fix(resolve): Improve multi-MSRV workspaces

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)]
@@ -236,9 +238,9 @@ mod test {
     }
 
     #[test]
-    fn test_max_rust_version() {
+    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),
@@ -267,6 +269,38 @@ mod test {
         );
     }
 
+    #[test]
+    fn test_multiple_rust_versions() {
+        let mut vp = VersionPreferences::default();
+        vp.rust_versions(vec!["1.45".parse().unwrap(), "1.55".parse().unwrap()]);
+
+        let mut summaries = vec![
+            summ("foo", "1.2.4", None),
+            summ("foo", "1.2.3", Some("1.60")),
+            summ("foo", "1.2.2", None),
+            summ("foo", "1.2.1", Some("1.50")),
+            summ("foo", "1.2.0", None),
+            summ("foo", "1.1.0", Some("1.40")),
+            summ("foo", "1.0.9", None),
+        ];
+
+        vp.version_ordering(VersionOrdering::MaximumVersionsFirst);
+        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/1.2.1, foo/1.2.3"
+                .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/1.2.0, foo/1.2.2, foo/1.2.4, foo/1.2.1, foo/1.2.3"
+                .to_string()
+        );
+    }
+
     #[test]
     fn test_empty_summaries() {
         let vp = VersionPreferences::default();

src/cargo/core/workspace.rs

@@ -664,7 +664,7 @@ impl<'gctx> Workspace<'gctx> {
 
     /// Get the lowest-common denominator `package.rust-version` within the workspace, if specified
     /// anywhere
-    pub fn rust_version(&self) -> Option<&RustVersion> {
+    pub fn lowest_rust_version(&self) -> Option<&RustVersion> {
         self.members().filter_map(|pkg| pkg.rust_version()).min()
     }
 

src/cargo/ops/cargo_update.rs

@@ -739,7 +739,7 @@ fn required_rust_version(ws: &Workspace<'_>) -> Option<PartialVersion> {
         return None;
     }
 
-    if let Some(ver) = ws.rust_version() {
+    if let Some(ver) = ws.lowest_rust_version() {
         Some(ver.clone().into_partial())
     } else {
         let rustc = ws.gctx().load_global_rustc(Some(ws)).ok()?;

src/cargo/ops/lockfile.rs

@@ -71,7 +71,7 @@ pub fn write_pkg_lockfile(ws: &Workspace<'_>, resolve: &mut Resolve) -> CargoRes
     // out lock file updates as they're otherwise already updated, and changes
     // which don't touch dependencies won't seemingly spuriously update the lock
     // file.
-    let default_version = ResolveVersion::with_rust_version(ws.rust_version());
+    let default_version = ResolveVersion::with_rust_version(ws.lowest_rust_version());
     let current_version = resolve.version();
     let next_lockfile_bump = ws.gctx().cli_unstable().next_lockfile_bump;
     tracing::debug!("lockfile - current: {current_version:?}, default: {default_version:?}");

src/cargo/ops/registry/info/mod.rs

@@ -217,6 +217,6 @@ fn try_get_msrv_from_nearest_manifest_or_ws(
     let rust_version = nearest_package.and_then(|p| p.rust_version().map(|v| v.as_partial()));
     // If the nearest manifest does not have a specific Rust version, try to get it from the workspace.
     rust_version
-        .or_else(|| ws.and_then(|ws| ws.rust_version().map(|v| v.as_partial())))
+        .or_else(|| ws.and_then(|ws| ws.lowest_rust_version().map(|v| v.as_partial())))
         .cloned()
 }

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.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 {
@@ -422,7 +425,7 @@ pub fn resolve_with_previous<'gctx>(
         &replace,
         registry,
         &version_prefs,
-        ResolveVersion::with_rust_version(ws.rust_version()),
+        ResolveVersion::with_rust_version(ws.lowest_rust_version()),
         Some(ws.gctx()),
     )?;
 

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

@@ -409,22 +409,36 @@ foo v0.0.1 ([ROOT]/foo)
 
 #[cargo_test]
 fn resolve_with_multiple_rust_versions() {
-    Package::new("only-newer", "1.6.0")
+    Package::new(&format!("shared-only-newer"), "1.65.0")
         .rust_version("1.65.0")
         .file("src/lib.rs", "fn other_stuff() {}")
         .publish();
-    Package::new("newer-and-older", "1.5.0")
-        .rust_version("1.45.0")
-        .file("src/lib.rs", "fn other_stuff() {}")
-        .publish();
-    Package::new("newer-and-older", "1.5.1")
-        .rust_version("1.55.0")
+    for ver in ["1.45.0", "1.55.0", "1.65.0"] {
+        Package::new(&format!("shared-newer-and-older"), ver)
+            .rust_version(ver)
+            .file("src/lib.rs", "fn other_stuff() {}")
+            .publish();
+    }
+    Package::new(&format!("lower-only-newer"), "1.65.0")
+        .rust_version("1.65.0")
         .file("src/lib.rs", "fn other_stuff() {}")
         .publish();
-    Package::new("newer-and-older", "1.6.0")
+    for ver in ["1.45.0", "1.55.0"] {
+        Package::new(&format!("lower-newer-and-older"), ver)
+            .rust_version(ver)
+            .file("src/lib.rs", "fn other_stuff() {}")
+            .publish();
+    }
+    Package::new(&format!("higher-only-newer"), "1.65.0")
         .rust_version("1.65.0")
         .file("src/lib.rs", "fn other_stuff() {}")
         .publish();
+    for ver in ["1.55.0", "1.65.0"] {
+        Package::new(&format!("higher-newer-and-older"), ver)
+            .rust_version(ver)
+            .file("src/lib.rs", "fn other_stuff() {}")
+            .publish();
+    }
 
     let p = project()
         .file(
@@ -441,8 +455,10 @@ fn resolve_with_multiple_rust_versions() {
             rust-version = "1.60.0"
 
             [dependencies]
-            only-newer = "1.0.0"
-            newer-and-older = "1.0.0"
+            higher-only-newer = "1"
+            higher-newer-and-older = "1"
+            shared-only-newer = "1"
+            shared-newer-and-older = "1"
         "#,
         )
         .file("src/main.rs", "fn main() {}")
@@ -457,8 +473,10 @@ fn resolve_with_multiple_rust_versions() {
             rust-version = "1.50.0"
 
             [dependencies]
-            only-newer = "1.0.0"
-            newer-and-older = "1.0.0"
+            lower-only-newer = "1"
+            lower-newer-and-older = "1"
+            shared-only-newer = "1"
+            shared-newer-and-older = "1"
         "#,
         )
         .file("lower/src/main.rs", "fn main() {}")
@@ -470,15 +488,17 @@ fn resolve_with_multiple_rust_versions() {
         .masquerade_as_nightly_cargo(&["msrv-policy"])
         .with_stderr_data(str![[r#"
 [UPDATING] `dummy-registry` index
-[LOCKING] 2 packages to latest compatible versions
+[LOCKING] 6 packages to latest compatible versions
 
 "#]])
         .run();
     p.cargo("tree")
         .with_stdout_data(str![[r#"
 higher v0.0.1 ([ROOT]/foo)
-├── newer-and-older v1.6.0
-└── only-newer v1.6.0
+├── higher-newer-and-older v1.65.0
+├── higher-only-newer v1.65.0
+├── shared-newer-and-older v1.65.0
+└── shared-only-newer v1.65.0
 
 "#]])
         .run();
@@ -489,17 +509,23 @@ higher v0.0.1 ([ROOT]/foo)
         .masquerade_as_nightly_cargo(&["msrv-policy"])
         .with_stderr_data(str![[r#"
 [UPDATING] `dummy-registry` index
-[LOCKING] 2 packages to latest Rust 1.50.0 compatible versions
-[ADDING] newer-and-older v1.5.0 (available: v1.6.0, requires Rust 1.65.0)
-[ADDING] only-newer v1.6.0 (requires Rust 1.65.0)
+[LOCKING] 6 packages to latest Rust 1.50.0 compatible versions
+[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)
+[ADDING] shared-newer-and-older v1.45.0 (available: v1.65.0, requires Rust 1.65.0)
+[ADDING] shared-only-newer v1.65.0 (requires Rust 1.65.0)
 
 "#]])
         .run();
     p.cargo("tree")
         .with_stdout_data(str![[r#"
 higher v0.0.1 ([ROOT]/foo)
-├── newer-and-older v1.5.0
-└── only-newer v1.6.0
+├── 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
 
 "#]])
         .run();