diff --git a/src/bootstrap/dist.rs b/src/bootstrap/dist.rs
index 500d576..076bcd8 100644
--- a/src/bootstrap/dist.rs
+++ b/src/bootstrap/dist.rs
@@ -2000,6 +2000,8 @@
     }
 
     fn run(self, builder: &Builder<'_>) {
+        // This gets called by `promote-release`
+        // (https://github.com/rust-lang/rust-central-station/tree/master/promote-release).
         let mut cmd = builder.tool_cmd(Tool::BuildManifest);
         if builder.config.dry_run {
             return;
@@ -2010,10 +2012,14 @@
         let addr = builder.config.dist_upload_addr.as_ref().unwrap_or_else(|| {
             panic!("\n\nfailed to specify `dist.upload-addr` in `config.toml`\n\n")
         });
-        let file = builder.config.dist_gpg_password_file.as_ref().unwrap_or_else(|| {
-            panic!("\n\nfailed to specify `dist.gpg-password-file` in `config.toml`\n\n")
-        });
-        let pass = t!(fs::read_to_string(&file));
+        let pass = if env::var("BUILD_MANIFEST_DISABLE_SIGNING").is_err() {
+            let file = builder.config.dist_gpg_password_file.as_ref().unwrap_or_else(|| {
+                panic!("\n\nfailed to specify `dist.gpg-password-file` in `config.toml`\n\n")
+            });
+            t!(fs::read_to_string(&file))
+        } else {
+            String::new()
+        };
 
         let today = output(Command::new("date").arg("+%Y-%m-%d"));
 
diff --git a/src/tools/build-manifest/src/main.rs b/src/tools/build-manifest/src/main.rs
index 9ffa939..eab23f3 100644
--- a/src/tools/build-manifest/src/main.rs
+++ b/src/tools/build-manifest/src/main.rs
@@ -1,3 +1,9 @@
+//! Build a dist manifest, hash and sign everything.
+//! This gets called by `promote-release`
+//! (https://github.com/rust-lang/rust-central-station/tree/master/promote-release)
+//! via `x.py dist hash-and-sign`; the cmdline arguments are set up
+//! by rustbuild (in `src/bootstrap/dist.rs`).
+
 use toml;
 use serde::Serialize;
 
@@ -270,6 +276,7 @@
     // Do not ask for a passphrase while manually testing
     let mut passphrase = String::new();
     if should_sign {
+        // `x.py` passes the passphrase via stdin.
         t!(io::stdin().read_to_string(&mut passphrase));
     }
 
@@ -362,6 +369,7 @@
         }
     }
 
+    /// Hash all files, compute their signatures, and collect the hashes in `self.digests`.
     fn digest_and_sign(&mut self) {
         for file in t!(self.input.read_dir()).map(|e| t!(e).path()) {
             let filename = file.file_name().unwrap().to_str().unwrap();
@@ -532,19 +540,20 @@
             .as_ref()
             .cloned()
             .map(|version| (version, true))
-            .unwrap_or_default();
+            .unwrap_or_default(); // `is_present` defaults to `false` here.
 
-        // miri needs to build std with xargo, which doesn't allow stable/beta:
-        // <https://github.com/japaric/xargo/pull/204#issuecomment-374888868>
+        // Miri is nightly-only; never ship it for other trains.
         if pkgname == "miri-preview" && self.rust_release != "nightly" {
-            is_present = false; // ignore it
+            is_present = false; // Pretend the component is entirely missing.
         }
 
         let targets = targets.iter().map(|name| {
             if is_present {
+                // The component generally exists, but it might still be missing for this target.
                 let filename = self.filename(pkgname, name);
                 let digest = match self.digests.remove(&filename) {
                     Some(digest) => digest,
+                    // This component does not exist for this target -- skip it.
                     None => return (name.to_string(), Target::unavailable()),
                 };
                 let xz_filename = filename.replace(".tar.gz", ".tar.xz");