fix(install): Suggest an alternative version on MSRV failure

src/cargo/ops/cargo_install.rs

@@ -68,6 +68,7 @@ impl<'cfg> InstallablePackage<'cfg> {
         force: bool,
         no_track: bool,
         needs_update_if_source_is_index: bool,
+        current_rust_version: Option<&semver::Version>,
     ) -> CargoResult<Option<Self>> {
         if let Some(name) = krate {
             if name == "." {
@@ -105,6 +106,7 @@ impl<'cfg> InstallablePackage<'cfg> {
                     dep,
                     |git: &mut GitSource<'_>| git.read_packages(),
                     config,
+                    current_rust_version,
                 )?
             } else if source_id.is_path() {
                 let mut src = path_source(source_id, config)?;
@@ -142,6 +144,7 @@ impl<'cfg> InstallablePackage<'cfg> {
                     dep,
                     |path: &mut PathSource<'_>| path.read_packages(),
                     config,
+                    current_rust_version,
                 )?
             } else if let Some(dep) = dep {
                 let mut source = map.load(source_id, &HashSet::new())?;
@@ -161,7 +164,13 @@ impl<'cfg> InstallablePackage<'cfg> {
                     config.shell().status("Ignored", &msg)?;
                     return Ok(None);
                 }
-                select_dep_pkg(&mut source, dep, config, needs_update_if_source_is_index)?
+                select_dep_pkg(
+                    &mut source,
+                    dep,
+                    config,
+                    needs_update_if_source_is_index,
+                    current_rust_version,
+                )?
             } else {
                 bail!(
                     "must specify a crate to install from \
@@ -616,14 +625,40 @@ pub fn install(
     let dst = root.join("bin").into_path_unlocked();
     let map = SourceConfigMap::new(config)?;
 
+    let current_rust_version = if opts.honor_rust_version {
+        let rustc = config.load_global_rustc(None)?;
+
+        // Remove any pre-release identifiers for easier comparison
+        let current_version = &rustc.version;
+        let untagged_version = semver::Version::new(
+            current_version.major,
+            current_version.minor,
+            current_version.patch,
+        );
+        Some(untagged_version)
+    } else {
+        None
+    };
+
     let (installed_anything, scheduled_error) = if krates.len() <= 1 {
         let (krate, vers) = krates
             .iter()
             .next()
             .map(|(k, v)| (Some(k.as_str()), v.as_ref()))
             .unwrap_or((None, None));
         let installable_pkg = InstallablePackage::new(
-            config, root, map, krate, source_id, from_cwd, vers, opts, force, no_track, true,
+            config,
+            root,
+            map,
+            krate,
+            source_id,
+            from_cwd,
+            vers,
+            opts,
+            force,
+            no_track,
+            true,
+            current_rust_version.as_ref(),
         )?;
         let mut installed_anything = true;
         if let Some(installable_pkg) = installable_pkg {
@@ -654,6 +689,7 @@ pub fn install(
                     force,
                     no_track,
                     !did_update,
+                    current_rust_version.as_ref(),
                 ) {
                     Ok(Some(installable_pkg)) => {
                         did_update = true;
@@ -773,7 +809,7 @@ where
     // expensive network call in the case that the package is already installed.
     // If this fails, the caller will possibly do an index update and try again, this is just a
     // best-effort check to see if we can avoid hitting the network.
-    if let Ok(pkg) = select_dep_pkg(source, dep, config, false) {
+    if let Ok(pkg) = select_dep_pkg(source, dep, config, false, None) {
         let (_ws, rustc, target) =
             make_ws_rustc_target(config, opts, &source.source_id(), pkg.clone())?;
         if let Ok(true) = is_installed(&pkg, config, opts, &rustc, &target, root, dst, force) {

src/cargo/ops/cargo_uninstall.rs

@@ -90,6 +90,7 @@ fn uninstall_cwd(root: &Filesystem, bins: &[String], config: &Config) -> CargoRe
         None,
         |path: &mut PathSource<'_>| path.read_packages(),
         config,
+        None,
     )?;
     let pkgid = pkg.package_id();
     uninstall_pkgid(root, tracker, pkgid, bins, config)

src/cargo/ops/common_for_install_and_uninstall.rs

@@ -532,6 +532,7 @@ pub fn select_dep_pkg<T>(
     dep: Dependency,
     config: &Config,
     needs_update: bool,
+    current_rust_version: Option<&semver::Version>,
 ) -> CargoResult<Package>
 where
     T: Source,
@@ -551,9 +552,56 @@ where
             Poll::Pending => source.block_until_ready()?,
         }
     };
-    match deps.iter().map(|p| p.package_id()).max() {
-        Some(pkgid) => {
-            let pkg = Box::new(source).download_now(pkgid, config)?;
+    match deps.iter().max_by_key(|p| p.package_id()) {
+        Some(summary) => {
+            if let (Some(current), Some(msrv)) = (current_rust_version, summary.rust_version()) {
+                let msrv_req = msrv.caret_req();
+                if !msrv_req.matches(current) {
+                    let name = summary.name();
+                    let ver = summary.version();
+                    let extra = if dep.source_id().is_registry() {
+                        // Match any version, not just the selected
+                        let msrv_dep =
+                            Dependency::parse(dep.package_name(), None, dep.source_id())?;
+                        let msrv_deps = loop {
+                            match source.query_vec(&msrv_dep, QueryKind::Exact)? {
+                                Poll::Ready(deps) => break deps,
+                                Poll::Pending => source.block_until_ready()?,
+                            }
+                        };
+                        if let Some(alt) = msrv_deps
+                            .iter()
+                            .filter(|summary| {
+                                summary
+                                    .rust_version()
+                                    .map(|msrv| msrv.caret_req().matches(current))
+                                    .unwrap_or(true)
+                            })
+                            .max_by_key(|s| s.package_id())
+                        {
+                            if let Some(rust_version) = alt.rust_version() {
+                                format!(
+                                    "\n`{name} {}` supports rustc {rust_version}",
+                                    alt.version()
+                                )
+                            } else {
+                                format!(
+                                    "\n`{name} {}` has an unspecified minimum rustc version",
+                                    alt.version()
+                                )
+                            }
+                        } else {
+                            String::new()
+                        }
+                    } else {
+                        String::new()
+                    };
+                    bail!("\
+cannot install package `{name} {ver}`, it requires rustc {msrv} or newer, while the currently active rustc version is {current}{extra}"
+)
+                }
+            }
+            let pkg = Box::new(source).download_now(summary.package_id(), config)?;
             Ok(pkg)
         }
         None => {
@@ -599,6 +647,7 @@ pub fn select_pkg<T, F>(
     dep: Option<Dependency>,
     mut list_all: F,
     config: &Config,
+    current_rust_version: Option<&semver::Version>,
 ) -> CargoResult<Package>
 where
     T: Source,
@@ -612,7 +661,7 @@ where
     source.invalidate_cache();
 
     return if let Some(dep) = dep {
-        select_dep_pkg(source, dep, config, false)
+        select_dep_pkg(source, dep, config, false, current_rust_version)
     } else {
         let candidates = list_all(source)?;
         let binaries = candidates

tests/testsuite/install.rs

@@ -2463,3 +2463,23 @@ For more information, try '--help'.
         .with_status(1)
         .run();
 }
+
+#[cargo_test]
+fn install_incompat_msrv() {
+    Package::new("foo", "0.1.0")
+        .file("src/main.rs", "fn main() {}")
+        .rust_version("1.30")
+        .publish();
+    Package::new("foo", "0.2.0")
+        .file("src/main.rs", "fn main() {}")
+        .rust_version("1.9876.0")
+        .publish();
+
+    cargo_process("install foo")
+        .with_stderr("\
+[UPDATING] `dummy-registry` index
+[ERROR] cannot install package `foo 0.2.0`, it requires rustc 1.9876.0 or newer, while the currently active rustc version is [..]
+`foo 0.1.0` supports rustc 1.30
+")
+        .with_status(101).run();
+}