Don't downgrade on prerelease VersionReq when update with --breaking.

src/cargo/util/toml_mut/upgrade.rs

@@ -15,12 +15,16 @@ pub(crate) fn upgrade_requirement(
         // Empty matches everything, no-change.
         Ok(None)
     } else {
-        let comparators: CargoResult<Vec<_>> = raw_req
+        let comparators: Vec<_> = raw_req
             .comparators
             .into_iter()
+            // Don't downgrade if pre-release was used, see https://github.com/rust-lang/cargo/issues/14178 and https://github.com/rust-lang/cargo/issues/13290.
+            .filter(|p| p.pre.is_empty() || matches_greater(p, version))
             .map(|p| set_comparator(p, version))
-            .collect();
-        let comparators = comparators?;
+            .collect::<CargoResult<_>>()?;
+        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 +78,33 @@ fn set_comparator(
     }
 }
 
+// See https://github.com/dtolnay/semver/blob/69efd3cc770ead273a06ad1788477b3092996d29/src/eval.rs#L64-L88
+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,
@@ -217,5 +248,17 @@ mod test {
             assert_req_bump("1.1.1", "=1.0.0", "=1.1.1");
             assert_req_bump("2.0.0", "=1.0.0", "=2.0.0");
         }
+
+        #[test]
+        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

@@ -2619,3 +2619,94 @@ fn update_breaking_mixed_pinning_renaming() {
             "#]],
     );
 }
+
+#[cargo_test]
+fn update_breaking_pre_release_downgrade() {
+    Package::new("bar", "2.0.0-beta.21").publish();
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+        [package]
+        name  =  "foo"
+        version  =  "0.0.1"
+        edition  =  "2015"
+        authors  =  []
+
+        [dependencies]
+        bar = "2.0.0-beta.21"
+    "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    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
+
+"#]])
+        .run();
+}
+
+#[cargo_test]
+fn update_breaking_pre_release_upgrade() {
+    Package::new("bar", "2.0.0-beta.21").publish();
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+        [package]
+        name  =  "foo"
+        version  =  "0.0.1"
+        edition  =  "2015"
+        authors  =  []
+
+        [dependencies]
+        bar = "2.0.0-beta.21"
+    "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("generate-lockfile").run();
+
+    // TODO: `2.0.0-beta.21` can be upgraded to `2.0.0-beta.22`
+    Package::new("bar", "2.0.0-beta.22").publish();
+    p.cargo("update -Zunstable-options --breaking bar")
+        .masquerade_as_nightly_cargo(&["update-breaking"])
+        .with_stderr_data(str![[r#"
+[UPDATING] `dummy-registry` index
+
+"#]])
+        .run();
+    // TODO: `2.0.0-beta.21` can be upgraded to `2.0.0`
+    Package::new("bar", "2.0.0").publish();
+    p.cargo("update -Zunstable-options --breaking bar")
+        .masquerade_as_nightly_cargo(&["update-breaking"])
+        .with_stderr_data(str![[r#"
+[UPDATING] `dummy-registry` index
+
+"#]])
+        .run();
+
+    Package::new("bar", "3.0.0").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 -> ^3.0.0
+[LOCKING] 1 package to latest compatible version
+[UPDATING] bar v2.0.0-beta.21 -> v3.0.0
+
+"#]])
+        .run();
+}