feat(resolver): Add CLI option to resolve minimal version dependencies

src/cargo/core/features.rs

@@ -285,6 +285,7 @@ pub struct CliUnstable {
     pub offline: bool,
     pub no_index_update: bool,
     pub avoid_dev_deps: bool,
+    pub minimal_versions: bool,
 }
 
 impl CliUnstable {
@@ -317,6 +318,7 @@ impl CliUnstable {
             "offline" => self.offline = true,
             "no-index-update" => self.no_index_update = true,
             "avoid-dev-deps" => self.avoid_dev_deps = true,
+            "minimal-versions" => self.minimal_versions = true,
             _ => bail!("unknown `-Z` flag specified: {}", k),
         }
 

src/cargo/core/resolver/mod.rs

@@ -430,7 +430,11 @@ pub fn resolve(
         warnings: RcList::new(),
     };
     let _p = profile::start("resolving");
-    let mut registry = RegistryQueryer::new(registry, replacements, try_to_use);
+    let minimal_versions = match config {
+        Some(config) => config.cli_unstable().minimal_versions,
+        None => false,
+    };
+    let mut registry = RegistryQueryer::new(registry, replacements, try_to_use, minimal_versions);
     let cx = activate_deps_loop(cx, &mut registry, summaries, config)?;
 
     let mut resolve = Resolve {
@@ -683,19 +687,25 @@ struct RegistryQueryer<'a> {
     try_to_use: &'a HashSet<&'a PackageId>,
     // TODO: with nll the Rc can be removed
     cache: HashMap<Dependency, Rc<Vec<Candidate>>>,
+    // If set the list of dependency candidates will be sorted by minimal
+    // versions first. That allows `cargo update -Z minimal-versions` which will
+    // specify minimum depedency versions to be used.
+    minimal_versions: bool,
 }
 
 impl<'a> RegistryQueryer<'a> {
     fn new(
         registry: &'a mut Registry,
         replacements: &'a [(PackageIdSpec, Dependency)],
         try_to_use: &'a HashSet<&'a PackageId>,
+        minimal_versions: bool,
     ) -> Self {
         RegistryQueryer {
             registry,
             replacements,
             cache: HashMap::new(),
             try_to_use,
+            minimal_versions,
         }
     }
 
@@ -795,9 +805,20 @@ impl<'a> RegistryQueryer<'a> {
         ret.sort_unstable_by(|a, b| {
             let a_in_previous = self.try_to_use.contains(a.summary.package_id());
             let b_in_previous = self.try_to_use.contains(b.summary.package_id());
-            let a = (a_in_previous, a.summary.version());
-            let b = (b_in_previous, b.summary.version());
-            a.cmp(&b).reverse()
+            let previous_cmp = a_in_previous.cmp(&b_in_previous).reverse();
+            match previous_cmp {
+                Ordering::Equal => {
+                    let cmp = a.summary.version().cmp(&b.summary.version());
+                    if self.minimal_versions == true {
+                        // Lower version ordered first.
+                        cmp
+                    } else {
+                        // Higher version ordered first.
+                        cmp.reverse()
+                    }
+                }
+                _ => previous_cmp,
+            }
         });
 
         let out = Rc::new(ret);

tests/testsuite/resolve.rs

@@ -5,13 +5,26 @@ use hamcrest::{assert_that, contains, is_not};
 use cargo::core::source::{GitReference, SourceId};
 use cargo::core::dependency::Kind::{self, Development};
 use cargo::core::{Dependency, PackageId, Registry, Summary};
-use cargo::util::{CargoResult, ToUrl};
+use cargo::util::{CargoResult, Config, ToUrl};
 use cargo::core::resolver::{self, Method};
 
+use cargotest::ChannelChanger;
+use cargotest::support::{execs, project};
+use cargotest::support::registry::Package;
+
 fn resolve(
     pkg: &PackageId,
     deps: Vec<Dependency>,
     registry: &[Summary],
+) -> CargoResult<Vec<PackageId>> {
+    resolve_with_config(pkg, deps, registry, None)
+}
+
+fn resolve_with_config(
+    pkg: &PackageId,
+    deps: Vec<Dependency>,
+    registry: &[Summary],
+    config: Option<&Config>,
 ) -> CargoResult<Vec<PackageId>> {
     struct MyRegistry<'a>(&'a [Summary]);
     impl<'a> Registry for MyRegistry<'a> {
@@ -38,7 +51,7 @@ fn resolve(
         &[],
         &mut registry,
         &HashSet::new(),
-        None,
+        config,
         false,
     )?;
     let res = resolve.iter().cloned().collect();
@@ -320,6 +333,87 @@ fn test_resolving_maximum_version_with_transitive_deps() {
     assert_that(&res, is_not(contains(names(&[("util", "1.1.1")]))));
 }
 
+#[test]
+fn test_resolving_minimum_version_with_transitive_deps() {
+    // When the minimal-versions config option is specified then the lowest
+    // possible version of a package should be selected. "util 1.0.0" can't be
+    // selected because of the requirements of "bar", so the minimum version
+    // must be 1.1.1.
+    let reg = registry(vec![
+        pkg!(("util", "1.2.2")),
+        pkg!(("util", "1.0.0")),
+        pkg!(("util", "1.1.1")),
+        pkg!("foo" => [dep_req("util", "1.0.0")]),
+        pkg!("bar" => [dep_req("util", ">=1.0.1")]),
+    ]);
+
+    let mut config = Config::default().unwrap();
+    config
+        .configure(
+            1,
+            None,
+            &None,
+            false,
+            false,
+            &["minimal-versions".to_string()],
+        )
+        .unwrap();
+
+    let res = resolve_with_config(
+        &pkg_id("root"),
+        vec![dep_req("foo", "1.0.0"), dep_req("bar", "1.0.0")],
+        &reg,
+        Some(&config),
+    ).unwrap();
+
+    assert_that(
+        &res,
+        contains(names(&[
+            ("root", "1.0.0"),
+            ("foo", "1.0.0"),
+            ("bar", "1.0.0"),
+            ("util", "1.1.1"),
+        ])),
+    );
+    assert_that(&res, is_not(contains(names(&[("util", "1.2.2")]))));
+    assert_that(&res, is_not(contains(names(&[("util", "1.0.0")]))));
+}
+
+// Ensure that the "-Z minimal-versions" CLI option works and the minimal
+// version of a dependency ends up in the lock file.
+#[test]
+fn minimal_version_cli() {
+    Package::new("dep", "1.0.0").publish();
+    Package::new("dep", "1.1.0").publish();
+
+    let p = project("foo")
+        .file(
+            "Cargo.toml",
+            r#"
+            [package]
+            name = "foo"
+            authors = []
+            version = "0.0.1"
+
+            [dependencies]
+            dep = "1.0"
+        "#,
+        )
+        .file("src/main.rs", "fn main() {}")
+        .build();
+
+    assert_that(
+        p.cargo("generate-lockfile")
+            .masquerade_as_nightly_cargo()
+            .arg("-Zminimal-versions"),
+        execs().with_status(0),
+    );
+
+    let lock = p.read_lockfile();
+
+    assert!(lock.contains("dep 1.0.0"));
+}
+
 #[test]
 fn resolving_incompat_versions() {
     let reg = registry(vec![