fix(install): Don't suggest --locked for MSRV when its root package

This will also report the error without having to download the `.crate`
first.

If installing multiple packages, this will also report it immediately,
rather than waiting for the other packages to be installed first.

This also offers us more flexibility in the error we report,
like suggesting more appropriate fixes.

src/bin/cargo/commands/install.rs

@@ -203,6 +203,7 @@ pub fn exec(config: &mut Config, args: &ArgMatches) -> CliResult {
             &compile_opts,
             args.flag("force"),
             args.flag("no-track"),
+            !args.flag("ignore-rust-version"),
         )?;
     }
     Ok(())

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 \
@@ -611,19 +620,46 @@ pub fn install(
     opts: &ops::CompileOptions,
     force: bool,
     no_track: bool,
+    honor_rust_version: bool,
 ) -> CargoResult<()> {
     let root = resolve_root(root, config)?;
     let dst = root.join("bin").into_path_unlocked();
     let map = SourceConfigMap::new(config)?;
 
+    let current_rust_version = if 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 +690,7 @@ pub fn install(
                     force,
                     no_track,
                     !did_update,
+                    current_rust_version.as_ref(),
                 ) {
                     Ok(Some(installable_pkg)) => {
                         did_update = true;
@@ -773,7 +810,7 @@ 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 +552,19 @@ 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();
+                    bail!("\
+cannot install package `{name} {ver}`, it requires rustc {msrv} or newer, while the currently active rustc version is {current}"
+)
+                }
+            }
+            let pkg = Box::new(source).download_now(summary.package_id(), config)?;
             Ok(pkg)
         }
         None => {
@@ -599,6 +610,7 @@ 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 +624,7 @@ 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

@@ -2478,15 +2478,7 @@ fn install_incompat_msrv() {
     cargo_process("install foo")
         .with_stderr("\
 [UPDATING] `dummy-registry` index
-[DOWNLOADING] crates ...
-[DOWNLOADED] foo v0.2.0 (registry `[..]`)
-[INSTALLING] foo v0.2.0
-[ERROR] failed to compile `foo v0.2.0`, intermediate artifacts can be found at `[..]`.
-To reuse those artifacts with a future compilation, set the environment variable `CARGO_TARGET_DIR` to that path.
-
-Caused by:
-  package `foo v0.2.0` cannot be built because it requires rustc 1.9876.0 or newer, while the currently active rustc version is [..]
-  Try re-running cargo install with `--locked`
+[ERROR] cannot install package `foo 0.2.0`, it requires rustc 1.9876.0 or newer, while the currently active rustc version is [..]
 ")
         .with_status(101).run();
 }