Auto merge of #10064 - arlosi:poll, r=Eh2406

Registry functions return Poll to enable parallel fetching of index data

Adds `Poll` as a return type for several registry functions to enable parallel fetching of crate metadata with a future http-based registry.

Work is scheduled by calling the `query` and related functions, then waited on with `block_until_ready`.

This PR is based on the draft PR started by eh2406 here [#8985](#8985).

r? `@Eh2406`
cc `@alexcrichton`
cc `@jonhoo`

Author: bors

crates/resolver-tests/src/lib.rs

@@ -7,6 +7,7 @@ use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};
 use std::fmt;
 use std::fmt::Write;
 use std::rc::Rc;
+use std::task::Poll;
 use std::time::Instant;
 
 use cargo::core::dependency::DepKind;
@@ -129,14 +130,14 @@ pub fn resolve_with_config_raw(
             dep: &Dependency,
             f: &mut dyn FnMut(Summary),
             fuzzy: bool,
-        ) -> CargoResult<()> {
+        ) -> Poll<CargoResult<()>> {
             for summary in self.list.iter() {
                 if fuzzy || dep.matches(summary) {
                     self.used.insert(summary.package_id());
                     f(summary.clone());
                 }
             }
-            Ok(())
+            Poll::Ready(Ok(()))
         }
 
         fn describe_source(&self, _src: SourceId) -> String {
@@ -146,6 +147,10 @@ pub fn resolve_with_config_raw(
         fn is_replaced(&self, _src: SourceId) -> bool {
             false
         }
+
+        fn block_until_ready(&mut self) -> CargoResult<()> {
+            Ok(())
+        }
     }
     impl<'a> Drop for MyRegistry<'a> {
         fn drop(&mut self) {

src/cargo/core/compiler/future_incompat.rs

@@ -9,6 +9,7 @@ use serde::{Deserialize, Serialize};
 use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};
 use std::fmt::Write as _;
 use std::io::{Read, Write};
+use std::task::Poll;
 
 pub const REPORT_PREAMBLE: &str = "\
 The following warnings were discovered during the build. These warnings are an
@@ -264,7 +265,7 @@ fn get_updates(ws: &Workspace<'_>, package_ids: &BTreeSet<PackageId>) -> Option<
     let _lock = ws.config().acquire_package_cache_lock().ok()?;
     // Create a set of updated registry sources.
     let map = SourceConfigMap::new(ws.config()).ok()?;
-    let package_ids: BTreeSet<_> = package_ids
+    let mut package_ids: BTreeSet<_> = package_ids
         .iter()
         .filter(|pkg_id| pkg_id.source_id().is_registry())
         .collect();
@@ -279,15 +280,35 @@ fn get_updates(ws: &Workspace<'_>, package_ids: &BTreeSet<PackageId>) -> Option<
             Some((sid, source))
         })
         .collect();
-    // Query the sources for new versions.
+
+    // Query the sources for new versions, mapping `package_ids` into `summaries`.
+    let mut summaries = Vec::new();
+    while !package_ids.is_empty() {
+        package_ids.retain(|&pkg_id| {
+            let source = match sources.get_mut(&pkg_id.source_id()) {
+                Some(s) => s,
+                None => return false,
+            };
+            let dep = match Dependency::parse(pkg_id.name(), None, pkg_id.source_id()) {
+                Ok(dep) => dep,
+                Err(_) => return false,
+            };
+            match source.query_vec(&dep) {
+                Poll::Ready(Ok(sum)) => {
+                    summaries.push((pkg_id, sum));
+                    false
+                }
+                Poll::Ready(Err(_)) => false,
+                Poll::Pending => true,
+            }
+        });
+        for (_, source) in sources.iter_mut() {
+            source.block_until_ready().ok()?;
+        }
+    }
+
     let mut updates = String::new();
-    for pkg_id in package_ids {
-        let source = match sources.get_mut(&pkg_id.source_id()) {
-            Some(s) => s,
-            None => continue,
-        };
-        let dep = Dependency::parse(pkg_id.name(), None, pkg_id.source_id()).ok()?;
-        let summaries = source.query_vec(&dep).ok()?;
+    for (pkg_id, summaries) in summaries {
         let mut updated_versions: Vec<_> = summaries
             .iter()
             .map(|summary| summary.version())