feat: Add -Zhint-msrv flag

src/cargo/core/compiler/mod.rs

@@ -1478,6 +1478,12 @@ fn build_base_args(
             .env("RUSTC_BOOTSTRAP", "1");
     }
 
+    if let Some(version) = unit.pkg.manifest().rust_version()
+        && bcx.gctx.cli_unstable().hint_msrv
+    {
+        cmd.arg("-Z").arg(format!("hint-msrv={version}"));
+    }
+
     Ok(())
 }
 

src/cargo/core/features.rs

@@ -885,6 +885,7 @@ unstable_cli_options!(
     git: Option<GitFeatures> = ("Enable support for shallow git fetch operations"),
     #[serde(deserialize_with = "deserialize_gitoxide_features")]
     gitoxide: Option<GitoxideFeatures> = ("Use gitoxide for the given git interactions, or all of them if no argument is given"),
+    hint_msrv: bool = ("Enable passing `package.rust-version` to rustc for lints"),
     host_config: bool = ("Enable the `[host]` section in the .cargo/config.toml file"),
     json_target_spec: bool = ("Enable `.json` target spec files"),
     min_publish_age: bool = ("Enable the `min-publish-age` configuration for dependency version age filtering"),
@@ -1433,6 +1434,7 @@ impl CliUnstable {
             "host-config" => self.host_config = parse_empty(k, v)?,
             "json-target-spec" => self.json_target_spec = parse_empty(k, v)?,
             "min-publish-age" => self.min_publish_age = parse_empty(k, v)?,
+            "hint-msrv" => self.hint_msrv = parse_empty(k, v)?,
             "next-lockfile-bump" => self.next_lockfile_bump = parse_empty(k, v)?,
             "minimal-versions" => self.minimal_versions = parse_empty(k, v)?,
             "msrv-policy" => self.msrv_policy = parse_empty(k, v)?,

src/doc/src/reference/unstable.md

@@ -102,6 +102,7 @@ Each new feature described below should explain how to use it.
     * [compile-time-deps](#compile-time-deps) --- Perma-unstable feature for rust-analyzer
     * [fine-grain-locking](#fine-grain-locking) --- Use fine grain locking instead of locking the entire build cache
     * [json-target-spec](#json-target-spec) --- Allows the use of `.json` custom target specs.
+    * [hint-msrv](#hint-msrv) --- Allows Cargo to set `-Zhint-msrv`
 * rustdoc
     * [rustdoc-map](#rustdoc-map) --- Provides mappings for documentation to link to external sites like [docs.rs](https://docs.rs/).
     * [scrape-examples](#scrape-examples) --- Shows examples within documentation.
@@ -2013,6 +2014,20 @@ cargo +nightly build --target my-target.json -Z json-target-spec
 
 This usually must be combined with [build-std](#build-std).
 
+## hint-msrv
+* Tracking Issue: [rust-lang/rust#157574](https://github.com/rust-lang/rust/issues/157574)
+
+The -`Z hint-msrv` CLI flag enables Cargo to pass `package.rust-version` to rustc which can affect which lints are emitted.
+
+You can set this via your global `~/.cargo/config.toml`, and nightly Cargo will
+automatically use it, while stable Cargo will silently ignore the unstable
+option:
+
+```toml
+[unstable]
+hint-msrv = true
+```
+
 ## min-publish-age
 
 * Tracking Issue: [#17009](https://github.com/rust-lang/cargo/issues/17009)

tests/testsuite/hint_msrv.rs

@@ -0,0 +1,62 @@
+//! Tests for `hint-msrv`
+
+use crate::prelude::*;
+use cargo_test_support::{basic_manifest, project, str};
+
+// Test that `hint-msrv` is not available on stable
+#[cargo_test]
+fn hint_msrv_stable() {
+    let p = project().file("src/lib.rs", "").build();
+
+    p.cargo("build -Zhint-msrv").with_status(101).with_stderr_data(str![[r#"
+[ERROR] the `-Z` flag is only accepted on the nightly channel 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.
+
+"#]]).run();
+}
+
+fn msrv_manifest(name: &str, version: &str, msrv: &str) -> String {
+    format!(
+        r#"
+        [package]
+        name = "{}"
+        version = "{}"
+        authors = []
+        edition = "2015"
+        rust-version = "{}"
+    "#,
+        name, version, msrv
+    )
+}
+
+// Test that `hint-msrv` passes `package.rust-version`
+#[cargo_test(nightly, reason = "tests nightly flag")]
+fn hint_msrv() {
+    let p = project()
+        .file("src/lib.rs", "")
+        .file("Cargo.toml", &msrv_manifest("foo", "0.0.0", "1.78.0"))
+        .build();
+
+    p.cargo("build --verbose -Zhint-msrv")
+        .masquerade_as_nightly_cargo(&["hint-msrv"])
+        .with_stderr_data(str![[r#"
+[COMPILING] foo v0.0.0 ([ROOT]/foo)
+[RUNNING] `rustc [..]-Z hint-msrv=1.78.0[..]`
+[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+
+"#]])
+        .with_status(0)
+        .run();
+
+    // Test that a missing package.rust-version is not an error, and does not pass a value
+    let p = project()
+        .file("src/lib.rs", "")
+        .file("Cargo.toml", &basic_manifest("foo", "0.0.0"))
+        .build();
+
+    p.cargo("build --verbose -Zhint-msrv")
+        .masquerade_as_nightly_cargo(&["hint-msrv"])
+        .with_stderr_does_not_contain("hint-msrv")
+        .with_status(0)
+        .run();
+}

tests/testsuite/main.rs

@@ -112,6 +112,7 @@ mod git_shallow;
 mod glob_targets;
 mod global_cache_tracker;
 mod help;
+mod hint_msrv;
 mod hints;
 mod https;
 mod inheritable_workspace_fields;