perf(config): avoid full manifest parsing for release hints

src/config.rs

@@ -1122,8 +1122,8 @@ impl<'a> Cfg<'a> {
             Err(e) => return Err(anyhow!(e).context("while looking for local stable toolchain")),
         };
         let release_date_str = match stable.get_manifestation() {
-            Ok(manifestation) => match manifestation.load_manifest() {
-                Ok(Some(manifest)) => manifest.date,
+            Ok(manifestation) => match manifestation.load_manifest_date() {
+                Ok(Some(date)) => date,
                 Ok(None) | Err(_) => FALLBACK_RELEASE_DATE.to_owned(),
             },
             Err(_) => FALLBACK_RELEASE_DATE.to_owned(),

src/dist/manifest.rs

@@ -332,6 +332,10 @@ impl Manifest {
         Ok(manifest)
     }
 
+    pub(crate) fn parse_date(header: &str) -> anyhow::Result<String> {
+        Ok(toml::from_str::<Self>(header)?.date)
+    }
+
     pub fn stringify(self) -> anyhow::Result<String> {
         Ok(toml::to_string(&self)?)
     }

src/dist/manifestation.rs

@@ -6,6 +6,8 @@ mod tests;
 
 use std::{
     collections::VecDeque,
+    fs,
+    io::{BufRead, BufReader, Read},
     path::Path,
     pin::Pin,
     sync::Arc,
@@ -36,8 +38,6 @@ use crate::{
     utils,
 };
 
-pub(crate) const CONFIG_FILE: &str = "multirust-config.toml";
-
 #[derive(Debug)]
 pub struct Manifestation {
     installation: Components,
@@ -378,6 +378,26 @@ impl Manifestation {
         }
     }
 
+    #[tracing::instrument(level = "trace")]
+    pub(crate) fn load_manifest_date(&self) -> anyhow::Result<Option<String>> {
+        let prefix = self.installation.prefix();
+        if let Some(manifest_path) = prefix.dist_manifest() {
+            let date = fs::File::open(&manifest_path)
+                .map_err(anyhow::Error::from)
+                .and_then(read_manifest_date);
+            match date {
+                Ok(date) => return Ok(Some(date)),
+                Err(error) => debug!(
+                    path = %manifest_path.display(),
+                    %error,
+                    "failed to read manifest date from header; falling back to full manifest parsing"
+                ),
+            }
+        }
+
+        Ok(self.load_manifest()?.map(|manifest| manifest.date))
+    }
+
     /// Installation using the legacy v1 manifest format
     pub(crate) async fn update_v1(
         &self,
@@ -471,6 +491,26 @@ impl Manifestation {
     }
 }
 
+/// Parses the manifest date from the root-level fields before the first line
+/// whose first non-whitespace character is `[`, reading at most
+/// `MANIFEST_HEADER_SIZE_LIMIT` bytes.
+fn read_manifest_date(reader: impl Read) -> anyhow::Result<String> {
+    let mut reader = BufReader::new(reader.take(MANIFEST_HEADER_SIZE_LIMIT));
+    let mut header = String::new();
+    loop {
+        let line_start = header.len();
+        if reader.read_line(&mut header)? == 0 {
+            break;
+        }
+        if header[line_start..].trim_start().starts_with('[') {
+            header.truncate(line_start);
+            break;
+        }
+    }
+
+    Manifest::parse_date(&header)
+}
+
 struct InstallEvents<'a, F> {
     manifestation: Arc<Manifestation>,
     components: vec::IntoIter<ComponentBinary<'a>>,
@@ -839,5 +879,12 @@ impl ComponentInstall {
     }
 }
 
+pub(crate) const CONFIG_FILE: &str = "multirust-config.toml";
+
+// The serialized manifest currently places `date` within the first 64 bytes.
+// Leave some headroom for reordered or additional root keys; if the header
+// exceeds this limit, loading falls back to parsing the full manifest.
+const MANIFEST_HEADER_SIZE_LIMIT: u64 = 128;
+
 #[cfg(feature = "test")]
 pub const CHECKPOINT_UPDATE_BEFORE_METADATA: &str = "update-before-metadata";

src/dist/manifestation/tests.rs

@@ -14,6 +14,7 @@ use std::{
 use anyhow::anyhow;
 use url::Url;
 
+use super::read_manifest_date;
 use crate::{
     config::Cfg,
     dist::{
@@ -38,6 +39,17 @@ use crate::{
 
 const SHA256_HASH_LEN: usize = 64;
 
+#[test]
+fn read_manifest_date_from_serialized_manifest() -> anyhow::Result<()> {
+    let serialized = Manifest::parse(include_str!(
+        "../manifest/tests/channel-rust-nightly-example.toml"
+    ))?
+    .stringify()?;
+
+    assert_eq!(read_manifest_date(serialized.as_bytes())?, "2015-10-10");
+    Ok(())
+}
+
 // Creates a mock dist server populated with some test data
 fn create_mock_dist_server(
     path: &Path,