feat: respect rust-version when generating lockfile

MSRV-aware resolver has been implemented in PR 12560.
Based on that effort, generating lockfile can now respect `rust-version`
so that a package with an old MSRV will never get an incompatible
lockfile, even using the latest Cargo.

src/cargo/core/resolver/mod.rs

@@ -150,6 +150,9 @@ pub fn resolve(
         Some(config) => config.cli_unstable().direct_minimal_versions,
         None => false,
     };
+
+    let resolve_version = ResolveVersion::with_rust_version(max_rust_version);
+
     if !config
         .map(|c| c.cli_unstable().msrv_policy)
         .unwrap_or(false)
@@ -203,7 +206,7 @@ pub fn resolve(
         cksums,
         BTreeMap::new(),
         Vec::new(),
-        ResolveVersion::default(),
+        resolve_version,
         summaries,
     );
 

src/cargo/core/resolver/resolve.rs

@@ -4,6 +4,7 @@ use crate::core::{Dependency, PackageId, PackageIdSpec, Summary, Target};
 use crate::util::errors::CargoResult;
 use crate::util::interning::InternedString;
 use crate::util::Graph;
+use crate::util::RustVersion;
 use std::borrow::Borrow;
 use std::collections::{HashMap, HashSet};
 use std::fmt;
@@ -97,6 +98,21 @@ impl ResolveVersion {
     pub fn max_stable() -> ResolveVersion {
         ResolveVersion::V3
     }
+
+    /// Gets the default lockfile version for the given Rust version.
+    pub fn with_rust_version(rust_version: Option<&RustVersion>) -> Self {
+        let Some(rust_version) = rust_version else {
+            return ResolveVersion::default();
+        };
+
+        if rust_version >= &"1.53".parse::<RustVersion>().unwrap() {
+            ResolveVersion::V3
+        } else if rust_version >= &"1.41".parse::<RustVersion>().unwrap() {
+            ResolveVersion::V2
+        } else {
+            ResolveVersion::V1
+        }
+    }
 }
 
 impl Resolve {

src/cargo/ops/lockfile.rs

@@ -64,9 +64,10 @@ 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::default();
+    let default_version = ResolveVersion::with_rust_version(ws.rust_version());
     let current_version = resolve.version();
     let next_lockfile_bump = ws.config().cli_unstable().next_lockfile_bump;
+    tracing::debug!("lockfile - current: {current_version:?}, default: {default_version:?}");
 
     if current_version < default_version {
         resolve.set_version(default_version);

tests/testsuite/lockfile_compat.rs

@@ -1220,23 +1220,23 @@ dependencies = [
 
     let cases = [
         // v1 is the default
-        ("1.37", None, 3),
+        ("1.37", None, 1),
         ("1.37", Some(1), 1),
         ("1.37", Some(2), 2),
         ("1.37", Some(3), 3),
         // v2 introduced
-        ("1.38", None, 3),
+        ("1.38", None, 1),
         // last version of v1 as the default
-        ("1.40", None, 3),
+        ("1.40", None, 1),
         // v2 is the default
-        ("1.41", None, 3),
+        ("1.41", None, 2),
         ("1.41", Some(1), 1),
         ("1.41", Some(2), 2),
         ("1.41", Some(3), 3),
         // v3 introduced
-        ("1.47", None, 3),
+        ("1.47", None, 2),
         // last version of v2 as the default
-        ("1.48", None, 3),
+        ("1.48", None, 2),
         // v3 is the default
         ("1.53", None, 3),
         ("1.53", Some(1), 1),