-Z linker-flavor

This patch adds a `-Z linker-flavor` flag to rustc which can be used to invoke
the linker using a different interface.

For example, by default rustc assumes that all the Linux targets will be linked
using GCC. This makes it impossible to use LLD as a linker using just `-C
linker=ld.lld` because that will invoke LLD with invalid command line
arguments. (e.g. rustc will pass -Wl,--gc-sections to LLD but LLD doesn't
understand that; --gc-sections would be the right argument)

With this patch one can pass `-Z linker-flavor=ld` to rustc to invoke the linker
using a LD-like interface. This way, `rustc -C linker=ld.lld -Z
linker-flavor=ld` will invoke LLD with the right arguments.

`-Z linker-flavor` accepts 4 different arguments: `em` (emcc), `ld`,
`gcc`, `msvc` (link.exe). `em`, `gnu` and `msvc` cover all the existing linker
interfaces. `ld` is a new flavor for interfacing GNU's ld and LLD.

This patch also changes target specifications. `linker-flavor` is now a
mandatory field that specifies the *default* linker flavor that the target will
use. This change also makes the linker interface *explicit*; before, it used to
be derived from other fields like linker-is-gnu, is-like-msvc,
is-like-emscripten, etc.

Another change to target specifications is that the fields `pre-link-args`,
`post-link-args` and `late-link-args` now expect a map from flavor to linker
arguments.

``` diff
-    "pre-link-args": ["-Wl,--as-needed", "-Wl,-z,-noexecstack"],
+    "pre-link-args": {
+        "gcc": ["-Wl,--as-needed", "-Wl,-z,-noexecstack"],
+        "ld": ["--as-needed", "-z,-noexecstack"],
+    },
```

[breaking-change]  for users of custom targets specifications

src/librustc/session/config.rs

@@ -19,7 +19,7 @@ pub use self::DebugInfoLevel::*;
 use session::{early_error, early_warn, Session};
 use session::search_paths::SearchPaths;
 
-use rustc_back::PanicStrategy;
+use rustc_back::{LinkerFlavor, PanicStrategy};
 use rustc_back::target::Target;
 use lint;
 use middle::cstore;
@@ -641,12 +641,14 @@ macro_rules! options {
             Some("either `panic` or `abort`");
         pub const parse_sanitizer: Option<&'static str> =
             Some("one of: `address`, `leak`, `memory` or `thread`");
+        pub const parse_linker_flavor: Option<&'static str> =
+            Some(::rustc_back::LinkerFlavor::one_of());
     }
 
     #[allow(dead_code)]
     mod $mod_set {
         use super::{$struct_name, Passes, SomePasses, AllPasses, Sanitizer};
-        use rustc_back::PanicStrategy;
+        use rustc_back::{LinkerFlavor, PanicStrategy};
 
         $(
             pub fn $opt(cg: &mut $struct_name, v: Option<&str>) -> bool {
@@ -777,6 +779,14 @@ macro_rules! options {
             }
             true
         }
+
+        fn parse_linker_flavor(slote: &mut Option<LinkerFlavor>, v: Option<&str>) -> bool {
+            match v.and_then(LinkerFlavor::from_str) {
+                Some(lf) => *slote = Some(lf),
+                _ => return false,
+            }
+            true
+        }
     }
 ) }
 
@@ -979,6 +989,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
           "pass `-install_name @rpath/...` to the macOS linker"),
     sanitizer: Option<Sanitizer> = (None, parse_sanitizer, [TRACKED],
                                    "Use a sanitizer"),
+    linker_flavor: Option<LinkerFlavor> = (None, parse_linker_flavor, [UNTRACKED],
+                                           "Linker flavor"),
 }
 
 pub fn default_lib_output() -> CrateType {

src/librustc/session/mod.rs

@@ -36,7 +36,7 @@ use syntax::{ast, codemap};
 use syntax::feature_gate::AttributeType;
 use syntax_pos::{Span, MultiSpan};
 
-use rustc_back::PanicStrategy;
+use rustc_back::{LinkerFlavor, PanicStrategy};
 use rustc_back::target::Target;
 use rustc_data_structures::flock;
 use llvm;
@@ -363,6 +363,9 @@ impl Session {
     pub fn panic_strategy(&self) -> PanicStrategy {
         self.opts.cg.panic.unwrap_or(self.target.target.options.panic_strategy)
     }
+    pub fn linker_flavor(&self) -> LinkerFlavor {
+        self.opts.debugging_opts.linker_flavor.unwrap_or(self.target.target.linker_flavor)
+    }
     pub fn no_landing_pads(&self) -> bool {
         self.opts.debugging_opts.no_landing_pads || self.panic_strategy() == PanicStrategy::Abort
     }

src/librustc_back/lib.rs

@@ -52,6 +52,48 @@ pub mod dynamic_lib;
 
 use serialize::json::{Json, ToJson};
 
+macro_rules! linker_flavor {
+    ($(($variant:ident, $string:expr),)+) => {
+        #[derive(Clone, Copy, Debug, Eq, Ord, PartialEq, PartialOrd, Hash,
+                 RustcEncodable, RustcDecodable)]
+        pub enum LinkerFlavor {
+            $($variant,)+
+        }
+
+        impl LinkerFlavor {
+            pub const fn one_of() -> &'static str {
+                concat!("one of: ", $($string, " ",)+)
+            }
+
+            pub fn from_str(s: &str) -> Option<Self> {
+                Some(match s {
+                    $($string => LinkerFlavor::$variant,)+
+                    _ => return None,
+                })
+            }
+
+            pub fn desc(&self) -> &str {
+                match *self {
+                    $(LinkerFlavor::$variant => $string,)+
+                }
+            }
+        }
+
+        impl ToJson for LinkerFlavor {
+            fn to_json(&self) -> Json {
+                self.desc().to_json()
+            }
+        }
+    }
+}
+
+linker_flavor! {
+    (Em, "em"),
+    (Gcc, "gcc"),
+    (Ld, "ld"),
+    (Msvc, "msvc"),
+}
+
 #[derive(Clone, Copy, Debug, PartialEq, Hash, RustcEncodable, RustcDecodable)]
 pub enum PanicStrategy {
     Unwind,

src/librustc_back/target/aarch64_apple_ios.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use LinkerFlavor;
 use target::{Target, TargetOptions, TargetResult};
 use super::apple_ios_base::{opts, Arch};
 
@@ -22,6 +23,7 @@ pub fn target() -> TargetResult {
         target_os: "ios".to_string(),
         target_env: "".to_string(),
         target_vendor: "apple".to_string(),
+        linker_flavor: LinkerFlavor::Gcc,
         options: TargetOptions {
             features: "+neon,+fp-armv8,+cyclone".to_string(),
             eliminate_frame_pointer: false,

src/librustc_back/target/aarch64_linux_android.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use LinkerFlavor;
 use target::{Target, TargetOptions, TargetResult};
 
 // See https://developer.android.com/ndk/guides/abis.html#arm64-v8a
@@ -28,6 +29,7 @@ pub fn target() -> TargetResult {
         target_os: "android".to_string(),
         target_env: "".to_string(),
         target_vendor: "unknown".to_string(),
+        linker_flavor: LinkerFlavor::Gcc,
         options: TargetOptions {
             abi_blacklist: super::arm_base::abi_blacklist(),
             .. base

src/librustc_back/target/aarch64_unknown_freebsd.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use LinkerFlavor;
 use target::{Target, TargetOptions, TargetResult};
 
 pub fn target() -> TargetResult {
@@ -26,6 +27,7 @@ pub fn target() -> TargetResult {
         target_os: "freebsd".to_string(),
         target_env: "".to_string(),
         target_vendor: "unknown".to_string(),
+        linker_flavor: LinkerFlavor::Gcc,
         options: TargetOptions {
             abi_blacklist: super::arm_base::abi_blacklist(),
             .. base

src/librustc_back/target/aarch64_unknown_fuchsia.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use LinkerFlavor;
 use target::{Target, TargetOptions, TargetResult};
 
 pub fn target() -> TargetResult {
@@ -23,6 +24,7 @@ pub fn target() -> TargetResult {
         target_os: "fuchsia".to_string(),
         target_env: "".to_string(),
         target_vendor: "unknown".to_string(),
+        linker_flavor: LinkerFlavor::Gcc,
         options: TargetOptions {
             abi_blacklist: super::arm_base::abi_blacklist(),
             .. base

src/librustc_back/target/aarch64_unknown_linux_gnu.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use LinkerFlavor;
 use target::{Target, TargetOptions, TargetResult};
 
 pub fn target() -> TargetResult {
@@ -26,6 +27,7 @@ pub fn target() -> TargetResult {
         arch: "aarch64".to_string(),
         target_os: "linux".to_string(),
         target_vendor: "unknown".to_string(),
+        linker_flavor: LinkerFlavor::Gcc,
         options: TargetOptions {
             abi_blacklist: super::arm_base::abi_blacklist(),
             .. base

src/librustc_back/target/android_base.rs

@@ -8,13 +8,15 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use LinkerFlavor;
 use target::TargetOptions;
 
 pub fn opts() -> TargetOptions {
     let mut base = super::linux_base::opts();
     // Many of the symbols defined in compiler-rt are also defined in libgcc.
     // Android's linker doesn't like that by default.
-    base.pre_link_args.push("-Wl,--allow-multiple-definition".to_string());
+    base.pre_link_args
+        .get_mut(&LinkerFlavor::Gcc).unwrap().push("-Wl,--allow-multiple-definition".to_string());
     base.is_like_android = true;
     base.position_independent_executables = true;
     base.has_elf_tls = false;

src/librustc_back/target/apple_base.rs

@@ -10,7 +10,7 @@
 
 use std::env;
 
-use target::TargetOptions;
+use target::{LinkArgs, TargetOptions};
 
 pub fn opts() -> TargetOptions {
     // ELF TLS is only available in macOS 10.7+. If you try to compile for 10.6
@@ -43,7 +43,7 @@ pub fn opts() -> TargetOptions {
         dll_prefix: "lib".to_string(),
         dll_suffix: ".dylib".to_string(),
         archive_format: "bsd".to_string(),
-        pre_link_args: Vec::new(),
+        pre_link_args: LinkArgs::new(),
         exe_allocation_crate: super::maybe_jemalloc(),
         has_elf_tls: version >= (10, 7),
         .. Default::default()

src/librustc_back/target/apple_ios_base.rs

@@ -8,9 +8,10 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use LinkerFlavor;
 use std::io;
 use std::process::Command;
-use target::TargetOptions;
+use target::{LinkArgs, TargetOptions};
 
 use self::Arch::*;
 
@@ -60,7 +61,7 @@ pub fn get_sdk_root(sdk_name: &str) -> Result<String, String> {
     }
 }
 
-fn build_pre_link_args(arch: Arch) -> Result<Vec<String>, String> {
+fn build_pre_link_args(arch: Arch) -> Result<LinkArgs, String> {
     let sdk_name = match arch {
         Armv7 | Armv7s | Arm64 => "iphoneos",
         I386 | X86_64 => "iphonesimulator"
@@ -70,8 +71,14 @@ fn build_pre_link_args(arch: Arch) -> Result<Vec<String>, String> {
 
     let sdk_root = get_sdk_root(sdk_name)?;
 
-    Ok(vec!["-arch".to_string(), arch_name.to_string(),
-         "-Wl,-syslibroot".to_string(), sdk_root])
+    let mut args = LinkArgs::new();
+    args.insert(LinkerFlavor::Gcc,
+                vec!["-arch".to_string(),
+                     arch_name.to_string(),
+                     "-Wl,-syslibroot".to_string(),
+                     sdk_root]);
+
+    Ok(args)
 }
 
 fn target_cpu(arch: Arch) -> String {

src/librustc_back/target/arm_linux_androideabi.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use LinkerFlavor;
 use target::{Target, TargetOptions, TargetResult};
 
 pub fn target() -> TargetResult {
@@ -24,6 +25,7 @@ pub fn target() -> TargetResult {
         target_os: "android".to_string(),
         target_env: "".to_string(),
         target_vendor: "unknown".to_string(),
+        linker_flavor: LinkerFlavor::Gcc,
         options: TargetOptions {
             abi_blacklist: super::arm_base::abi_blacklist(),
             .. base

src/librustc_back/target/arm_unknown_linux_gnueabi.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use LinkerFlavor;
 use target::{Target, TargetOptions, TargetResult};
 
 pub fn target() -> TargetResult {
@@ -22,6 +23,7 @@ pub fn target() -> TargetResult {
         target_os: "linux".to_string(),
         target_env: "gnu".to_string(),
         target_vendor: "unknown".to_string(),
+        linker_flavor: LinkerFlavor::Gcc,
 
         options: TargetOptions {
             features: "+v6".to_string(),

src/librustc_back/target/arm_unknown_linux_gnueabihf.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use LinkerFlavor;
 use target::{Target, TargetOptions, TargetResult};
 
 pub fn target() -> TargetResult {
@@ -22,6 +23,7 @@ pub fn target() -> TargetResult {
         target_os: "linux".to_string(),
         target_env: "gnu".to_string(),
         target_vendor: "unknown".to_string(),
+        linker_flavor: LinkerFlavor::Gcc,
 
         options: TargetOptions {
             features: "+v6,+vfp2".to_string(),

src/librustc_back/target/arm_unknown_linux_musleabi.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use LinkerFlavor;
 use target::{Target, TargetOptions, TargetResult};
 
 pub fn target() -> TargetResult {
@@ -29,6 +30,7 @@ pub fn target() -> TargetResult {
         target_os: "linux".to_string(),
         target_env: "musl".to_string(),
         target_vendor: "unknown".to_string(),
+        linker_flavor: LinkerFlavor::Gcc,
         options: TargetOptions {
             abi_blacklist: super::arm_base::abi_blacklist(),
             .. base

src/librustc_back/target/arm_unknown_linux_musleabihf.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use LinkerFlavor;
 use target::{Target, TargetOptions, TargetResult};
 
 pub fn target() -> TargetResult {
@@ -29,6 +30,7 @@ pub fn target() -> TargetResult {
         target_os: "linux".to_string(),
         target_env: "musl".to_string(),
         target_vendor: "unknown".to_string(),
+        linker_flavor: LinkerFlavor::Gcc,
         options: TargetOptions {
             abi_blacklist: super::arm_base::abi_blacklist(),
             .. base

src/librustc_back/target/armv5te_unknown_linux_gnueabi.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use LinkerFlavor;
 use target::{Target, TargetOptions, TargetResult};
 
 pub fn target() -> TargetResult {
@@ -21,6 +22,7 @@ pub fn target() -> TargetResult {
         target_os: "linux".to_string(),
         target_env: "gnu".to_string(),
         target_vendor: "unknown".to_string(),
+        linker_flavor: LinkerFlavor::Gcc,
 
         options: TargetOptions {
             features: "+soft-float".to_string(),
@@ -31,4 +33,3 @@ pub fn target() -> TargetResult {
         }
     })
 }
-

src/librustc_back/target/armv7_apple_ios.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use LinkerFlavor;
 use target::{Target, TargetOptions, TargetResult};
 use super::apple_ios_base::{opts, Arch};
 
@@ -22,6 +23,7 @@ pub fn target() -> TargetResult {
         target_os: "ios".to_string(),
         target_env: "".to_string(),
         target_vendor: "apple".to_string(),
+        linker_flavor: LinkerFlavor::Gcc,
         options: TargetOptions {
             features: "+v7,+vfp3,+neon".to_string(),
             max_atomic_width: Some(64),