Add escape hatch for nightly version hashing.

Author: ehuss

src/cargo/core/compiler/context/compilation_files.rs

@@ -644,7 +644,7 @@ fn compute_metadata<'a, 'cfg>(
 
 fn hash_rustc_version(bcx: &BuildContext<'_, '_>, hasher: &mut SipHasher) {
     let vers = &bcx.rustc().version;
-    if vers.pre.is_empty() {
+    if vers.pre.is_empty() || bcx.config.cli_unstable().separate_nightlies {
         // For stable, keep the artifacts separate. This helps if someone is
         // testing multiple versions, to avoid recompiles.
         bcx.rustc().verbose_version.hash(hasher);

src/cargo/core/features.rs

@@ -343,6 +343,7 @@ pub struct CliUnstable {
     pub jobserver_per_rustc: bool,
     pub features: Option<Vec<String>>,
     pub crate_versions: bool,
+    pub separate_nightlies: bool,
 }
 
 impl CliUnstable {
@@ -420,6 +421,7 @@ impl CliUnstable {
             "jobserver-per-rustc" => self.jobserver_per_rustc = parse_empty(k, v)?,
             "features" => self.features = Some(parse_features(v)),
             "crate-versions" => self.crate_versions = parse_empty(k, v)?,
+            "separate-nightlies" => self.separate_nightlies = parse_empty(k, v)?,
             _ => bail!("unknown `-Z` flag specified: {}", k),
         }