[beta] Automaticaly calculate beta prerelease numbers

This commit automatically calculates the beta prerelease number meaning we'll no
longer need to manually change the beta version. Instead beta will automatically
deploy any time a backport is merged, ensuring that backports are released for
testing ASAP. More details about this can be found on the internal [forums]

The only bit of trickiness here was that on CI we do shallow clones by default
but the git history probing here requires some more information. Do cope with
that this commit chooses the strategy of converting the repository to a full
clone via the `--unshallow` flag to `git`. That way this should work for local
developers as well as CI changes.

Note that this commit is coming first to the beta branch to test it, and if
successful we can go back and land it on master.

[forums]: https://internals.rust-lang.org/t/tweaking-how-betas-are-produced/6526

Author: alexcrichton

Diff for: src/bootstrap/channel.rs

@@ -26,11 +26,6 @@ use config::Config;
 // The version number
 pub const CFG_RELEASE_NUM: &str = "1.24.0";
 
-// An optional number to put after the label, e.g. '.2' -> '-beta.2'
-// Be sure to make this starts with a dot to conform to semver pre-release
-// versions (section 9)
-pub const CFG_PRERELEASE_VERSION: &str = ".3";
-
 pub struct GitInfo {
     inner: Option<Info>,
 }

Diff for: src/bootstrap/dist.rs

@@ -1650,7 +1650,6 @@ fn add_env(build: &Build, cmd: &mut Command, target: Interned<String>) {
     cmd.env("CFG_RELEASE_INFO", build.rust_version())
        .env("CFG_RELEASE_NUM", channel::CFG_RELEASE_NUM)
        .env("CFG_RELEASE", build.rust_release())
-       .env("CFG_PRERELEASE_VERSION", channel::CFG_PRERELEASE_VERSION)
        .env("CFG_VER_MAJOR", parts.next().unwrap())
        .env("CFG_VER_MINOR", parts.next().unwrap())
        .env("CFG_VER_PATCH", parts.next().unwrap())

Diff for: src/bootstrap/lib.rs

@@ -134,7 +134,7 @@ extern crate toml;
 #[cfg(unix)]
 extern crate libc;
 
-use std::cell::RefCell;
+use std::cell::{RefCell, Cell};
 use std::collections::{HashSet, HashMap};
 use std::env;
 use std::fs::{self, File};
@@ -250,6 +250,7 @@ pub struct Build {
     is_sudo: bool,
     ci_env: CiEnv,
     delayed_failures: RefCell<Vec<String>>,
+    prerelease_version: Cell<Option<u32>>,
 }
 
 #[derive(Debug)]
@@ -335,6 +336,7 @@ impl Build {
             is_sudo,
             ci_env: CiEnv::current(),
             delayed_failures: RefCell::new(Vec::new()),
+            prerelease_version: Cell::new(None),
         }
     }
 
@@ -769,12 +771,55 @@ impl Build {
     fn release(&self, num: &str) -> String {
         match &self.config.channel[..] {
             "stable" => num.to_string(),
-            "beta" => format!("{}-beta{}", num, channel::CFG_PRERELEASE_VERSION),
+            "beta" => format!("{}-beta.{}", num, self.beta_prerelease_version()),
             "nightly" => format!("{}-nightly", num),
             _ => format!("{}-dev", num),
         }
     }
 
+    fn beta_prerelease_version(&self) -> u32 {
+        if let Some(s) = self.prerelease_version.get() {
+            return s
+        }
+
+        let beta = output(
+            Command::new("git")
+                .arg("ls-remote")
+                .arg("origin")
+                .arg("beta")
+        );
+        let beta = beta.trim().split_whitespace().next().unwrap();
+        let master = output(
+            Command::new("git")
+                .arg("ls-remote")
+                .arg("origin")
+                .arg("master")
+        );
+        let master = master.trim().split_whitespace().next().unwrap();
+
+        // Figure out where the current beta branch started.
+        let base = output(
+            Command::new("git")
+                .arg("merge-base")
+                .arg(beta)
+                .arg(master),
+        );
+        let base = base.trim();
+
+        // Next figure out how many merge commits happened since we branched off
+        // beta. That's our beta number!
+        let count = output(
+            Command::new("git")
+                .arg("rev-list")
+                .arg("--count")
+                .arg("--merges")
+                .arg(format!("{}...HEAD", base)),
+        );
+        let n = count.trim().parse().unwrap();
+        self.prerelease_version.set(Some(n));
+        return n
+    }
+
     /// Returns the value of `release` above for Rust itself.
     fn rust_release(&self) -> String {
         self.release(channel::CFG_RELEASE_NUM)

Diff for: src/ci/init_repo.sh

@@ -36,6 +36,12 @@ fi
 rm -rf "$CACHE_DIR"
 mkdir "$CACHE_DIR"
 
+# On the beta channel we'll be automatically calculating the prerelease version
+# via the git history, so unshallow our shallow clone from CI.
+if grep -q RUST_RELEASE_CHANNEL=beta src/ci/run.sh; then
+  git fetch origin --unshallow beta master
+fi
+
 travis_fold start update_cache
 travis_time_start