T66: cmd doubles the backslashes ending a dropped path, since a program's parser reads one before the closing quote as a quote, as C:\ ends, and a path holding !, which cmd expands inside quotes under /v:on, stops the drop

src/term/drop.rs

@@ -20,7 +20,7 @@ pub enum Quote {
     Posix,
     /// Single quotes, which expand nothing.
     PowerShell,
-    /// Double quotes, past which cmd still expands `%`.
+    /// Double quotes, past which cmd still expands `%` and, under `/v:on`, `!`.
     Cmd,
 }
 
@@ -55,10 +55,12 @@ pub fn typed(paths: &[PathBuf], quote: Quote) -> anyhow::Result<String> {
                 }
                 Quote::Cmd => {
                     anyhow::ensure!(
-                        !text.contains(['%', '"']),
-                        "{text:?} holds % or \", which cmd reads inside quotes"
+                        !text.contains(['%', '!', '"']),
+                        "{text:?} holds %, ! or \", which cmd reads inside quotes"
                     );
-                    out = format!("\"{text}\"");
+                    // A program reads a backslash before the closing quote as a quote.
+                    let trailing = text.len() - text.trim_end_matches('\\').len();
+                    out = format!("\"{text}{}\"", "\\".repeat(trailing));
                 }
             }
             out.push(' ');

src/term/drop/tests.rs

@@ -40,14 +40,22 @@ fn a_powershell_path_is_single_quoted_with_its_quotes_doubled() {
 }
 
 #[test]
-fn a_cmd_path_is_double_quoted_and_one_with_a_percent_types_nothing() {
+fn a_cmd_path_is_double_quoted_its_trailing_backslashes_doubled_and_one_cmd_expands_types_nothing()
+{
     let typed = |all: &[&str]| typed_as(Quote::Cmd, all);
     assert_eq!(
         typed(&["C:\\My Files\\a&b^c.txt", "D:\\c"]),
         "\"C:\\My Files\\a&b^c.txt\" \"D:\\c\" "
     );
-    let percent = super::typed(&[PathBuf::from("C:\\%PATH%.txt")], Quote::Cmd);
-    assert!(percent.is_err(), "{percent:?}");
+    assert_eq!(
+        typed(&["C:\\", "D:\\clip.mp4"]),
+        "\"C:\\\\\" \"D:\\clip.mp4\" ",
+        "a root's backslash"
+    );
+    for name in ["C:\\%PATH%.txt", "C:\\!DROP!.txt"] {
+        let refused = super::typed(&[PathBuf::from(name)], Quote::Cmd);
+        assert!(refused.is_err(), "{name}: {refused:?}");
+    }
 }
 
 #[test]