default to $ARCH-apple-macosx10.7.0 LLVM triple for darwin targets

Over in #60378, we made `rustc` switch LLVM target triples dynamically
based on the `MACOSX_DEPLOYMENT_TARGET` environment variable.  This
change was made to align with `clang`'s behavior, and therefore make
cross-language LTO feasible on OS X.  Otherwise, `rustc` would produce
LLVM bitcode files with a target triple of `x86_64-apple-darwin`,
`clang` would produce LLVM bitcode files with a target triple of
`x86_64-apple-macosx$VERSION`, and the linker would complain.

This change worked fine, except for one corner case: if you didn't have
`MACOSX_DEPLOYMENT_TARGET` set, and you wanted to do LTO on just Rust
code, you'd get warning messages similar to:

```
warning: Linking two modules of different target triples: ' is 'x86_64-apple-macosx10.7.0' whereas 'main.7rcbfp3g-cgu.4' is 'x86_64-apple-darwin'
```

This message occurs because libstd is compiled with
`MACOSX_DEPLOYMENT_TARGET` set to 10.7.  The LLVM bitcode distributed in
libstd's rlibs, then, is tagged with the target triple of
`x86_64-apple-macosx10.7.0`, while the bitcode `rustc` produces for
"user" code is tagged with the target triple of `x86_64-apple-darwin`.

It's not good to have LTO on just Rust code (probably much more common
than cross-language LTO) warn by default.  These warnings also break
Cargo's testsuite.

This change defaults to acting as though `MACOSX_DEPLOYMENT_TARGET` was
set to 10.7.  "user" code will then be given a target triple that is
equivalent to the target triple libstd bitcode is already using.  The
above warning will therefore go away.

`rustc` already assumes that compiling without
`MACOSX_DEPLOYMENT_TARGET` means that we're compiling for a target
compatible with OS X 10.7 (e.g. that things like TLS work properly).  So
this change is really just making things conform more closely to the
status quo.

(It's also worth noting that before and after this patch, compiling with
`MACOSX_DEPLOYMENT_TARGET` set to, say, 10.9, works just fine: target
triples with an "apple" version ignore OS versions when checking
compatibility, so bitcode with a `x86_64-apple-macosx10.7.0` triple works just
fine with bitcode with a `x86_64-apple-macosx10.9.0` triple.)

2 files changed, 2 insertions, 2 deletions

diff --git a/src/test/codegen/i686-no-macosx-deployment-target.rs b/src/test/codegen/i686-no-macosx-deployment-target.rs
index eb826590523..1cebc49236f 100644
--- a/src/test/codegen/i686-no-macosx-deployment-target.rs
+++ b/src/test/codegen/i686-no-macosx-deployment-target.rs
@@ -19,7 +19,7 @@ pub struct Bool {
     b: bool,
 }
 
-// CHECK: target triple = "i686-apple-darwin"
+// CHECK: target triple = "i686-apple-macosx10.7.0"
 #[no_mangle]
 pub extern "C" fn structbool() -> Bool {
     Bool { b: true }
diff --git a/src/test/codegen/x86_64-no-macosx-deployment-target.rs b/src/test/codegen/x86_64-no-macosx-deployment-target.rs
index 58a11d1095b..c5ac73b54e1 100644
--- a/src/test/codegen/x86_64-no-macosx-deployment-target.rs
+++ b/src/test/codegen/x86_64-no-macosx-deployment-target.rs
@@ -19,7 +19,7 @@ pub struct Bool {
     b: bool,
 }
 
-// CHECK: target triple = "x86_64-apple-darwin"
+// CHECK: target triple = "x86_64-apple-macosx10.7.0"
 #[no_mangle]
 pub extern "C" fn structbool() -> Bool {
     Bool { b: true }