Support for machine wide installs: fallback-settings file

This provides a way for snaps, distro packages and other such installs
to set the default-toolchain for users that have not gone through the
rustup-init one-time set of questions.

/etc/rustup/settings.toml can more now just provide a default toolchain
for users of rustup on the machine. More keys will be supported as
needed. No equivalent file is defined for Windows OS machines at this
time.

src/config.rs

@@ -108,10 +108,13 @@ impl Display for PgpPublicKey {
     }
 }
 
+pub const UNIX_FALLBACK_SETTINGS: &str = "/etc/rustup/settings.toml";
+
 pub struct Cfg {
     pub profile_override: Option<dist::Profile>,
     pub rustup_dir: PathBuf,
     pub settings_file: SettingsFile,
+    pub fallback_settings: Option<Settings>,
     pub toolchains_dir: PathBuf,
     pub update_hash_dir: PathBuf,
     pub download_dir: PathBuf,
@@ -133,6 +136,20 @@ impl Cfg {
 
         let settings_file = SettingsFile::new(rustup_dir.join("settings.toml"));
 
+        // Centralised file for multi-user systems to provide admin/distributor set initial values.
+        let fallback_settings = if cfg!(not(windows)) {
+            let f = SettingsFile::new(PathBuf::from(UNIX_FALLBACK_SETTINGS));
+            // Users cannot fix issues with missing/unreadable/invalid centralised files, so we simply trap all errors.
+            // Ideally we would separate these into missing file vs others, so that we could log a message about the bad
+            // situation rather than entirely ignoring it.
+            match f.with(|s| Ok(Some(s.clone()))) {
+                Ok(res) => res,
+                Err(_) => None,
+            }
+        } else {
+            None
+        };
+
         let toolchains_dir = rustup_dir.join("toolchains");
         let update_hash_dir = rustup_dir.join("update-hashes");
         let download_dir = rustup_dir.join("downloads");
@@ -191,6 +208,7 @@ impl Cfg {
             profile_override: None,
             rustup_dir,
             settings_file,
+            fallback_settings,
             toolchains_dir,
             update_hash_dir,
             download_dir,
@@ -365,9 +383,7 @@ impl Cfg {
     }
 
     pub fn find_default(&self) -> Result<Option<Toolchain<'_>>> {
-        let opt_name = self
-            .settings_file
-            .with(|s| Ok(s.default_toolchain.clone()))?;
+        let opt_name = self.get_default()?;
 
         if let Some(name) = opt_name {
             let toolchain = self
@@ -509,7 +525,14 @@ impl Cfg {
     }
 
     pub fn get_default(&self) -> Result<Option<String>> {
-        self.settings_file.with(|s| Ok(s.default_toolchain.clone()))
+        let user_opt = self.settings_file.with(|s| Ok(s.default_toolchain.clone()));
+        if let Some(fallback_settings) = &self.fallback_settings {
+            match user_opt {
+                Err(_) | Ok(None) => return Ok(fallback_settings.default_toolchain.clone()),
+                _ => {}
+            };
+        };
+        user_opt
     }
 
     pub fn list_toolchains(&self) -> Result<Vec<String>> {