Add a warning to cargo clean --dry-run

This makes it more consistent with other `--dry-run` commands, and
makes it clearer to the user that cargo did not do anything.

src/cargo/ops/cargo_clean.rs

@@ -417,7 +417,13 @@ impl<'cfg> CleanContext<'cfg> {
         };
         self.config
             .shell()
-            .status(status, format!("{file_count}{byte_count}"))
+            .status(status, format!("{file_count}{byte_count}"))?;
+        if self.dry_run {
+            self.config
+                .shell()
+                .warn("no files deleted due to --dry-run")?;
+        }
+        Ok(())
     }
 
     /// Deletes all of the given paths, showing a progress bar as it proceeds.

tests/testsuite/clean.rs

@@ -817,13 +817,19 @@ fn clean_dry_run() {
     // Start with no files.
     p.cargo("clean --dry-run")
         .with_stdout("")
-        .with_stderr("[SUMMARY] 0 files")
+        .with_stderr(
+            "[SUMMARY] 0 files\n\
+             [WARNING] no files deleted due to --dry-run",
+        )
         .run();
     p.cargo("check").run();
     let before = ls_r();
     p.cargo("clean --dry-run")
         .with_stdout("[CWD]/target")
-        .with_stderr("[SUMMARY] [..] files, [..] total")
+        .with_stderr(
+            "[SUMMARY] [..] files, [..] total\n\
+             [WARNING] no files deleted due to --dry-run",
+        )
         .run();
     // Verify it didn't delete anything.
     let after = ls_r();
@@ -833,7 +839,10 @@ fn clean_dry_run() {
     // Verify the verbose output.
     p.cargo("clean --dry-run -v")
         .with_stdout_unordered(expected)
-        .with_stderr("[SUMMARY] [..] files, [..] total")
+        .with_stderr(
+            "[SUMMARY] [..] files, [..] total\n\
+             [WARNING] no files deleted due to --dry-run",
+        )
         .run();
 }