fix(resolver): Prefer MSRV, rather than ignore incompatible

This is another experiment for rust-lang#9930.

Comparing preferring over exclusively using MSRV compatible:

Benefits
- Better error messages
- `--ignore-rust-version` is implicitly sticky

Downsides
- Can't backtrack for MSRV compatible version
- Still requires workspace-wide MSRV (compared to our desired end state of declaring MSRV as yet another dependency)

This builds on rust-lang#12930

src/cargo/core/resolver/version_prefs.rs

@@ -58,10 +58,10 @@ impl VersionPreferences {
     ///
     /// Sort order:
     /// 1. Preferred packages
-    /// 2. `first_version`, falling back to [`VersionPreferences::version_ordering`] when `None`
+    /// 2. [`VersionPreferences::max_rust_version`]
+    /// 3. `first_version`, falling back to [`VersionPreferences::version_ordering`] when `None`
     ///
     /// Filtering:
-    /// - [`VersionPreferences::max_rust_version`]
     /// - `first_version`
     pub fn sort_summaries(
         &self,
@@ -76,9 +76,6 @@ impl VersionPreferences {
                     .map(|deps| deps.iter().any(|d| d.matches_id(*pkg_id)))
                     .unwrap_or(false)
         };
-        if self.max_rust_version.is_some() {
-            summaries.retain(|s| s.rust_version() <= self.max_rust_version.as_ref());
-        }
         summaries.sort_unstable_by(|a, b| {
             let prefer_a = should_prefer(&a.package_id());
             let prefer_b = should_prefer(&b.package_id());
@@ -87,6 +84,15 @@ impl VersionPreferences {
                 return previous_cmp;
             }
 
+            if self.max_rust_version.is_some() {
+                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();
+                if msrv_cmp != Ordering::Equal {
+                    return msrv_cmp;
+                }
+            }
+
             let cmp = a.version().cmp(b.version());
             match first_version.unwrap_or(self.version_ordering) {
                 VersionOrdering::MaximumVersionsFirst => cmp.reverse(),
@@ -236,14 +242,14 @@ mod test {
         vp.sort_summaries(&mut summaries, None);
         assert_eq!(
             describe(&summaries),
-            "foo/1.2.3, foo/1.1.0, foo/1.0.9".to_string()
+            "foo/1.2.3, foo/1.1.0, foo/1.0.9, foo/1.2.4".to_string()
         );
 
         vp.version_ordering(VersionOrdering::MinimumVersionsFirst);
         vp.sort_summaries(&mut summaries, None);
         assert_eq!(
             describe(&summaries),
-            "foo/1.0.9, foo/1.1.0, foo/1.2.3".to_string()
+            "foo/1.0.9, foo/1.1.0, foo/1.2.3, foo/1.2.4".to_string()
         );
     }
 }

tests/testsuite/cargo_add/rust_version_ignore/mod.rs

@@ -26,7 +26,7 @@ fn case() {
         .current_dir(cwd)
         .masquerade_as_nightly_cargo(&["msrv-policy"])
         .assert()
-        .code(101)
+        .code(0)
         .stdout_matches_path(curr_dir!().join("stdout.log"))
         .stderr_matches_path(curr_dir!().join("stderr.log"));
 

tests/testsuite/cargo_add/rust_version_ignore/stderr.log

@@ -1,7 +1,2 @@
     Updating `dummy-registry` index
       Adding rust-version-user v0.2.1 to dependencies.
-error: failed to select a version for the requirement `rust-version-user = "^0.2.1"`
-candidate versions found which didn't match: 0.2.1, 0.1.0
-location searched: `dummy-registry` index (which is replacing registry `crates-io`)
-required by package `cargo-list-test-fixture v0.0.0 ([ROOT]/case)`
-perhaps a crate was updated and forgotten to be re-vendored?

tests/testsuite/rust_version.rs

@@ -245,37 +245,13 @@ fn dependency_rust_version_newer_than_package() {
         .file("src/main.rs", "fn main(){}")
         .build();
 
-    p.cargo("check --ignore-rust-version")
+    p.cargo("check")
         .arg("-Zmsrv-policy")
         .masquerade_as_nightly_cargo(&["msrv-policy"])
-        // This shouldn't fail
-        .with_status(101)
-        .with_stderr(
-            "\
-[UPDATING] `dummy-registry` index
-[ERROR] failed to select a version for the requirement `bar = \"^1.0.0\"`
-candidate versions found which didn't match: 1.6.0
-location searched: `dummy-registry` index (which is replacing registry `crates-io`)
-required by package `foo v0.0.1 ([CWD])`
-perhaps a crate was updated and forgotten to be re-vendored?
-",
-        )
         .run();
-    p.cargo("check")
+    p.cargo("check --ignore-rust-version")
         .arg("-Zmsrv-policy")
         .masquerade_as_nightly_cargo(&["msrv-policy"])
-        .with_status(101)
-        // This should have a better error message
-        .with_stderr(
-            "\
-[UPDATING] `dummy-registry` index
-[ERROR] failed to select a version for the requirement `bar = \"^1.0.0\"`
-candidate versions found which didn't match: 1.6.0
-location searched: `dummy-registry` index (which is replacing registry `crates-io`)
-required by package `foo v0.0.1 ([CWD])`
-perhaps a crate was updated and forgotten to be re-vendored?
-",
-        )
         .run();
 }
 
@@ -369,8 +345,10 @@ fn dependency_rust_version_backtracking() {
             "\
 [UPDATING] `dummy-registry` index
 [DOWNLOADING] crates ...
-[DOWNLOADED] no-rust-version v2.1.0 (registry `dummy-registry`)
-[CHECKING] no-rust-version v2.1.0
+[DOWNLOADED] no-rust-version v2.2.0 (registry `dummy-registry`)
+[DOWNLOADED] has-rust-version v1.6.0 (registry `dummy-registry`)
+[CHECKING] has-rust-version v1.6.0
+[CHECKING] no-rust-version v2.2.0
 [CHECKING] [..]
 [FINISHED] [..]
 ",