Auto merge of #584 - ehuss:dont-uninstall-stable, r=pietroalbini

Don't uninstall the stable toolchain.

#572 added some code to uninstall existing toolchains.  However, this caused the `stable` toolchain to always be uninstalled. This toolchain is used for installing tools like rustup-toolchain-install-master.  Uninstalling it causes it to be redownloaded every time crater is run locally, making it difficult to use it locally since it can take about a minute for me to download.

Pietro said to add a test for stable here.  Unfortunately rustwide doesn't provide public access to anything other than the display impl to test this.  I can make some changes to rustwide if a cleaner approach is desired.

Author: bors

Cargo.lock

@@ -60,7 +60,7 @@ hmac = "0.7"
 sha-1 = "0.8"
 rust_team_data = { git = "https://github.com/rust-lang/team" }
 systemstat = "0.1.4"
-rustwide = { version = "0.13.1", features = ["unstable", "unstable-toolchain-ci"] }
+rustwide = { version = "0.14.0", features = ["unstable", "unstable-toolchain-ci"] }
 percent-encoding = "2.1.0"
 remove_dir_all = "0.5.2"
 ctrlc = "3.1.3"

Cargo.toml

@@ -70,7 +70,9 @@ pub fn run_ex<DB: WriteResults + Sync>(
     // proceed slightly faster than it would otherwise.
     for tc in workspace.installed_toolchains()? {
         // But don't uninstall it if we're going to reinstall in a couple lines.
-        if !ex.toolchains.iter().any(|t| tc == t.source) {
+        // And don't uninstall stable, since that is mainly used for
+        // installing tools.
+        if !tc.is_needed_by_rustwide() && !ex.toolchains.iter().any(|t| tc == t.source) {
             tc.uninstall(workspace)?;
         }
     }