support rust.channel = "auto-detect"

config.example.toml

@@ -608,11 +608,12 @@
 
 # The "channel" for the Rust build to produce. The stable/beta channels only
 # allow using stable features, whereas the nightly and dev channels allow using
-# nightly features
+# nightly features.
 #
-# If using tarball sources, default value for `channel` is taken from the `src/ci/channel` file;
-# otherwise, it's "dev".
-#channel = if "is a tarball source" { content of `src/ci/channel` file } else { "dev" }
+# You can set the channel to "auto-detect" to load the channel name from `src/ci/channel`.
+#
+# If using tarball sources, default value is "auto-detect", otherwise, it's "dev".
+#channel = if "is a tarball source" { "auto-detect" } else { "dev" }
 
 # A descriptive string to be appended to `rustc --version` output, which is
 # also used in places like debuginfo `DW_AT_producer`. This may be useful for

src/bootstrap/src/core/config/config.rs

@@ -1777,7 +1777,11 @@ impl Config {
 
         let is_user_configured_rust_channel =
             if let Some(channel) = toml.rust.as_ref().and_then(|r| r.channel.clone()) {
-                config.channel = channel;
+                if channel == "auto-detect" {
+                    config.channel = ci_channel.into();
+                } else {
+                    config.channel = channel;
+                }
                 true
             } else {
                 false

src/bootstrap/src/utils/change_tracker.rs

@@ -360,4 +360,9 @@ pub const CONFIG_CHANGE_HISTORY: &[ChangeInfo] = &[
         severity: ChangeSeverity::Info,
         summary: "Added `build.test-stage = 2` to 'tools' profile defaults",
     },
+    ChangeInfo {
+        change_id: 137220,
+        severity: ChangeSeverity::Info,
+        summary: "`rust.channel` now supports \"auto-detect\" to load the channel from `src/ci/channel`",
+    },
 ];