Abstract away an assertions details

This will make it easier to change the implementation in the future.

src/cargo/core/registry.rs

@@ -800,11 +800,9 @@ impl<'gctx> Registry for PackageRegistry<'gctx> {
 
     #[tracing::instrument(skip_all)]
     fn block_until_ready(&mut self) -> CargoResult<()> {
-        if cfg!(debug_assertions) {
+        // Ensure `shell` is not already in use,
-            // Force borrow to catch invalid borrows, regardless of which source is used and how it
+        // regardless of which source is used and how it happens to behave this time
-            // happens to behave this time
+        self.gctx.debug_assert_shell_not_borrowed();
-            self.gctx.shell().verbosity();
-        }
         for (source_id, source) in self.sources.sources_mut() {
             source
                 .block_until_ready()

src/cargo/util/context/mod.rs

@@ -412,6 +412,17 @@ impl GlobalContext {
         self.shell.borrow_mut()
     }
 
+    /// Assert [`Self::shell`] is not in use
+    ///
+    /// Testing might not identify bugs with two accesses to `shell` at once
+    /// due to conditional logic,
+    /// so place this outside of the conditions to catch these bugs in more situations.
+    pub fn debug_assert_shell_not_borrowed(&self) {
+        if cfg!(debug_assertions) {
+            self.shell().verbosity();
+        }
+    }
+
     /// Gets the path to the `rustdoc` executable.
     pub fn rustdoc(&self) -> CargoResult<&Path> {
         self.rustdoc

src/cargo/util/flock.rs

@@ -389,10 +389,9 @@ fn acquire(
     lock_try: &dyn Fn() -> Result<(), TryLockError>,
     lock_block: &dyn Fn() -> io::Result<()>,
 ) -> CargoResult<()> {
-    if cfg!(debug_assertions) {
+    // Ensure `shell` is not already in use,
-        // Force borrow to catch invalid borrows outside of contention situations
+    // regardless of whether we hit contention or not
-        gctx.shell().verbosity();
+    gctx.debug_assert_shell_not_borrowed();
-    }
     if try_acquire(path, lock_try)? {
         return Ok(());
     }