Add -Zskip-rustdoc-fingerprint

This is a hidden flag intended to only be used by rustbuild which will
skip the rustdoc fingerprint check. rustbuild does some funky things
with sharing the doc directory across multiple target directories via
symlinks, and that causes problems where after building in one target
directory, then switching to the second one, it will clear the contents.

src/cargo/core/compiler/build_context/target_info.rs

@@ -780,6 +780,9 @@ impl RustDocFingerprint {
     /// versions of the `js/html/css` files that `rustdoc` autogenerates which do not have
     /// any versioning.
     pub fn check_rustdoc_fingerprint(cx: &Context<'_, '_>) -> CargoResult<()> {
+        if cx.bcx.config.cli_unstable().skip_rustdoc_fingerprint {
+            return Ok(());
+        }
         let actual_rustdoc_target_data = RustDocFingerprint {
             rustc_vv: cx.bcx.rustc().verbose_version.clone(),
         };

src/cargo/core/features.rs

@@ -542,8 +542,8 @@ macro_rules! unstable_cli_options {
     (
         $(
             $(#[$meta:meta])?
-            $element: ident: $ty: ty = ($help: expr )
-        ),*
+            $element: ident: $ty: ty = ($help: expr ),
+        )*
     ) => {
         /// A parsed representation of all unstable flags that Cargo accepts.
         ///
@@ -603,7 +603,8 @@ unstable_cli_options!(
     terminal_width: Option<Option<usize>>  = ("Provide a terminal width to rustc for error truncation"),
     timings: Option<Vec<String>>  = ("Display concurrency information"),
     unstable_options: bool = ("Allow the usage of unstable options"),
-    weak_dep_features: bool = ("Allow `dep_name?/feature` feature syntax")
+    weak_dep_features: bool = ("Allow `dep_name?/feature` feature syntax"),
+    skip_rustdoc_fingerprint: bool = (HIDDEN),
 );
 
 const STABILIZED_COMPILE_PROGRESS: &str = "The progress bar is now always \
@@ -813,6 +814,7 @@ impl CliUnstable {
             "weak-dep-features" => self.weak_dep_features = parse_empty(k, v)?,
             "extra-link-arg" => self.extra_link_arg = parse_empty(k, v)?,
             "credential-process" => self.credential_process = parse_empty(k, v)?,
+            "skip-rustdoc-fingerprint" => self.skip_rustdoc_fingerprint = parse_empty(k, v)?,
             "compile-progress" => stabilized_warn(k, "1.30", STABILIZED_COMPILE_PROGRESS),
             "offline" => stabilized_err(k, "1.36", STABILIZED_OFFLINE)?,
             "cache-messages" => stabilized_warn(k, "1.40", STABILIZED_CACHE_MESSAGES),