Auto merge of #28578 - gandro:nodefaultlibs, r=alexcrichton

This patch basically adds a target option for omitting the `-nodefaultlibs` flag when invoking the linker. I am not sure if this is the correct or only way to approach this problem, so any feedback is welcome.

Motivation: I'm currently working on a Rust target specification for the [rumprun](/rumpkernel/rumprun) unikernel. rumprun is based on rump kernels and uses NetBSDs libc and drivers to provide a POSIXy environment. It provides its own linker wrapper that generates binaries which can be "baked" into a unikernel after configuration. Using `-nodefaultlibs` on the rumprun linker will prevent it from selecting the search paths for the rumprun libraries. My current target implementation for rumprun is here: gandro/rust@295744b

Currently, only a target that `is_like_windows` will omit the `-nodefaultlibs` flag, but since rumprun is not like Windows otherwise, I think a separate flag makes more sense. This might be a breaking change for target specifications that have the `is_like_windows` option set to true. Such targets need to set `no_default_libraries` to false in order to restore the old behavior.

src/librustc_back/target/mod.rs

@@ -157,6 +157,9 @@ pub struct TargetOptions {
     /// Whether to disable linking to compiler-rt. Defaults to false, as LLVM
     /// will emit references to the functions that compiler-rt provides.
     pub no_compiler_rt: bool,
+    /// Whether to disable linking to the default libraries, typically corresponds
+    /// to `-nodefaultlibs`. Defaults to true.
+    pub no_default_libraries: bool,
     /// Dynamically linked executables can be compiled as position independent
     /// if the default relocation model of position independent code is not
     /// changed. This is a requirement to take advantage of ASLR, as otherwise
@@ -212,6 +215,7 @@ impl Default for TargetOptions {
             linker_is_gnu: false,
             has_rpath: false,
             no_compiler_rt: false,
+            no_default_libraries: true,
             position_independent_executables: false,
             pre_link_objects: Vec::new(),
             post_link_objects: Vec::new(),
@@ -319,6 +323,7 @@ impl Target {
         key!(linker_is_gnu, bool);
         key!(has_rpath, bool);
         key!(no_compiler_rt, bool);
+        key!(no_default_libraries, bool);
         key!(pre_link_args, list);
         key!(post_link_args, list);
         key!(allow_asm, bool);

src/librustc_back/target/windows_base.rs

@@ -23,6 +23,10 @@ pub fn opts() -> TargetOptions {
         exe_suffix: ".exe".to_string(),
         staticlib_prefix: "".to_string(),
         staticlib_suffix: ".lib".to_string(),
+        // Unfortunately right now passing -nodefaultlibs to gcc on windows
+        // doesn't work so hot (in terms of native dependencies). This flag
+        // should hopefully be removed one day though!
+        no_default_libraries: false,
         is_like_windows: true,
         archive_format: "gnu".to_string(),
         pre_link_args: vec!(

src/librustc_trans/back/link.rs

@@ -970,7 +970,9 @@ fn link_args(cmd: &mut Linker,
     // default. Note that this does not happen for windows because windows pulls
     // in some large number of libraries and I couldn't quite figure out which
     // subset we wanted.
-    cmd.no_default_libraries();
+    if t.options.no_default_libraries {
+        cmd.no_default_libraries();
+    }
 
     // Take careful note of the ordering of the arguments we pass to the linker
     // here. Linkers will assume that things on the left depend on things to the

src/librustc_trans/back/linker.rs

@@ -159,12 +159,7 @@ impl<'a> Linker for GnuLinker<'a> {
     }
 
     fn no_default_libraries(&mut self) {
-        // Unfortunately right now passing -nodefaultlibs to gcc on windows
-        // doesn't work so hot (in terms of native dependencies). This if
-        // statement should hopefully be removed one day though!
-        if !self.sess.target.target.options.is_like_windows {
-            self.cmd.arg("-nodefaultlibs");
-        }
+        self.cmd.arg("-nodefaultlibs");
     }
 
     fn build_dylib(&mut self, out_filename: &Path) {