rustup_mode: add toolchain install --allow-downgrade option

In case you have an already installed nightly toolchain
and you want to install a nightly toolchain with a component
which is not available in the installed toolchain,
rustup is searching for the most recent nightly supporting
all requested components and which is not older than the currently
installed nightly.

With this option set it will also search for toolchains older
than the currently installed one.

src/cli/rustup_mode.rs

@@ -319,6 +319,12 @@ pub fn cli() -> App<'static, 'static> {
                                 .help("Force an update, even if some components are missing")
                                 .long("force")
                                 .takes_value(false),
+                        )
+                        .arg(
+                            Arg::with_name("allow-downgrade")
+                                .help("Allow rustup to downgrade the toolchain to satisfy your component choice")
+                                .long("allow-downgrade")
+                                .takes_value(false),
                         ),
                 )
                 .subcommand(
@@ -822,7 +828,12 @@ fn update(cfg: &mut Cfg, m: &ArgMatches<'_>) -> Result<()> {
                     .values_of("targets")
                     .map(|v| v.collect())
                     .unwrap_or_else(Vec::new);
-                Some(toolchain.install_from_dist(m.is_present("force"), &components, &targets)?)
+                Some(toolchain.install_from_dist(
+                    m.is_present("force"),
+                    m.is_present("allow-downgrade"),
+                    &components,
+                    &targets,
+                )?)
             } else if !toolchain.exists() {
                 return Err(ErrorKind::InvalidToolchainName(toolchain.name().to_string()).into());
             } else {

src/cli/self_update.rs

@@ -782,7 +782,7 @@ fn maybe_install_rust(
         println!();
     } else if cfg.find_default()?.is_none() {
         let toolchain = cfg.get_toolchain(toolchain_str, false)?;
-        let status = toolchain.install_from_dist(true, components, targets)?;
+        let status = toolchain.install_from_dist(true, false, components, targets)?;
         cfg.set_default(toolchain_str)?;
         println!();
         common::show_channel_update(&cfg, toolchain_str, Ok(status))?;

src/config.rs

@@ -438,7 +438,7 @@ impl Cfg {
                             ErrorKind::OverrideToolchainNotInstalled(name.to_string())
                         })
                     } else {
-                        toolchain.install_from_dist(true, &[], &[])?;
+                        toolchain.install_from_dist(true, false, &[], &[])?;
                         Ok(Some((toolchain, reason)))
                     }
                 }
@@ -547,7 +547,7 @@ impl Cfg {
         // Update toolchains and collect the results
         let channels = channels.map(|(n, t)| {
             let t = t.and_then(|t| {
-                let t = t.install_from_dist(force_update, &[], &[]);
+                let t = t.install_from_dist(force_update, false, &[], &[]);
                 if let Err(ref e) = t {
                     (self.notify_handler)(Notification::NonFatalError(e));
                 }
@@ -599,7 +599,7 @@ impl Cfg {
     ) -> Result<Command> {
         let toolchain = self.get_toolchain(toolchain, false)?;
         if install_if_missing && !toolchain.exists() {
-            toolchain.install_from_dist(true, &[], &[])?;
+            toolchain.install_from_dist(true, false, &[], &[])?;
         }
 
         if let Some(cmd) = self.maybe_do_cargo_fallback(&toolchain, binary)? {

src/dist/dist.rs

@@ -580,6 +580,7 @@ pub fn update_from_dist<'a>(
     profile: Option<Profile>,
     prefix: &InstallPrefix,
     force_update: bool,
+    allow_downgrade: bool,
     old_date: Option<&str>,
     components: &[&str],
     targets: &[&str],
@@ -601,6 +602,7 @@ pub fn update_from_dist<'a>(
         profile,
         prefix,
         force_update,
+        allow_downgrade,
         old_date,
         components,
         targets,
@@ -622,6 +624,7 @@ fn update_from_dist_<'a>(
     profile: Option<Profile>,
     prefix: &InstallPrefix,
     force_update: bool,
+    allow_downgrade: bool,
     old_date: Option<&str>,
     components: &[&str],
     targets: &[&str],
@@ -644,21 +647,24 @@ fn update_from_dist_<'a>(
         Some(if provided < 1 { 1 } else { provided })
     };
 
-    // We never want to backtrack further back than the nightly that's already installed.
+    // In case there is no allow-downgrade option set
+    // we never want to backtrack further back than the nightly that's already installed.
     //
     // If no nightly is installed, it makes no sense to backtrack beyond the first ever manifest,
     // which is 2014-12-20 according to
     // https://static.rust-lang.org/cargo-dist/index.html.
     //
     // We could arguably use the date of the first rustup release here, but that would break a
     // bunch of the tests, which (inexplicably) use 2015-01-01 as their manifest dates.
-    let first_manifest = old_date
-        .map(|date| {
-            Utc.from_utc_date(
-                &NaiveDate::parse_from_str(date, "%Y-%m-%d").expect("Malformed manifest date"),
-            )
-        })
-        .unwrap_or_else(|| Utc.from_utc_date(&NaiveDate::from_ymd(2014, 12, 20)));
+    let first_manifest = Utc.from_utc_date(&NaiveDate::from_ymd(2014, 12, 20));
+    let old_manifest = old_date
+        .and_then(|date| utc_from_manifest_date(date))
+        .unwrap_or(first_manifest);
+    let last_manifest = if allow_downgrade {
+        first_manifest
+    } else {
+        old_manifest
+    };
 
     loop {
         match try_update_from_dist_(
@@ -713,22 +719,12 @@ fn update_from_dist_<'a>(
                 // the components that the user currently has installed. Let's try the previous
                 // nightlies in reverse chronological order until we find a nightly that does,
                 // starting at one date earlier than the current manifest's date.
-                let try_next = Utc
-                    .from_utc_date(
-                        &NaiveDate::parse_from_str(
-                            toolchain.date.as_ref().unwrap_or(&fetched),
-                            "%Y-%m-%d",
-                        )
-                        .unwrap_or_else(|_| {
-                            panic!(
-                                "Malformed manifest date: {:?}",
-                                toolchain.date.as_ref().unwrap_or(&fetched)
-                            )
-                        }),
-                    )
+                let toolchain_date = toolchain.date.as_ref().unwrap_or(&fetched);
+                let try_next = utc_from_manifest_date(toolchain_date)
+                    .unwrap_or_else(|| panic!("Malformed manifest date: {:?}", toolchain_date))
                     .pred();
 
-                if try_next < first_manifest {
+                if try_next < last_manifest {
                     // Wouldn't be an update if we go further back than the user's current nightly.
                     if let Some(e) = first_err {
                         break Err(e);
@@ -926,3 +922,9 @@ fn dl_v1_manifest<'a>(download: DownloadCfg<'a>, toolchain: &ToolchainDesc) -> R
 
     Ok(urls)
 }
+
+fn utc_from_manifest_date(date_str: &str) -> Option<Date<Utc>> {
+    NaiveDate::parse_from_str(date_str, "%Y-%m-%d")
+        .ok()
+        .map(|date| Utc.from_utc_date(&date))
+}

src/install.rs

@@ -24,6 +24,8 @@ pub enum InstallMethod<'a> {
         DownloadCfg<'a>,
         // --force
         bool,
+        // --allow-downgrade
+        bool,
         // toolchain already exists
         bool,
         // currently installed date
@@ -66,6 +68,7 @@ impl<'a> InstallMethod<'a> {
                 update_hash,
                 dl_cfg,
                 force_update,
+                allow_downgrade,
                 exists,
                 old_date,
                 components,
@@ -79,6 +82,7 @@ impl<'a> InstallMethod<'a> {
                     if exists { None } else { Some(profile) },
                     prefix,
                     force_update,
+                    allow_downgrade,
                     old_date,
                     components,
                     targets,

src/toolchain.rs

@@ -166,6 +166,7 @@ impl<'a> Toolchain<'a> {
     pub fn install_from_dist(
         &self,
         force_update: bool,
+        allow_downgrade: bool,
         components: &[&str],
         targets: &[&str],
     ) -> Result<UpdateStatus> {
@@ -177,6 +178,7 @@ impl<'a> Toolchain<'a> {
             update_hash.as_ref().map(|p| &**p),
             self.download_cfg(),
             force_update,
+            allow_downgrade,
             self.exists(),
             old_date.as_ref().map(|s| &**s),
             components,
@@ -193,6 +195,7 @@ impl<'a> Toolchain<'a> {
             self.download_cfg(),
             false,
             false,
+            false,
             None,
             &[],
             &[],

tests/cli-v2.rs

@@ -1392,3 +1392,60 @@ fn check_pgp_keys() {
         );
     })
 }
+
+#[test]
+fn install_allow_downgrade() {
+    clitools::setup(Scenario::MissingComponent, &|config| {
+        let trip = TargetTriple::from_build();
+
+        // this dist has no rls and there is no newer one
+        set_current_dist_date(config, "2019-09-14");
+        expect_ok(
+            config,
+            &[
+                "rustup",
+                "toolchain",
+                "install",
+                "nightly",
+                "--no-self-update",
+            ],
+        );
+        expect_stdout_ok(config, &["rustc", "--version"], "hash-nightly-3");
+        expect_component_not_executable(config, "rls");
+
+        expect_err(
+            config,
+            &[
+                "rustup",
+                "toolchain",
+                "install",
+                "nightly",
+                "--no-self-update",
+                "-c",
+                "rls",
+            ],
+            &format!(
+                "component 'rls' for target '{}' is unavailable for download for channel nightly",
+                trip,
+            ),
+        );
+        expect_stdout_ok(config, &["rustc", "--version"], "hash-nightly-3");
+        expect_component_not_executable(config, "rls");
+
+        expect_ok(
+            config,
+            &[
+                "rustup",
+                "toolchain",
+                "install",
+                "nightly",
+                "--no-self-update",
+                "-c",
+                "rls",
+                "--allow-downgrade",
+            ],
+        );
+        expect_stdout_ok(config, &["rustc", "--version"], "hash-nightly-2");
+        expect_component_executable(config, "rls");
+    });
+}