load tbb stub library on windows again

NEWS.md

@@ -1,5 +1,12 @@
 # RcppParallel (development version)
 
+* On Windows, RcppParallel once again loads its compatibility stub library
+  (`tbb.dll`) when the package is loaded. Packages linking with `-ltbb`
+  (e.g. via StanHeaders) record a load-time dependency on `tbb.dll`, which
+  can only be resolved if RcppParallel has already loaded it; with
+  RcppParallel 6.0.0, such packages would fail to load with "LoadLibrary
+  failure: The specified module could not be found".
+
 * The bundled oneTBB headers now guard against GCC's `<cpuid.h>` being
   included before `<intrin.h>` on Windows (mingw). Previously, translation
   units including `<cpuid.h>` before any TBB header would fail to compile,

R/zzz.R

@@ -11,10 +11,23 @@
 .tbbMallocProxyDllInfo <- NULL
 
 loadTbbLibrary <- function(name) {
-   # TBB is statically linked on Windows
+
+   # On Windows, TBB is statically linked into RcppParallel.dll, but we
+   # still ship a stub tbb.dll for compatibility with packages linking via
+   # '-ltbb' (e.g. through StanHeaders). Such packages record a load-time
+   # dependency on 'tbb.dll', which the Windows loader can only resolve if
+   # we've already loaded it -- the library directory itself is not on the
+   # DLL search path.
    if (is_windows()) {
-      return(NULL)
+
+      path <- file.path(tbbRoot(), paste0(name, ".dll"))
+      if (!file.exists(path))
+         return(NULL)
+
+      return(dyn.load(path, local = FALSE, now = TRUE))
+
    }
+
    path <- tbbLibraryPath(name)
    if (is.null(path))
       return(NULL)