Export ZERO_AR_DATE for macos linker invocations

This commit attempts to improve reproducibility of builds on macOS by
exporting the `ZERO_AR_DATE=1` environment variable for all invocations
of the linker. While it looks like this env var is targeted at just the
`ar` command (which does actually read this) it appears that recent-ish
versions of the linker *also* read this environment variable. This
env var forces the linker to set a deterministic zero value for the
mtime in the N_OSO field of the object file.

Currently it's believe that older versions of the linker will simply
ignore this env var, while newer versions will read it and produce a
deterministic output for compilations with debuginfo.

Closes #47086
Closes #66568

src/librustc_target/spec/apple_base.rs

@@ -31,6 +31,17 @@ pub fn opts() -> TargetOptions {
         has_elf_tls: version >= (10, 7),
         abi_return_struct_as_int: true,
         emit_debug_gdb_scripts: false,
+
+        // This environment variable is pretty magical but is intended for
+        // producing deterministic builds. This was first discovered to be used
+        // by the `ar` tool as a way to control whether or not mtime entries in
+        // the archive headers were set to zero or not. It appears that
+        // eventually the linker got updated to do the same thing and now reads
+        // this environment variable too in recent versions.
+        //
+        // For some more info see the commentary on #47086
+        link_env: vec![("ZERO_AR_DATE".to_string(), "1".to_string())],
+
         ..Default::default()
     }
 }

src/test/run-make-fulldeps/reproducible-build-2/Makefile

@@ -2,7 +2,6 @@
 
 # ignore-musl
 # ignore-windows
-# ignore-macos (rust-lang/rust#66568)
 # Objects are reproducible but their path is not.
 
 all:  \
@@ -21,7 +20,7 @@ sysroot:
 	rm -rf $(TMPDIR) && mkdir $(TMPDIR)
 	$(RUSTC) reproducible-build-aux.rs
 	$(RUSTC) reproducible-build.rs --crate-type rlib --sysroot $(shell $(RUSTC) --print sysroot) --remap-path-prefix=$(shell $(RUSTC) --print sysroot)=/sysroot
-	cp -r $(shell $(RUSTC) --print sysroot) $(TMPDIR)/sysroot
+	cp -R $(shell $(RUSTC) --print sysroot) $(TMPDIR)/sysroot
 	cp $(TMPDIR)/libreproducible_build.rlib $(TMPDIR)/libfoo.rlib
 	$(RUSTC) reproducible-build.rs --crate-type rlib --sysroot $(TMPDIR)/sysroot --remap-path-prefix=$(TMPDIR)/sysroot=/sysroot
 	cmp "$(TMPDIR)/libreproducible_build.rlib" "$(TMPDIR)/libfoo.rlib" || exit 1