Infer the default host target from the host toolchain if possible

This fixes ongoing issues where x.py will detect the wrong host triple
between MSVC and GNU.

- Add line to changelog

src/bootstrap/CHANGELOG.md

@@ -8,6 +8,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 - `x.py check` needs opt-in to check tests (--all-targets) [#77473](https://github.com/rust-lang/rust/pull/77473)
 - The default bootstrap profiles are now located at `bootstrap/defaults/config.$PROFILE.toml` (previously they were located at `bootstrap/defaults/config.toml.$PROFILE`) [#77558](https://github.com/rust-lang/rust/pull/77558)
+- If you have Rust already installed, `x.py` will now infer the host target
+  from the default rust toolchain. [#78513](https://github.com/rust-lang/rust/pull/78513)
 
 
 ## [Version 2] - 2020-09-25

src/bootstrap/bootstrap.py

@@ -187,8 +187,23 @@ def format_build_time(duration):
     return str(datetime.timedelta(seconds=int(duration)))
 
 
-def default_build_triple():
+def default_build_triple(verbose):
     """Build triple as in LLVM"""
+    # If the user already has a host build triple with an existing `rustc`
+    # install, use their preference. This fixes most issues with Windows builds
+    # being detected as GNU instead of MSVC.
+    try:
+        version = subprocess.check_output(["rustc", "--version", "--verbose"])
+        host = next(x for x in version.split('\n') if x.startswith("host: "))
+        triple = host.split("host: ")[1]
+        if verbose:
+            print("detected default triple {}".format(triple))
+        return triple
+    except Exception as e:
+        if verbose:
+            print("rustup not detected: {}".format(e))
+            print("falling back to auto-detect")
+
     default_encoding = sys.getdefaultencoding()
     required = sys.platform != 'win32'
     ostype = require(["uname", "-s"], exit=required)
@@ -831,7 +846,7 @@ class RustBuild(object):
         config = self.get_toml('build')
         if config:
             return config
-        return default_build_triple()
+        return default_build_triple(self.verbose)
 
     def check_submodule(self, module, slow_submodules):
         if not slow_submodules:

src/bootstrap/configure.py

@@ -266,7 +266,7 @@ config = {}
 def build():
     if 'build' in known_args:
         return known_args['build'][-1][1]
-    return bootstrap.default_build_triple()
+    return bootstrap.default_build_triple(verbose=False)
 
 
 def set(key, value):