Don't trigger an update when looking for suggestions.

src/cargo/core/compiler/future_incompat.rs

@@ -245,12 +245,7 @@ fn render_suggestions(
     let mut sources: HashMap<_, _> = source_ids
         .into_iter()
         .filter_map(|sid| {
-            let unlocked = sid.clone().with_precise(None);
-            let mut source = map.load(unlocked, &HashSet::new()).ok()?;
-            // Ignore errors updating.
-            if let Err(e) = source.update() {
-                log::debug!("failed to update source: {:?}", e);
-            }
+            let source = map.load(sid, &HashSet::new()).ok()?;
             Some((sid, source))
         })
         .collect();

tests/testsuite/future_incompat_report.rs

@@ -325,6 +325,15 @@ fn suggestions_for_updates() {
         .file("src/lib.rs", "")
         .publish();
 
+    // This is a hack to force cargo to update the index. Cargo can't do this
+    // automatically because doing a network update on every build would be a
+    // bad idea. Under normal circumstances, we'll hope the user has done
+    // something else along the way to trigger an update (building some other
+    // project or something). This could use some more consideration of how to
+    // handle this better (maybe only trigger an update if it hasn't updated
+    // in a long while?).
+    p.cargo("update -p without_updates").run();
+
     p.cargo("check -Zfuture-incompat-report")
         .masquerade_as_nightly_cargo()
         .with_stderr_contains("[..]cargo report future-incompatibilities --id 1[..]")