fix(resolve): Improve multi-MSRV workspaces

We do this by resolving for a package version that is compatible
with the most number of MSRVs within a workspace.

If a version requirement is just right, every package will get the
highest MSRV-compatible dependency.

If its too high, packages will get MSRV-incompatible dependency
versions.
This will happen regardless of what we do due to the nature of
`"fallback"`.

If its too low, packages with higher MSRVs will get older-than-necessary
dependency versions.
This is similar to the "some with and without MSRV" workspaces.
When locking dependencies, we do report to users when newer MSRV/SemVer
compatible dependencies are available to help guide them to upgrading if
desired.

Fixes #14414

src/cargo/core/resolver/version_prefs.rs

@@ -21,7 +21,7 @@ pub struct VersionPreferences {
     try_to_use: HashSet<PackageId>,
     prefer_patch_deps: HashMap<InternedString, HashSet<Dependency>>,
     version_ordering: VersionOrdering,
-    max_rust_version: Option<PartialVersion>,
+    rust_versions: Vec<PartialVersion>,
 }
 
 #[derive(Copy, Clone, Default, PartialEq, Eq, Hash, Debug)]
@@ -49,8 +49,8 @@ impl VersionPreferences {
         self.version_ordering = ordering;
     }
 
-    pub fn max_rust_version(&mut self, ver: Option<PartialVersion>) {
-        self.max_rust_version = ver;
+    pub fn rust_versions(&mut self, vers: Vec<PartialVersion>) {
+        self.rust_versions = vers;
     }
 
     /// Sort (and filter) the given vector of summaries in-place
@@ -59,7 +59,7 @@ impl VersionPreferences {
     ///
     /// Sort order:
     /// 1. Preferred packages
-    /// 2. [`VersionPreferences::max_rust_version`]
+    /// 2. Most compatible [`VersionPreferences::rust_versions`]
     /// 3. `first_version`, falling back to [`VersionPreferences::version_ordering`] when `None`
     ///
     /// Filtering:
@@ -85,20 +85,11 @@ impl VersionPreferences {
                 return previous_cmp;
             }
 
-            if let Some(max_rust_version) = &self.max_rust_version {
-                let a_is_compat = a
-                    .rust_version()
-                    .map(|a| a.is_compatible_with(max_rust_version))
-                    .unwrap_or(true);
-                let b_is_compat = b
-                    .rust_version()
-                    .map(|b| b.is_compatible_with(max_rust_version))
-                    .unwrap_or(true);
-                match (a_is_compat, b_is_compat) {
-                    (true, true) => {}   // fallback
-                    (false, false) => {} // fallback
-                    (true, false) => return Ordering::Less,
-                    (false, true) => return Ordering::Greater,
+            if !self.rust_versions.is_empty() {
+                let a_compat_count = self.msrv_compat_count(a);
+                let b_compat_count = self.msrv_compat_count(b);
+                if b_compat_count != a_compat_count {
+                    return b_compat_count.cmp(&a_compat_count);
                 }
             }
 
@@ -112,6 +103,17 @@ impl VersionPreferences {
             let _ = summaries.split_off(1);
         }
     }
+
+    fn msrv_compat_count(&self, summary: &Summary) -> usize {
+        let Some(rust_version) = summary.rust_version() else {
+            return self.rust_versions.len();
+        };
+
+        self.rust_versions
+            .iter()
+            .filter(|max| rust_version.is_compatible_with(max))
+            .count()
+    }
 }
 
 #[cfg(test)]
@@ -238,7 +240,7 @@ mod test {
     #[test]
     fn test_single_rust_version() {
         let mut vp = VersionPreferences::default();
-        vp.max_rust_version(Some("1.50".parse().unwrap()));
+        vp.rust_versions(vec!["1.50".parse().unwrap()]);
 
         let mut summaries = vec![
             summ("foo", "1.2.4", None),
@@ -270,7 +272,7 @@ mod test {
     #[test]
     fn test_multiple_rust_versions() {
         let mut vp = VersionPreferences::default();
-        vp.max_rust_version(Some("1.45".parse().unwrap()));
+        vp.rust_versions(vec!["1.45".parse().unwrap(), "1.55".parse().unwrap()]);
 
         let mut summaries = vec![
             summ("foo", "1.2.4", None),
@@ -286,7 +288,7 @@ mod test {
         vp.sort_summaries(&mut summaries, None);
         assert_eq!(
             describe(&summaries),
-            "foo/1.2.4, foo/1.2.2, foo/1.2.0, foo/1.1.0, foo/1.0.9, foo/1.2.3, foo/1.2.1"
+            "foo/1.2.4, foo/1.2.2, foo/1.2.0, foo/1.1.0, foo/1.0.9, foo/1.2.1, foo/1.2.3"
                 .to_string()
         );
 

src/cargo/ops/resolve.rs

@@ -79,6 +79,7 @@ use crate::util::errors::CargoResult;
 use crate::util::CanonicalUrl;
 use anyhow::Context as _;
 use cargo_util::paths;
+use cargo_util_schemas::core::PartialVersion;
 use std::collections::{HashMap, HashSet};
 use tracing::{debug, trace};
 
@@ -357,14 +358,16 @@ pub fn resolve_with_previous<'gctx>(
         version_prefs.version_ordering(VersionOrdering::MinimumVersionsFirst)
     }
     if ws.resolve_honors_rust_version() {
-        let rust_version = if let Some(ver) = ws.lowest_rust_version() {
-            ver.clone().into_partial()
-        } else {
+        let mut rust_versions: Vec<_> = ws
+            .members()
+            .filter_map(|p| p.rust_version().map(|rv| rv.as_partial().clone()))
+            .collect();
+        if rust_versions.is_empty() {
             let rustc = ws.gctx().load_global_rustc(Some(ws))?;
-            let rustc_version = rustc.version.clone().into();
-            rustc_version
-        };
-        version_prefs.max_rust_version(Some(rust_version));
+            let rust_version: PartialVersion = rustc.version.clone().into();
+            rust_versions.push(rust_version);
+        }
+        version_prefs.rust_versions(rust_versions);
     }
 
     let avoid_patch_ids = if register_patches {

src/doc/src/reference/unstable.md

@@ -348,7 +348,9 @@ This was stabilized in 1.79 in [#13608](https://github.com/rust-lang/cargo/pull/
 - `package.edition = "2024"` (only in workspace root)
 
 The resolver will prefer dependencies with a `package.rust-version` that is the same or older than your project's MSRV.
-Your project's MSRV is determined by taking the lowest `package.rust-version` set among your workspace members.
+As the resolver is unable to determine which workspace members will eventually
+depend on a package when it is being selected, we prioritize versions based on
+how many workspace member MSRVs they are compatible with.
 If there is no MSRV set then your toolchain version will be used, allowing it to pick up the toolchain version from pinned in rustup (e.g. `rust-toolchain.toml`).
 
 #### `resolver.incompatible-rust-versions`

tests/testsuite/rust_version.rs

@@ -510,7 +510,7 @@ higher v0.0.1 ([ROOT]/foo)
         .with_stderr_data(str![[r#"
 [UPDATING] `dummy-registry` index
 [LOCKING] 6 packages to latest Rust 1.50.0 compatible versions
-[ADDING] higher-newer-and-older v1.65.0 (requires Rust 1.65.0)
+[ADDING] higher-newer-and-older v1.55.0 (available: v1.65.0, requires Rust 1.65.0)
 [ADDING] higher-only-newer v1.65.0 (requires Rust 1.65.0)
 [ADDING] lower-newer-and-older v1.45.0 (available: v1.55.0, requires Rust 1.55.0)
 [ADDING] lower-only-newer v1.65.0 (requires Rust 1.65.0)
@@ -522,7 +522,7 @@ higher v0.0.1 ([ROOT]/foo)
     p.cargo("tree")
         .with_stdout_data(str![[r#"
 higher v0.0.1 ([ROOT]/foo)
-├── higher-newer-and-older v1.65.0
+├── higher-newer-and-older v1.55.0
 ├── higher-only-newer v1.65.0
 ├── shared-newer-and-older v1.45.0
 └── shared-only-newer v1.65.0