feat(resolve): Fallback to 'rustc -V' for MSRV resolving

src/cargo/ops/resolve.rs

@@ -73,7 +73,6 @@ use crate::util::cache_lock::CacheLockMode;
 use crate::util::errors::CargoResult;
 use crate::util::CanonicalUrl;
 use anyhow::Context as _;
-use cargo_util_schemas::manifest::RustVersion;
 use std::collections::{HashMap, HashSet};
 use tracing::{debug, trace};
 
@@ -304,7 +303,14 @@ pub fn resolve_with_previous<'gctx>(
         version_prefs.version_ordering(VersionOrdering::MinimumVersionsFirst)
     }
     if ws.resolve_honors_rust_version() {
-        version_prefs.max_rust_version(ws.rust_version().cloned().map(RustVersion::into_partial));
+        let rust_version = if let Some(ver) = ws.rust_version() {
+            ver.clone().into_partial()
+        } else {
+            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 avoid_patch_ids = if register_patches {

tests/testsuite/rust_version.rs

@@ -370,6 +370,56 @@ fn dependency_rust_version_older_and_newer_than_package() {
         .run();
 }
 
+#[cargo_test]
+fn resolve_with_rustc() {
+    Package::new("bar", "1.5.0")
+        .rust_version("1.0")
+        .file("src/lib.rs", "fn other_stuff() {}")
+        .publish();
+    Package::new("bar", "1.6.0")
+        .rust_version("1.2345")
+        .file("src/lib.rs", "fn other_stuff() {}")
+        .publish();
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+            [package]
+            name = "foo"
+            version = "0.0.1"
+            edition = "2015"
+            authors = []
+            [dependencies]
+            bar = "1.0.0"
+        "#,
+        )
+        .file("src/main.rs", "fn main(){}")
+        .build();
+
+    p.cargo("generate-lockfile --ignore-rust-version")
+        .arg("-Zmsrv-policy")
+        .masquerade_as_nightly_cargo(&["msrv-policy"])
+        .with_stderr(
+            "\
+[UPDATING] `dummy-registry` index
+[LOCKING] 2 packages
+",
+        )
+        .run();
+    p.cargo("generate-lockfile")
+        .arg("-Zmsrv-policy")
+        .masquerade_as_nightly_cargo(&["msrv-policy"])
+        .with_stderr(
+            "\
+[UPDATING] `dummy-registry` index
+[LOCKING] 2 packages
+[ADDING] bar v1.5.0 (latest: v1.6.0)
+",
+        )
+        .run();
+}
+
 #[cargo_test]
 fn dependency_rust_version_backtracking() {
     Package::new("has-rust-version", "1.6.0")