Try installing exact versions before updating

When an exact version is being installed, if we already have that
version from the index, we don't need to update the index before
installing it. Don't do this if it's not an exact version, because the
update may find us a newer version.

This is particularly useful for scripts which unconditionally run
`cargo install some-crate --version=1.2.3`. Before install-update, I
wrote a crate to do this
(https://crates.io/crates/cargo-ensure-installed) which I'm trying to
replace with just `cargo install`, but the extra latency of updating the
index for a no-op is noticeable.

This introduces an interesting edge-case around yanked crates; the
yanked-ness of crates will no longer change on install for exact version
matches which were already downloaded. This feels niche enough to
hopefully not matter...

src/cargo/ops/cargo_install.rs

@@ -151,7 +151,7 @@ fn install_one(
             krate,
             vers,
             config,
-            true,
+            NeedsUpdate::True,
             &mut |git| git.read_packages(),
         )?
     } else if source_id.is_path() {
@@ -180,7 +180,7 @@ fn install_one(
             }
         }
         src.update()?;
-        select_pkg(src, krate, vers, config, false, &mut |path| {
+        select_pkg(src, krate, vers, config, NeedsUpdate::False, &mut |path| {
             path.read_packages()
         })?
     } else {
@@ -189,7 +189,11 @@ fn install_one(
             krate,
             vers,
             config,
-            is_first_install,
+            if is_first_install {
+                NeedsUpdate::TryWithoutFirst
+            } else {
+                NeedsUpdate::False
+            },
             &mut |_| {
                 bail!(
                     "must specify a crate to install from \

src/cargo/ops/cargo_uninstall.rs

@@ -85,7 +85,7 @@ fn uninstall_cwd(root: &Filesystem, bins: &[String], config: &Config) -> CargoRe
     let tracker = InstallTracker::load(config, root)?;
     let source_id = SourceId::for_path(config.cwd())?;
     let src = path_source(source_id, config)?;
-    let pkg = select_pkg(src, None, None, config, true, &mut |path| {
+    let pkg = select_pkg(src, None, None, config, NeedsUpdate::True, &mut |path| {
         path.read_packages()
     })?;
     let pkgid = pkg.package_id();

src/cargo/ops/common_for_install_and_uninstall.rs

@@ -525,13 +525,20 @@ pub fn path_source(source_id: SourceId, config: &Config) -> CargoResult<PathSour
     Ok(PathSource::new(&path, source_id, config))
 }
 
+#[derive(Clone, Copy, PartialEq, Eq)]
+pub enum NeedsUpdate {
+    True,
+    False,
+    TryWithoutFirst,
+}
+
 /// Gets a Package based on command-line requirements.
 pub fn select_pkg<'a, T>(
     mut source: T,
     name: Option<&str>,
     vers: Option<&str>,
     config: &Config,
-    needs_update: bool,
+    needs_update: NeedsUpdate,
     list_all: &mut dyn FnMut(&mut T) -> CargoResult<Vec<Package>>,
 ) -> CargoResult<Package>
 where
@@ -542,7 +549,7 @@ where
     // with other global Cargos
     let _lock = config.acquire_package_cache_lock()?;
 
-    if needs_update {
+    if let NeedsUpdate::True = needs_update {
         source.update()?;
     }
 
@@ -603,8 +610,21 @@ where
             vers
         };
         let dep = Dependency::parse_no_deprecated(name, vers_spec, source.source_id())?;
-        let deps = source.query_vec(&dep)?;
-        match deps.iter().map(|p| p.package_id()).max() {
+        let deps = (|| {
+            if let Some(vers_spec) = vers_spec {
+                if semver::VersionReq::parse(vers_spec).unwrap().is_exact() {
+                    let deps = source.query_vec(&dep)?;
+                    if deps.len() == 1 {
+                        return Ok(deps);
+                    }
+                }
+            }
+            if needs_update != NeedsUpdate::False {
+                source.update()?;
+            }
+            source.query_vec(&dep)
+        })();
+        match deps?.iter().map(|p| p.package_id()).max() {
             Some(pkgid) => {
                 let pkg = Box::new(&mut source).download_now(pkgid, config)?;
                 Ok(pkg)

tests/testsuite/install_upgrade.rs

@@ -225,8 +225,7 @@ fn ambiguous_version_no_longer_allowed() {
     cargo_process("install foo --version=1.0")
         .with_stderr(
             "\
-[UPDATING] `[..]` index
-[ERROR] the `--vers` provided, `1.0`, is not a valid semver version: cannot parse '1.0' as a semver
+error: the `--vers` provided, `1.0`, is not a valid semver version: cannot parse '1.0' as a semver
 
 if you want to specify semver range, add an explicit qualifier, like ^1.0
 ",