feat(resolver): Add v3 resolver for MSRV-aware resolving

This is a part of #9930 and is important for changing the default with
the new edition.

Author: epage

src/cargo/core/resolver/features.rs

@@ -196,7 +196,7 @@ impl FeatureOpts {
         }
         match ws.resolve_behavior() {
             ResolveBehavior::V1 => {}
-            ResolveBehavior::V2 => {
+            ResolveBehavior::V2 | ResolveBehavior::V3 => {
                 enable(&vec!["all".to_string()]).unwrap();
             }
         }
@@ -214,7 +214,7 @@ impl FeatureOpts {
     pub fn new_behavior(behavior: ResolveBehavior, has_dev_units: HasDevUnits) -> FeatureOpts {
         match behavior {
             ResolveBehavior::V1 => FeatureOpts::default(),
-            ResolveBehavior::V2 => FeatureOpts {
+            ResolveBehavior::V2 | ResolveBehavior::V3 => FeatureOpts {
                 decouple_host_deps: true,
                 decouple_dev_deps: has_dev_units == HasDevUnits::No,
                 ignore_inactive_targets: true,

src/cargo/core/resolver/types.rs

@@ -111,13 +111,16 @@ pub enum ResolveBehavior {
     V1,
     /// V2 adds the new feature resolver.
     V2,
+    /// V3 changes version preferences
+    V3,
 }
 
 impl ResolveBehavior {
     pub fn from_manifest(resolver: &str) -> CargoResult<ResolveBehavior> {
         match resolver {
             "1" => Ok(ResolveBehavior::V1),
             "2" => Ok(ResolveBehavior::V2),
+            "3" => Ok(ResolveBehavior::V3),
             s => anyhow::bail!(
                 "`resolver` setting `{}` is not valid, valid options are \"1\" or \"2\"",
                 s
@@ -129,6 +132,7 @@ impl ResolveBehavior {
         match self {
             ResolveBehavior::V1 => "1",
             ResolveBehavior::V2 => "2",
+            ResolveBehavior::V3 => "3",
         }
         .to_owned()
     }

src/cargo/core/workspace.rs

@@ -306,6 +306,17 @@ impl<'gctx> Workspace<'gctx> {
             MaybePackage::Virtual(vm) => vm.resolve_behavior().unwrap_or(ResolveBehavior::V1),
         };
 
+        match self.resolve_behavior() {
+            ResolveBehavior::V1 | ResolveBehavior::V2 => {}
+            ResolveBehavior::V3 => {
+                if self.resolve_behavior == ResolveBehavior::V3 {
+                    if !self.gctx().cli_unstable().msrv_policy {
+                        anyhow::bail!("`resolver=\"3\"` requires `-Zmsrv-policy`");
+                    }
+                    self.resolve_honors_rust_version = true;
+                }
+            }
+        }
         match self.gctx().get::<CargoResolverConfig>("resolver") {
             Ok(CargoResolverConfig {
                 something_like_precedence: Some(precedence),
@@ -869,7 +880,7 @@ impl<'gctx> Workspace<'gctx> {
         self.is_virtual()
             || match self.resolve_behavior() {
                 ResolveBehavior::V1 => false,
-                ResolveBehavior::V2 => true,
+                ResolveBehavior::V2 | ResolveBehavior::V3 => true,
             }
     }
 

src/cargo/util/toml/mod.rs

@@ -265,6 +265,12 @@ fn resolve_toml(
     warnings: &mut Vec<String>,
     _errors: &mut Vec<String>,
 ) -> CargoResult<manifest::TomlManifest> {
+    if let Some(workspace) = &original_toml.workspace {
+        if workspace.resolver.as_deref() == Some("3") {
+            features.require(Feature::edition2024())?;
+        }
+    }
+
     let mut resolved_toml = manifest::TomlManifest {
         cargo_features: original_toml.cargo_features.clone(),
         package: None,
@@ -299,7 +305,8 @@ fn resolve_toml(
     };
 
     if let Some(original_package) = original_toml.package() {
-        let resolved_package = resolve_package_toml(original_package, package_root, &inherit)?;
+        let resolved_package =
+            resolve_package_toml(original_package, features, package_root, &inherit)?;
         resolved_toml.package = Some(resolved_package);
 
         resolved_toml.dependencies = resolve_dependencies(
@@ -399,6 +406,7 @@ fn resolve_toml(
 #[tracing::instrument(skip_all)]
 fn resolve_package_toml<'a>(
     original_package: &manifest::TomlPackage,
+    features: &Features,
     package_root: &Path,
     inherit: &dyn Fn() -> CargoResult<&'a InheritableFields>,
 ) -> CargoResult<Box<manifest::TomlPackage>> {
@@ -526,6 +534,11 @@ fn resolve_package_toml<'a>(
         metadata: original_package.metadata.clone(),
         _invalid_cargo_features: Default::default(),
     };
+
+    if resolved_package.resolver.as_deref() == Some("3") {
+        features.require(Feature::edition2024())?;
+    }
+
     Ok(Box::new(resolved_package))
 }
 

src/doc/src/reference/unstable.md

@@ -338,6 +338,7 @@ This was stabilized in 1.79 in [#13608](https://github.com/rust-lang/cargo/pull/
 
 `-Zmsrv-policy` allows access to an MSRV-aware resolver which can be enabled with:
 - `resolver.something-like-precedence` config field
+- `workspace.resolver = "3"` / `package.resolver = "3"`
 
 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.

tests/testsuite/rust_version.rs

@@ -613,6 +613,133 @@ foo v0.0.1 ([CWD])
         .run();
 }
 
+#[cargo_test(nightly, reason = "edition2024 in rustc is unstable")]
+fn resolve_v3() {
+    Package::new("only-newer", "1.6.0")
+        .rust_version("1.65.0")
+        .file("src/lib.rs", "fn other_stuff() {}")
+        .publish();
+    Package::new("newer-and-older", "1.5.0")
+        .rust_version("1.55.0")
+        .file("src/lib.rs", "fn other_stuff() {}")
+        .publish();
+    Package::new("newer-and-older", "1.6.0")
+        .rust_version("1.65.0")
+        .file("src/lib.rs", "fn other_stuff() {}")
+        .publish();
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+            cargo-features = ["edition2024"]
+
+            [package]
+            name = "foo"
+            version = "0.0.1"
+            edition = "2015"
+            authors = []
+            rust-version = "1.60.0"
+            resolver = "3"
+
+            [dependencies]
+            only-newer = "1.0.0"
+            newer-and-older = "1.0.0"
+        "#,
+        )
+        .file("src/main.rs", "fn main(){}")
+        .build();
+
+    // v3 should resolve for MSRV
+    p.cargo("generate-lockfile")
+        .arg("-Zmsrv-policy")
+        .masquerade_as_nightly_cargo(&["edition2024", "msrv-policy"])
+        .with_stderr(
+            "\
+[UPDATING] `dummy-registry` index
+[LOCKING] 3 packages to latest Rust 1.60.0 compatible versions
+[ADDING] newer-and-older v1.5.0 (latest: v1.6.0)
+",
+        )
+        .run();
+    p.cargo("tree")
+        .arg("-Zmsrv-policy")
+        .masquerade_as_nightly_cargo(&["edition2024", "msrv-policy"])
+        .with_stdout(
+            "\
+foo v0.0.1 ([CWD])
+├── newer-and-older v1.5.0
+└── only-newer v1.6.0
+",
+        )
+        .run();
+
+    // `--ignore-rust-version` has precedence over v3
+    p.cargo("generate-lockfile --ignore-rust-version")
+        .with_stderr(
+            "\
+[UPDATING] `dummy-registry` index
+[LOCKING] 3 packages to latest compatible versions
+",
+        )
+        .arg("-Zmsrv-policy")
+        .masquerade_as_nightly_cargo(&["msrv-policy"])
+        .run();
+    p.cargo("tree")
+        .arg("-Zmsrv-policy")
+        .masquerade_as_nightly_cargo(&["edition2024", "msrv-policy"])
+        .with_stdout(
+            "\
+foo v0.0.1 ([CWD])
+├── newer-and-older v1.6.0
+└── only-newer v1.6.0
+",
+        )
+        .run();
+
+    // config has precedence over v3
+    p.cargo("generate-lockfile")
+        .env(
+            "CARGO_RESOLVER_SOMETHING_LIKE_PRECEDENCE",
+            "something-like-maximum",
+        )
+        .with_stderr(
+            "\
+[UPDATING] `dummy-registry` index
+[LOCKING] 3 packages to latest compatible versions
+",
+        )
+        .arg("-Zmsrv-policy")
+        .masquerade_as_nightly_cargo(&["msrv-policy"])
+        .run();
+    p.cargo("tree")
+        .arg("-Zmsrv-policy")
+        .masquerade_as_nightly_cargo(&["edition2024", "msrv-policy"])
+        .with_stdout(
+            "\
+foo v0.0.1 ([CWD])
+├── newer-and-older v1.6.0
+└── only-newer v1.6.0
+",
+        )
+        .run();
+
+    // unstable
+    p.cargo("generate-lockfile")
+        .with_status(101)
+        .with_stderr(
+            "\
+[ERROR] failed to parse manifest at `[CWD]/Cargo.toml`
+
+Caused by:
+  the cargo feature `edition2024` requires a nightly version of Cargo, but this is the `stable` channel
+  See https://doc.rust-lang.org/book/appendix-07-nightly-rust.html for more information about Rust release channels.
+  See https://doc.rust-lang.org/cargo/reference/unstable.html#edition-2024 for more information about using this feature.
+",
+        )
+        .run();
+}
+
 #[cargo_test]
 fn generate_lockfile_ignore_rust_version_is_unstable() {
     Package::new("bar", "1.5.0")