rustup: Prevent setting an invalid default host

Perform similar checks to when resolving toolchains in order to
reduce the possibility that the configuration will end up with an
unusable default-host value.  This finally addresses the initial
point in #745 and thus fixes it.

Signed-off-by: Daniel Silverstone <dsilvers@digital-scurf.org>

src/rustup/config.rs

@@ -478,9 +478,11 @@ impl Cfg {
     }
 
     pub fn set_default_host_triple(&self, host_triple: &str) -> Result<()> {
-        if dist::PartialTargetTriple::from_str(host_triple).is_none() {
-            return Err("Invalid host triple".into());
-        }
+        // Ensure that the provided host_triple is capable of resolving
+        // against the 'stable' toolchain.  This provides early errors
+        // if the supplied triple is insufficient / bad.
+        dist::PartialToolchainDesc::from_str("stable")?
+            .resolve(&dist::TargetTriple::from_str(host_triple))?;
         self.settings_file.with_mut(|s| {
             s.default_host_triple = Some(host_triple.to_owned());
             Ok(())

tests/cli-rustup.rs

@@ -900,7 +900,19 @@ fn set_default_host_invalid_triple() {
         expect_err(
             config,
             &["rustup", "set", "default-host", "foo"],
-            "Invalid host triple",
+            "error: Provided host 'foo' couldn't be converted to partial triple",
+        );
+    });
+}
+
+// #745
+#[test]
+fn set_default_host_invalid_triple_valid_partial() {
+    setup(&|config| {
+        expect_err(
+            config,
+            &["rustup", "set", "default-host", "x86_64-msvc"],
+            "error: Provided host 'x86_64-msvc' did not specify an operating system",
         );
     });
 }