From 3d68234e2ef7ff4a646a65b19a2fb96d8b6d14f1 Mon Sep 17 00:00:00 2001
From: Ed Page <eopage@gmail.com>
Date: Tue, 28 Nov 2023 14:14:03 -0600
Subject: [PATCH] fix(resolver): De-prioritize no-rust-version in MSRV resolver

This is a corner case without a good answer.
As such, this change leans on some happy-path entries existing and
preferring those.
---
 src/cargo/core/resolver/version_prefs.rs | 37 ++++++++++++++++++++++--
 1 file changed, 34 insertions(+), 3 deletions(-)

diff --git a/src/cargo/core/resolver/version_prefs.rs b/src/cargo/core/resolver/version_prefs.rs
index f621c62e9d55..a3fa8acf8914 100644
--- a/src/cargo/core/resolver/version_prefs.rs
+++ b/src/cargo/core/resolver/version_prefs.rs
@@ -84,7 +84,38 @@ impl VersionPreferences {
                 return previous_cmp;
             }
 
-            if self.max_rust_version.is_some() {
+            if let Some(max_rust_version) = &self.max_rust_version {
+                match (a.rust_version(), b.rust_version()) {
+                    // Fallback
+                    (None, None) => {}
+                    (Some(a), Some(b)) if a == b => {}
+                    // Primary comparison
+                    (Some(a), Some(b)) => {
+                        let a_is_compat = a <= max_rust_version;
+                        let b_is_compat = b <= max_rust_version;
+                        match (a_is_compat, b_is_compat) {
+                            (true, true) => {}   // fallback
+                            (false, false) => {} // fallback
+                            (true, false) => return Ordering::Less,
+                            (false, true) => return Ordering::Greater,
+                        }
+                    }
+                    // Prioritize `None` over incompatible
+                    (None, Some(b)) => {
+                        if b <= max_rust_version {
+                            return Ordering::Greater;
+                        } else {
+                            return Ordering::Less;
+                        }
+                    }
+                    (Some(a), None) => {
+                        if a <= max_rust_version {
+                            return Ordering::Less;
+                        } else {
+                            return Ordering::Greater;
+                        }
+                    }
+                }
                 let msrv_a = a.rust_version() <= self.max_rust_version.as_ref();
                 let msrv_b = b.rust_version() <= self.max_rust_version.as_ref();
                 let msrv_cmp = msrv_a.cmp(&msrv_b).reverse();
@@ -245,7 +276,7 @@ mod test {
         vp.sort_summaries(&mut summaries, None);
         assert_eq!(
             describe(&summaries),
-            "foo/1.2.4, foo/1.2.2, foo/1.2.1, foo/1.2.0, foo/1.1.0, foo/1.0.9, foo/1.2.3"
+            "foo/1.2.1, foo/1.1.0, foo/1.2.4, foo/1.2.2, foo/1.2.0, foo/1.0.9, foo/1.2.3"
                 .to_string()
         );
 
@@ -253,7 +284,7 @@ mod test {
         vp.sort_summaries(&mut summaries, None);
         assert_eq!(
             describe(&summaries),
-            "foo/1.0.9, foo/1.1.0, foo/1.2.0, foo/1.2.1, foo/1.2.2, foo/1.2.4, foo/1.2.3"
+            "foo/1.1.0, foo/1.2.1, foo/1.0.9, foo/1.2.0, foo/1.2.2, foo/1.2.4, foo/1.2.3"
                 .to_string()
         );
     }