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.

Author: Noratrieb

compiler/rustc_codegen_ssa/src/back/link.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: