Add install-upgrade.

src/bin/cargo/cli.rs

@@ -35,6 +35,7 @@ Available unstable (nightly-only) flags:
     -Z offline          -- Offline mode that does not perform network requests
     -Z unstable-options -- Allow the usage of unstable options such as --registry
     -Z config-profile   -- Read profiles from .cargo/config files
+    -Z install-upgrade  -- `cargo install` will upgrade instead of failing
 
 Run with 'cargo -Z [FLAG] [SUBCOMMAND]'"
         );

src/bin/cargo/commands/install.rs

@@ -46,6 +46,10 @@ pub fn cli() -> App {
         ))
         .arg_jobs()
         .arg(opt("force", "Force overwriting existing crates or binaries").short("f"))
+        .arg(opt(
+            "no-track",
+            "Do not save tracking information (unstable)",
+        ))
         .arg_features()
         .arg(opt("debug", "Build in debug mode instead of release mode"))
         .arg_targets_bins_examples(
@@ -148,6 +152,12 @@ pub fn exec(config: &mut Config, args: &ArgMatches<'_>) -> CliResult {
     let version = args.value_of("version");
     let root = args.value_of("root");
 
+    if args.is_present("no-track") && !config.cli_unstable().install_upgrade {
+        Err(failure::format_err!(
+            "`--no-track` flag is unstable, pass `-Z install-upgrade` to enable it"
+        ))?;
+    };
+
     if args.is_present("list") {
         ops::install_list(root, config)?;
     } else {
@@ -159,6 +169,7 @@ pub fn exec(config: &mut Config, args: &ArgMatches<'_>) -> CliResult {
             version,
             &compile_opts,
             args.is_present("force"),
+            args.is_present("no-track"),
         )?;
     }
     Ok(())

src/cargo/core/features.rs

@@ -328,6 +328,7 @@ pub struct CliUnstable {
     pub config_profile: bool,
     pub dual_proc_macros: bool,
     pub mtime_on_use: bool,
+    pub install_upgrade: bool,
 }
 
 impl CliUnstable {
@@ -372,6 +373,7 @@ impl CliUnstable {
             "config-profile" => self.config_profile = true,
             "dual-proc-macros" => self.dual_proc_macros = true,
             "mtime-on-use" => self.mtime_on_use = true,
+            "install-upgrade" => self.install_upgrade = true,
             _ => failure::bail!("unknown `-Z` flag specified: {}", k),
         }