feat(resolver): **Very** preliminary MSRV resolver support

benches/benchsuite/benches/resolve.rs

@@ -31,6 +31,7 @@ fn do_resolve<'cfg>(config: &'cfg Config, ws_root: &Path) -> ResolveInfo<'cfg> {
     let specs = pkgs.to_package_id_specs(&ws).unwrap();
     let has_dev_units = HasDevUnits::Yes;
     let force_all_targets = ForceAllTargets::No;
+    let max_rust_version = None;
     // Do an initial run to download anything necessary so that it does
     // not confuse criterion's warmup.
     let ws_resolve = cargo::ops::resolve_ws_with_opts(
@@ -41,6 +42,7 @@ fn do_resolve<'cfg>(config: &'cfg Config, ws_root: &Path) -> ResolveInfo<'cfg> {
         &specs,
         has_dev_units,
         force_all_targets,
+        max_rust_version,
     )
     .unwrap();
     ResolveInfo {
@@ -82,6 +84,7 @@ fn resolve_ws(c: &mut Criterion) {
                 force_all_targets,
                 ..
             } = lazy_info.get_or_insert_with(|| do_resolve(&config, &ws_root));
+            let max_rust_version = None;
             b.iter(|| {
                 cargo::ops::resolve_ws_with_opts(
                     ws,
@@ -91,6 +94,7 @@ fn resolve_ws(c: &mut Criterion) {
                     specs,
                     *has_dev_units,
                     *force_all_targets,
+                    max_rust_version,
                 )
                 .unwrap();
             })

crates/resolver-tests/src/lib.rs

@@ -188,13 +188,15 @@ pub fn resolve_with_config_raw(
     .unwrap();
     let opts = ResolveOpts::everything();
     let start = Instant::now();
+    let max_rust_version = None;
     let resolve = resolver::resolve(
         &[(summary, opts)],
         &[],
         &mut registry,
         &VersionPreferences::default(),
         Some(config),
         true,
+        max_rust_version,
     );
 
     // The largest test in our suite takes less then 30 sec.

src/cargo/core/compiler/standard_lib.rs

@@ -145,6 +145,7 @@ pub fn resolve_std<'cfg>(
     let cli_features = CliFeatures::from_command_line(
         &features, /*all_features*/ false, /*uses_default_features*/ false,
     )?;
+    let max_rust_version = ws.rust_version();
     let resolve = ops::resolve_ws_with_opts(
         &std_ws,
         target_data,
@@ -153,6 +154,7 @@ pub fn resolve_std<'cfg>(
         &specs,
         HasDevUnits::No,
         crate::core::resolver::features::ForceAllTargets::No,
+        max_rust_version,
     )?;
     Ok((
         resolve.pkg_set,

src/cargo/core/resolver/dep_cache.rs

@@ -21,6 +21,7 @@ use crate::core::{
 };
 use crate::util::errors::CargoResult;
 use crate::util::interning::InternedString;
+use crate::util::PartialVersion;
 
 use anyhow::Context as _;
 use std::collections::{BTreeSet, HashMap, HashSet};
@@ -36,6 +37,7 @@ pub struct RegistryQueryer<'a> {
     /// versions first. That allows `cargo update -Z minimal-versions` which will
     /// specify minimum dependency versions to be used.
     minimal_versions: bool,
+    max_rust_version: Option<PartialVersion>,
     /// a cache of `Candidate`s that fulfil a `Dependency` (and whether `first_minimal_version`)
     registry_cache: HashMap<(Dependency, bool), Poll<Rc<Vec<Summary>>>>,
     /// a cache of `Dependency`s that are required for a `Summary`
@@ -57,12 +59,14 @@ impl<'a> RegistryQueryer<'a> {
         replacements: &'a [(PackageIdSpec, Dependency)],
         version_prefs: &'a VersionPreferences,
         minimal_versions: bool,
+        max_rust_version: Option<PartialVersion>,
     ) -> Self {
         RegistryQueryer {
             registry,
             replacements,
             version_prefs,
             minimal_versions,
+            max_rust_version,
             registry_cache: HashMap::new(),
             summary_cache: HashMap::new(),
             used_replacements: HashMap::new(),
@@ -112,7 +116,9 @@ impl<'a> RegistryQueryer<'a> {
 
         let mut ret = Vec::new();
         let ready = self.registry.query(dep, QueryKind::Exact, &mut |s| {
-            ret.push(s);
+            if self.max_rust_version.is_none() || s.rust_version() <= self.max_rust_version {
+                ret.push(s);
+            }
         })?;
         if ready.is_pending() {
             self.registry_cache

src/cargo/core/resolver/mod.rs

@@ -71,6 +71,7 @@ use crate::util::config::Config;
 use crate::util::errors::CargoResult;
 use crate::util::network::PollExt;
 use crate::util::profile;
+use crate::util::PartialVersion;
 
 use self::context::Context;
 use self::dep_cache::RegistryQueryer;
@@ -138,6 +139,7 @@ pub fn resolve(
     version_prefs: &VersionPreferences,
     config: Option<&Config>,
     check_public_visible_dependencies: bool,
+    mut max_rust_version: Option<PartialVersion>,
 ) -> CargoResult<Resolve> {
     let _p = profile::start("resolving");
     let minimal_versions = match config {
@@ -148,8 +150,19 @@ pub fn resolve(
         Some(config) => config.cli_unstable().direct_minimal_versions,
         None => false,
     };
-    let mut registry =
-        RegistryQueryer::new(registry, replacements, version_prefs, minimal_versions);
+    if !config
+        .map(|c| c.cli_unstable().msrv_policy)
+        .unwrap_or(false)
+    {
+        max_rust_version = None;
+    }
+    let mut registry = RegistryQueryer::new(
+        registry,
+        replacements,
+        version_prefs,
+        minimal_versions,
+        max_rust_version,
+    );
     let cx = loop {
         let cx = Context::new(check_public_visible_dependencies);
         let cx = activate_deps_loop(

src/cargo/core/workspace.rs

@@ -23,6 +23,7 @@ use crate::util::edit_distance;
 use crate::util::errors::{CargoResult, ManifestError};
 use crate::util::interning::InternedString;
 use crate::util::toml::{read_manifest, InheritableFields, TomlDependency, TomlProfiles};
+use crate::util::PartialVersion;
 use crate::util::{config::ConfigRelativePath, Config, Filesystem, IntoUrl};
 use cargo_util::paths;
 use cargo_util::paths::normalize_path;
@@ -595,6 +596,12 @@ impl<'cfg> Workspace<'cfg> {
         self
     }
 
+    /// Get the lowest-common denominator `package.rust-version` within the workspace, if specified
+    /// anywhere
+    pub fn rust_version(&self) -> Option<PartialVersion> {
+        self.members().filter_map(|pkg| pkg.rust_version()).min()
+    }
+
     pub fn custom_metadata(&self) -> Option<&toml::Value> {
         self.custom_metadata.as_ref()
     }

src/cargo/ops/cargo_compile/mod.rs

@@ -261,6 +261,7 @@ pub fn create_bcx<'a, 'cfg>(
             HasDevUnits::No
         }
     };
+    let max_rust_version = ws.rust_version();
     let resolve = ops::resolve_ws_with_opts(
         ws,
         &mut target_data,
@@ -269,6 +270,7 @@ pub fn create_bcx<'a, 'cfg>(
         &specs,
         has_dev_units,
         crate::core::resolver::features::ForceAllTargets::No,
+        max_rust_version,
     )?;
     let WorkspaceResolve {
         mut pkg_set,

src/cargo/ops/cargo_generate_lockfile.rs

@@ -21,6 +21,7 @@ pub struct UpdateOptions<'a> {
 
 pub fn generate_lockfile(ws: &Workspace<'_>) -> CargoResult<()> {
     let mut registry = PackageRegistry::new(ws.config())?;
+    let max_rust_version = ws.rust_version();
     let mut resolve = ops::resolve_with_previous(
         &mut registry,
         ws,
@@ -30,6 +31,7 @@ pub fn generate_lockfile(ws: &Workspace<'_>) -> CargoResult<()> {
         None,
         &[],
         true,
+        max_rust_version,
     )?;
     ops::write_pkg_lockfile(ws, &mut resolve)?;
     Ok(())
@@ -48,6 +50,8 @@ pub fn update_lockfile(ws: &Workspace<'_>, opts: &UpdateOptions<'_>) -> CargoRes
     // that we're synchronized against other Cargos.
     let _lock = ws.config().acquire_package_cache_lock()?;
 
+    let max_rust_version = ws.rust_version();
+
     let previous_resolve = match ops::load_pkg_lockfile(ws)? {
         Some(resolve) => resolve,
         None => {
@@ -67,6 +71,7 @@ pub fn update_lockfile(ws: &Workspace<'_>, opts: &UpdateOptions<'_>) -> CargoRes
                         None,
                         &[],
                         true,
+                        max_rust_version,
                     )?
                 }
             }
@@ -125,6 +130,7 @@ pub fn update_lockfile(ws: &Workspace<'_>, opts: &UpdateOptions<'_>) -> CargoRes
         Some(&to_avoid),
         &[],
         true,
+        max_rust_version,
     )?;
 
     // Summarize what is changing for the user.

src/cargo/ops/cargo_output_metadata.rs

@@ -135,6 +135,8 @@ fn build_resolve_graph(
         crate::core::resolver::features::ForceAllTargets::No
     };
 
+    let max_rust_version = ws.rust_version();
+
     // Note that even with --filter-platform we end up downloading host dependencies as well,
     // as that is the behavior of download_accessible.
     let ws_resolve = ops::resolve_ws_with_opts(
@@ -145,6 +147,7 @@ fn build_resolve_graph(
         &specs,
         HasDevUnits::Yes,
         force_all,
+        max_rust_version,
     )?;
 
     let package_map: BTreeMap<PackageId, Package> = ws_resolve

src/cargo/ops/cargo_package.rs

@@ -423,6 +423,8 @@ fn build_lock(ws: &Workspace<'_>, orig_pkg: &Package) -> CargoResult<String> {
         TomlManifest::to_real_manifest(&toml_manifest, false, source_id, package_root, config)?;
     let new_pkg = Package::new(manifest, orig_pkg.manifest_path());
 
+    let max_rust_version = new_pkg.rust_version();
+
     // Regenerate Cargo.lock using the old one as a guide.
     let tmp_ws = Workspace::ephemeral(new_pkg, ws.config(), None, true)?;
     let mut tmp_reg = PackageRegistry::new(ws.config())?;
@@ -435,6 +437,7 @@ fn build_lock(ws: &Workspace<'_>, orig_pkg: &Package) -> CargoResult<String> {
         None,
         &[],
         true,
+        max_rust_version,
     )?;
     let pkg_set = ops::get_resolved_packages(&new_resolve, tmp_reg)?;
 

src/cargo/ops/fix.rs

@@ -246,6 +246,8 @@ fn check_resolver_change(ws: &Workspace<'_>, opts: &FixOptions) -> CargoResult<(
     let mut target_data =
         RustcTargetData::new(ws, &opts.compile_opts.build_config.requested_kinds)?;
     let mut resolve_differences = |has_dev_units| -> CargoResult<(WorkspaceResolve<'_>, DiffMap)> {
+        let max_rust_version = ws.rust_version();
+
         let ws_resolve = ops::resolve_ws_with_opts(
             ws,
             &mut target_data,
@@ -254,6 +256,7 @@ fn check_resolver_change(ws: &Workspace<'_>, opts: &FixOptions) -> CargoResult<(
             &specs,
             has_dev_units,
             crate::core::resolver::features::ForceAllTargets::No,
+            max_rust_version,
         )?;
 
         let feature_opts = FeatureOpts::new_behavior(ResolveBehavior::V2, has_dev_units);

src/cargo/ops/resolve.rs

@@ -69,6 +69,7 @@ use crate::core::{GitReference, PackageId, PackageIdSpec, PackageSet, SourceId,
 use crate::ops;
 use crate::sources::PathSource;
 use crate::util::errors::CargoResult;
+use crate::util::PartialVersion;
 use crate::util::{profile, CanonicalUrl};
 use anyhow::Context as _;
 use std::collections::{HashMap, HashSet};
@@ -106,8 +107,10 @@ version. This may also occur with an optional dependency that is not enabled.";
 /// This is a simple interface used by commands like `clean`, `fetch`, and
 /// `package`, which don't specify any options or features.
 pub fn resolve_ws<'a>(ws: &Workspace<'a>) -> CargoResult<(PackageSet<'a>, Resolve)> {
+    let max_rust_version = ws.rust_version();
+
     let mut registry = PackageRegistry::new(ws.config())?;
-    let resolve = resolve_with_registry(ws, &mut registry)?;
+    let resolve = resolve_with_registry(ws, &mut registry, max_rust_version)?;
     let packages = get_resolved_packages(&resolve, registry)?;
     Ok((packages, resolve))
 }
@@ -130,6 +133,7 @@ pub fn resolve_ws_with_opts<'cfg>(
     specs: &[PackageIdSpec],
     has_dev_units: HasDevUnits,
     force_all_targets: ForceAllTargets,
+    max_rust_version: Option<PartialVersion>,
 ) -> CargoResult<WorkspaceResolve<'cfg>> {
     let mut registry = PackageRegistry::new(ws.config())?;
     let mut add_patches = true;
@@ -138,7 +142,7 @@ pub fn resolve_ws_with_opts<'cfg>(
     } else if ws.require_optional_deps() {
         // First, resolve the root_package's *listed* dependencies, as well as
         // downloading and updating all remotes and such.
-        let resolve = resolve_with_registry(ws, &mut registry)?;
+        let resolve = resolve_with_registry(ws, &mut registry, max_rust_version)?;
         // No need to add patches again, `resolve_with_registry` has done it.
         add_patches = false;
 
@@ -184,6 +188,7 @@ pub fn resolve_ws_with_opts<'cfg>(
         None,
         specs,
         add_patches,
+        max_rust_version,
     )?;
 
     let pkg_set = get_resolved_packages(&resolved_with_overrides, registry)?;
@@ -235,6 +240,7 @@ pub fn resolve_ws_with_opts<'cfg>(
 fn resolve_with_registry<'cfg>(
     ws: &Workspace<'cfg>,
     registry: &mut PackageRegistry<'cfg>,
+    max_rust_version: Option<PartialVersion>,
 ) -> CargoResult<Resolve> {
     let prev = ops::load_pkg_lockfile(ws)?;
     let mut resolve = resolve_with_previous(
@@ -246,6 +252,7 @@ fn resolve_with_registry<'cfg>(
         None,
         &[],
         true,
+        max_rust_version,
     )?;
 
     if !ws.is_ephemeral() && ws.require_optional_deps() {
@@ -278,6 +285,7 @@ pub fn resolve_with_previous<'cfg>(
     to_avoid: Option<&HashSet<PackageId>>,
     specs: &[PackageIdSpec],
     register_patches: bool,
+    max_rust_version: Option<PartialVersion>,
 ) -> CargoResult<Resolve> {
     // We only want one Cargo at a time resolving a crate graph since this can
     // involve a lot of frobbing of the global caches.
@@ -505,6 +513,7 @@ pub fn resolve_with_previous<'cfg>(
         ws.unstable_features()
             .require(Feature::public_dependency())
             .is_ok(),
+        max_rust_version,
     )?;
     let patches: Vec<_> = registry
         .patches()

src/cargo/ops/tree/mod.rs

@@ -150,6 +150,7 @@ pub fn build_and_print(ws: &Workspace<'_>, opts: &TreeOptions) -> CargoResult<()
     } else {
         ForceAllTargets::No
     };
+    let max_rust_version = ws.rust_version();
     let ws_resolve = ops::resolve_ws_with_opts(
         ws,
         &mut target_data,
@@ -158,6 +159,7 @@ pub fn build_and_print(ws: &Workspace<'_>, opts: &TreeOptions) -> CargoResult<()
         &specs,
         has_dev,
         force_all,
+        max_rust_version,
     )?;
 
     let package_map: HashMap<PackageId, &Package> = ws_resolve

src/cargo/util/semver_ext.rs

@@ -108,7 +108,7 @@ impl From<VersionReq> for OptVersionReq {
     }
 }
 
-#[derive(PartialEq, Eq, Hash, Copy, Clone, Debug)]
+#[derive(PartialEq, Eq, PartialOrd, Ord, Hash, Copy, Clone, Debug)]
 pub struct PartialVersion {
     pub major: u64,
     pub minor: Option<u64>,

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()
-        .success()
+        .code(101)
         .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

@@ -1,2 +1,7 @@
     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?