Make rustup override and rust-toolchain.toml hierarchical

src/config.rs

@@ -580,14 +580,15 @@ impl Cfg {
         dir: &Path,
         settings: &Settings,
     ) -> Result<Option<(OverrideFile, OverrideReason)>> {
+        let mut override_ = None;
+
         let notify = self.notify_handler.as_ref();
-        let mut dir = Some(dir);
 
-        while let Some(d) = dir {
+        for d in std::iter::successors(Some(dir), |d| d.parent()) {
             // First check the override database
             if let Some(name) = settings.dir_override(d, notify) {
                 let reason = OverrideReason::OverrideDB(d.to_owned());
-                return Ok(Some((name.into(), reason)));
+                update_override(&mut override_, name.into(), reason);
             }
 
             // Then look for 'rust-toolchain' or 'rust-toolchain.toml'
@@ -662,13 +663,11 @@ impl Cfg {
                 }
 
                 let reason = OverrideReason::ToolchainFile(toolchain_file);
-                return Ok(Some((override_file, reason)));
+                update_override(&mut override_, override_file, reason);
             }
-
-            dir = d.parent();
         }
 
-        Ok(None)
+        Ok(override_)
     }
 
     fn parse_override_file<S: AsRef<str>>(