Include allowed features in fingerprint

If this weren't the case, a user that compiled, and then re-ran cargo
with a particular set of allowed features would not see any breakage
even if other unstable features were used.

The downside of this is that changing allow-features causes a full
re-compile, but that still _seems_ like the right thing to do.

Author: Jon Gjengset

src/cargo/core/compiler/fingerprint.rs

@@ -1327,14 +1327,18 @@ fn calculate_normal(cx: &mut Context<'_, '_>, unit: &Unit) -> CargoResult<Finger
     // Include metadata since it is exposed as environment variables.
     let m = unit.pkg.manifest().metadata();
     let metadata = util::hash_u64((&m.authors, &m.description, &m.homepage, &m.repository));
-    let config = if unit.mode.is_doc() && cx.bcx.config.cli_unstable().rustdoc_map {
-        cx.bcx
-            .config
-            .doc_extern_map()
-            .map_or(0, |map| util::hash_u64(map))
-    } else {
-        0
-    };
+    let mut config = 0u64;
+    if unit.mode.is_doc() && cx.bcx.config.cli_unstable().rustdoc_map {
+        config = config.wrapping_add(
+            cx.bcx
+                .config
+                .doc_extern_map()
+                .map_or(0, |map| util::hash_u64(map)),
+        );
+    }
+    if let Some(allow_features) = &cx.bcx.config.cli_unstable().allow_features {
+        config = config.wrapping_add(util::hash_u64(allow_features));
+    }
     let compile_kind = unit.kind.fingerprint_hash();
     Ok(Fingerprint {
         rustc: util::hash_u64(&cx.bcx.rustc().verbose_version),

src/cargo/core/features.rs

@@ -92,7 +92,7 @@
 //!    of that page. Update the rest of the documentation to add the new
 //!    feature.
 
-use std::collections::HashSet;
+use std::collections::BTreeSet;
 use std::env;
 use std::fmt;
 use std::str::FromStr;
@@ -554,7 +554,7 @@ impl Features {
 #[serde(default, rename_all = "kebab-case")]
 pub struct CliUnstable {
     pub print_im_a_teapot: bool,
-    pub allow_features: Option<HashSet<String>>,
+    pub allow_features: Option<BTreeSet<String>>,
 
     pub unstable_options: bool,
     pub no_index_update: bool,