fix: Do not downgrade with update --breaking if VersionReq is prerelease

Author: linyihai

src/cargo/util/toml_mut/upgrade.rs

@@ -18,9 +18,14 @@ pub(crate) fn upgrade_requirement(
         let comparators: CargoResult<Vec<_>> = raw_req
             .comparators
             .into_iter()
+            // Don't downgrade if pre-release was used, see rust-lang/cargo#14178 and rust-lang/cargo#13290.
+            .filter(|p| p.pre.is_empty() || matches_greater(p, version))
             .map(|p| set_comparator(p, version))
             .collect();
-        let comparators = comparators?;
+        let comparators: Vec<_> = comparators?;
+        if comparators.is_empty() {
+            return Ok(None);
+        }
         let new_req = semver::VersionReq { comparators };
         let mut new_req_text = new_req.to_string();
         if new_req_text.starts_with('^') && !req.starts_with('^') {
@@ -74,6 +79,33 @@ fn set_comparator(
     }
 }
 
+// See https://github.com/dtolnay/semver/blob/master/src/eval.rs#L64
+fn matches_greater(cmp: &semver::Comparator, ver: &semver::Version) -> bool {
+    if ver.major != cmp.major {
+        return ver.major > cmp.major;
+    }
+
+    match cmp.minor {
+        None => return false,
+        Some(minor) => {
+            if ver.minor != minor {
+                return ver.minor > minor;
+            }
+        }
+    }
+
+    match cmp.patch {
+        None => return false,
+        Some(patch) => {
+            if ver.patch != patch {
+                return ver.patch > patch;
+            }
+        }
+    }
+
+    ver.pre > cmp.pre
+}
+
 fn assign_partial_req(
     version: &semver::Version,
     mut pred: semver::Comparator,
@@ -219,8 +251,15 @@ mod test {
         }
 
         #[test]
-        fn caret_prerelease() {
-            assert_req_bump("1.7.0", "2.0.0-beta.21", "1.7.0");
+        fn greater_prerelease() {
+            assert_req_bump("1.7.0", "2.0.0-beta.21", None);
+            assert_req_bump("1.7.0", "=2.0.0-beta.21", None);
+            assert_req_bump("1.7.0", "~2.0.0-beta.21", None);
+            assert_req_bump("2.0.0-beta.20", "2.0.0-beta.21", None);
+            assert_req_bump("2.0.0-beta.21", "2.0.0-beta.21", None);
+            assert_req_bump("2.0.0-beta.22", "2.0.0-beta.21", "2.0.0-beta.22");
+            assert_req_bump("2.0.0", "2.0.0-beta.21", "2.0.0");
+            assert_req_bump("3.0.0", "2.0.0-beta.21", "3.0.0");
         }
     }
 }

tests/testsuite/update.rs

@@ -2640,15 +2640,14 @@ fn update_breaking_pre_release() {
 
     p.cargo("generate-lockfile").run();
 
+    // The purpose of this test is
+    // to demonstrate that `update --breaking` will not try to downgrade to the latest stable version (1.7.0),
+    // but will rather keep the latest pre-release (2.0.0-beta.21).
     Package::new("bar", "1.7.0").publish();
-
     p.cargo("update -Zunstable-options --breaking bar")
         .masquerade_as_nightly_cargo(&["update-breaking"])
         .with_stderr_data(str![[r#"
 [UPDATING] `dummy-registry` index
-[UPGRADING] bar ^2.0.0-beta.21 -> ^1.7.0
-[LOCKING] 1 package to latest compatible version
-[DOWNGRADING] bar v2.0.0-beta.21 -> v1.7.0
 
 "#]])
         .run();