlyWindows { span });
                                 }
+
                                 NativeLibKind::RawDylib
                             }
                             "link-arg" => {

compiler/rustc_session/src/utils.rs

@@ -34,6 +34,7 @@ pub enum NativeLibKind {
         as_needed: Option<bool>,
     },
     /// Dynamic library (e.g. `foo.dll` on Windows) without a corresponding import library.
+    /// On Linux, it refers to a generated shared library stub.
     RawDylib,
     /// A macOS-specific kind of dynamic libraries.
     Framework {

compiler/rustc_span/src/symbol.rs

@@ -1596,6 +1596,7 @@ symbols! {
         quote,
         range_inclusive_new,
         raw_dylib,
+        raw_dylib_elf,
         raw_eq,
         raw_identifiers,
         raw_ref_op,

compiler/rustc_target/src/spec/mod.rs

@@ -42,7 +42,7 @@ use std::path::{Path, PathBuf};
 use std::str::FromStr;
 use std::{fmt, io};
 
-use rustc_data_structures::fx::FxHashSet;
+use rustc_data_structures::fx::{FxHashSet, FxIndexSet};
 use rustc_fs_util::try_canonicalize;
 use rustc_macros::{Decodable, Encodable, HashStable_Generic};
 use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
@@ -69,6 +69,7 @@ mod base;
 mod json;
 
 pub use base::avr_gnu::ef_avr_arch;
+pub use object::BinaryFormat;
 
 /// Linker is called through a C/C++ compiler.
 #[derive(Clone, Copy, Debug, Eq, Ord, PartialEq, PartialOrd)]
@@ -3407,6 +3408,71 @@ impl Target {
             s => s.clone(),
         }
     }
+
+    pub fn binary_format(&self) -> object::BinaryFormat {
+        if self.is_like_osx {
+            object::BinaryFormat::MachO
+        } else if self.is_like_windows {
+            object::BinaryFormat::Coff
+        } else if self.is_like_aix {
+            object::BinaryFormat::Xcoff
+        } else {
+            object::BinaryFormat::Elf
+        }
+    }
+
+    pub fn object_architecture(
+        &self,
+        unstable_target_features: &FxIndexSet<Symbol>,
+    ) -> Option<(object::Architecture, Option<object::SubArchitecture>)> {
+        use object::Architecture;
+        Some(match self.arch.as_ref() {
+            "arm" => (Architecture::Arm, None),
+            "aarch64" => (
+                if self.pointer_width == 32 {
+                    Architecture::Aarch64_Ilp32
+                } else {
+                    Architecture::Aarch64
+                },
+                None,
+            ),
+            "x86" => (Architecture::I386, None),
+            "s390x" => (Architecture::S390x, None),
+            "mips" | "mips32r6" => (Architecture::Mips, None),
+            "mips64" | "mips64r6" => (Architecture::Mips64, None),
+            "x86_64" => (
+                if self.pointer_width == 32 {
+                    Architecture::X86_64_X32
+                } else {
+                    Architecture::X86_64
+                },
+                None,
+            ),
+            "powerpc" => (Architecture::PowerPc, None),
+            "powerpc64" => (Architecture::PowerPc64, None),
+            "riscv32" => (Architecture::Riscv32, None),
+            "riscv64" => (Architecture::Riscv64, None),
+            "sparc" => {
+                if unstable_target_features.contains(&sym::v8plus) {
+                    // Target uses V8+, aka EM_SPARC32PLUS, aka 64-bit V9 but in 32-bit mode
+                    (Architecture::Sparc32Plus, None)
+                } else {
+                    // Target uses V7 or V8, aka EM_SPARC
+                    (Architecture::Sparc, None)
+                }
+            }
+            "sparc64" => (Architecture::Sparc64, None),
+            "avr" => (Architecture::Avr, None),
+            "msp430" => (Architecture::Msp430, None),
+            "hexagon" => (Architecture::Hexagon, None),
+            "bpf" => (Architecture::Bpf, None),
+            "loongarch64" => (Architecture::LoongArch64, None),
+            "csky" => (Architecture::Csky, None),
+            "arm64ec" => (Architecture::Aarch64, Some(object::SubArchitecture::Arm64EC)),
+            // Unsupported architecture.
+            _ => return None,
+        })
+    }
 }
 
 /// Either a target tuple string or a path to a JSON file.

src/doc/rustc-dev-guide/src/tests/directives.md

@@ -142,6 +142,7 @@ Some examples of `X` in `ignore-X` or `only-X`:
   matches that target as well as the emscripten targets.
 - Pointer width: `32bit`, `64bit`
 - Endianness: `endian-big`
+- Binary format: `elf`
 - Stage: `stage0`, `stage1`, `stage2`
 - Channel: `stable`, `beta`
 - When cross compiling: `cross-compile`

src/tools/compiletest/src/directive-list.rs

@@ -48,6 +48,7 @@ const KNOWN_DIRECTIVE_NAMES: &[&str] = &[
     "ignore-coverage-run",
     "ignore-cross-compile",
     "ignore-eabi",
+    "ignore-elf",
     "ignore-emscripten",
     "ignore-endian-big",
     "ignore-enzyme",
@@ -177,6 +178,7 @@ const KNOWN_DIRECTIVE_NAMES: &[&str] = &[
     "only-bpf",
     "only-cdb",
     "only-dist",
+    "only-elf",
     "only-emscripten",
     "only-gnu",
     "only-i686-pc-windows-gnu",

src/tools/compiletest/src/header/cfg.rs

@@ -166,6 +166,15 @@ fn parse_cfg_name_directive<'a>(
         message: "when the target vendor is Apple"
     }
 
+    condition! {
+        name: "elf",
+        condition: !config.target.contains("windows")
+            && !config.target.contains("apple")
+            && !config.target.contains("aix")
+            && !config.target.contains("uefi"),
+        message: "when the target binary format is ELF"
+    }
+
     condition! {
         name: "enzyme",
         condition: config.has_enzyme,

tests/run-make/raw-dylib-elf-verbatim-absolute/main.rs

@@ -0,0 +1,11 @@
+#![feature(raw_dylib_elf)]
+#![allow(incomplete_features)]
+
+#[link(name = "/absolute-path/liblibrary.so.1", kind = "raw-dylib", modifiers = "+verbatim")]
+unsafe extern "C" {
+    safe fn this_is_a_library_function() -> core::ffi::c_int;
+}
+
+fn main() {
+    println!("{}", this_is_a_library_function())
+}

tests/run-make/raw-dylib-elf-verbatim-absolute/output.txt

@@ -0,0 +1 @@
+42

tests/run-make/raw-dylib-elf-verbatim-absolute/rmake.rs

@@ -0,0 +1,15 @@
+//@ only-elf
+
+//! Ensure ELF raw-dylib is able to link against a non-existent verbatim absolute path
+//! by embedding the absolute path in the DT_SONAME and passing a different path for
+//! the linker for the stub.
+
+use run_make_support::{build_native_dynamic_lib, cwd, diff, rfs, run, rustc};
+
+fn main() {
+    // We compile the binary without having the library present.
+    // The verbatim library name is an absolute path.
+    rustc().crate_type("bin").input("main.rs").run();
+
+    // FIXME(raw_dylib_elf): Read the NEEDED of the library to ensure it's the absolute path.
+}

tests/run-make/raw-dylib-elf-verbatim/library.c

@@ -0,0 +1,3 @@
+int this_is_a_library_function() {
+    return 42;
+}

tests/run-make/raw-dylib-elf-verbatim/main.rs

@@ -0,0 +1,11 @@
+#![feature(raw_dylib_elf)]
+#![allow(incomplete_features)]
+
+#[link(name = "liblibrary.so.1", kind = "raw-dylib", modifiers = "+verbatim")]
+unsafe extern "C" {
+    safe fn this_is_a_library_function() -> core::ffi::c_int;
+}
+
+fn main() {
+    println!("{}", this_is_a_library_function())
+}

tests/run-make/raw-dylib-elf-verbatim/output.txt

@@ -0,0 +1 @@
+42