diff --git a/src/bootstrap/channel.rs b/src/bootstrap/channel.rs
index 4e3f3a00b15..e412dd9e3e6 100644
--- a/src/bootstrap/channel.rs
+++ b/src/bootstrap/channel.rs
@@ -26,11 +26,6 @@
 // The version number
 pub const CFG_RELEASE_NUM: &str = "1.25.0";
 
-// 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 = ".1";
-
 pub struct GitInfo {
     inner: Option<Info>,
 }
diff --git a/src/bootstrap/dist.rs b/src/bootstrap/dist.rs
index 7f0613aabe6..224b31ef268 100644
--- a/src/bootstrap/dist.rs
+++ b/src/bootstrap/dist.rs
@@ -1652,7 +1652,6 @@ 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 --git a/src/bootstrap/lib.rs b/src/bootstrap/lib.rs
index 948bf29bbac..3738828a4ba 100644
--- a/src/bootstrap/lib.rs
+++ b/src/bootstrap/lib.rs
@@ -134,7 +134,7 @@
 #[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 @@ pub fn new(config: Config) -> Build {
             is_sudo,
             ci_env: CiEnv::current(),
             delayed_failures: RefCell::new(Vec::new()),
+            prerelease_version: Cell::new(None),
         }
     }
 
@@ -774,12 +776,59 @@ fn openssl_install_dir(&self, target: Interned<String>) -> Option<PathBuf> {
     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")
+                .current_dir(&self.src)
+        );
+        let beta = beta.trim().split_whitespace().next().unwrap();
+        let master = output(
+            Command::new("git")
+                .arg("ls-remote")
+                .arg("origin")
+                .arg("master")
+                .current_dir(&self.src)
+        );
+        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)
+                .current_dir(&self.src),
+        );
+        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))
+                .current_dir(&self.src),
+        );
+        let n = count.trim().parse().unwrap();
+        self.prerelease_version.set(Some(n));
+        n
+    }
+
     /// Returns the value of `release` above for Rust itself.
     fn rust_release(&self) -> String {
         self.release(channel::CFG_RELEASE_NUM)
diff --git a/src/ci/init_repo.sh b/src/ci/init_repo.sh
index e073a3d99c1..14a1906ff42 100755
--- a/src/ci/init_repo.sh
+++ b/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