vendor: implement --explicit-version

Implement --explicit-version from standalone cargo-vendor. This helps with
vendoring performance as it avoids redundantly deleting and re-copying
already vendored packages.

For example, when re-vendoring cargo's dependencies it makes a big
improvement on wallclock time. For initial vendoring it makes no
difference, but re-vendoring (ie, when most or all dependencies haven't
changed) without explicit versions is actually slightly slower (5.8s ->
6s), but with explicit versions it goes from 5.8s -> 1.6s.

Timings:

Without explicit versions, initial vendor
real	0m5.810s
user	0m0.924s
sys	0m2.491s

Re-vendor:
real	0m6.083s
user	0m0.937s
sys	0m2.654s

With explicit versions, initial vendor:
real	0m5.810s
user	0m0.937s
sys	0m2.461s

Re-vendor:
real	0m1.567s
user	0m0.578s
sys	0m0.967s

The summaries of syscalls executed shows why:

Revendoring without explicit versions:
```
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 25.17    1.104699          18     59432      1065 openat
 19.86    0.871574          21     41156     13825 unlink
 13.64    0.598739           2    210510           lstat
  9.02    0.395948          29     13208           copy_file_range
  8.00    0.351242          11     30245           read
  6.36    0.279005           3     72487      4476 statx
  5.35    0.235027           6     37219           write
  4.02    0.176267           3     58368           close
```
with explicit versions:
```
 29.38    0.419068          15     27798     13825 unlink
 25.52    0.364021           1    209586           lstat
 20.67    0.294788          16     17967      1032 openat
 10.42    0.148586           4     35646           write
  3.53    0.050350           3     13825           chmod
  3.14    0.044786           2     16701      1622 statx
  2.19    0.031171           1     16936           close
  1.86    0.026538          24      1078           rmdir
```

Specifically, there are a lot fewer opens, copy_file_ranges, and unlinks.

Author: jsgf

src/bin/cargo/commands/vendor.rs

@@ -27,6 +27,12 @@ pub fn cli() -> App {
                 .help("Respect `[source]` config in `.cargo/config`")
                 .multiple(true),
         )
+        .arg(
+            Arg::with_name("explicit-version")
+                .short("-x")
+                .long("explicit-version")
+                .help("Always include version in subdir name"),
+        )
         .arg(
             Arg::with_name("no-merge-sources")
                 .long("no-merge-sources")
@@ -42,12 +48,6 @@ pub fn cli() -> App {
                 .long("only-git-deps")
                 .hidden(true),
         )
-        .arg(
-            Arg::with_name("explicit-version")
-                .short("-x")
-                .long("explicit-version")
-                .hidden(true),
-        )
         .arg(
             Arg::with_name("disallow-duplicates")
                 .long("disallow-duplicates")
@@ -85,8 +85,6 @@ pub fn exec(config: &mut Config, args: &ArgMatches<'_>) -> CliResult {
         Some("--relative-path")
     } else if args.is_present("only-git-deps") {
         Some("--only-git-deps")
-    } else if args.is_present("explicit-version") {
-        Some("--explicit-version")
     } else if args.is_present("disallow-duplicates") {
         Some("--disallow-duplicates")
     } else {
@@ -116,6 +114,7 @@ https://github.com/rust-lang/cargo/issues/new
         &ops::VendorOptions {
             no_delete: args.is_present("no-delete"),
             destination: &path,
+            explicit_version: args.is_present("explicit-version"),
             extra: args
                 .values_of_os("tomls")
                 .unwrap_or_default()

src/cargo/ops/vendor.rs

@@ -14,6 +14,7 @@ use std::path::{Path, PathBuf};
 
 pub struct VendorOptions<'a> {
     pub no_delete: bool,
+    pub explicit_version: bool,
     pub destination: &'a Path,
     pub extra: Vec<PathBuf>,
 }
@@ -186,7 +187,7 @@ fn sync(
             .parent()
             .expect("manifest_path should point to a file");
         let max_version = *versions[&id.name()].iter().rev().next().unwrap().0;
-        let dir_has_version_suffix = id.version() != max_version;
+        let dir_has_version_suffix = opts.explicit_version || id.version() != max_version;
         let dst_name = if dir_has_version_suffix {
             // Eg vendor/futures-0.1.13
             format!("{}-{}", id.name(), id.version())

tests/testsuite/vendor.rs

@@ -88,6 +88,51 @@ fn two_versions() {
     p.cargo("build").run();
 }
 
+#[cargo_test]
+fn two_explicit_versions() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.1.0"
+
+                [dependencies]
+                bitflags = "0.8.0"
+                bar = { path = "bar" }
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .file(
+            "bar/Cargo.toml",
+            r#"
+                [package]
+                name = "bar"
+                version = "0.1.0"
+
+                [dependencies]
+                bitflags = "0.7.0"
+            "#,
+        )
+        .file("bar/src/lib.rs", "")
+        .build();
+
+    Package::new("bitflags", "0.7.0").publish();
+    Package::new("bitflags", "0.8.0").publish();
+
+    p.cargo("vendor --respect-source-config --explicit-version")
+        .run();
+
+    let lock = p.read_file("vendor/bitflags-0.8.0/Cargo.toml");
+    assert!(lock.contains("version = \"0.8.0\""));
+    let lock = p.read_file("vendor/bitflags-0.7.0/Cargo.toml");
+    assert!(lock.contains("version = \"0.7.0\""));
+
+    add_vendor_config(&p);
+    p.cargo("build").run();
+}
+
 #[cargo_test]
 fn help() {
     let p = project().build();