Document why the stored options carry no user header

The resolver strips both header spellings from cpp_options and only
$compile() reinserts the selected one, so precompile_cpp_options_ never
carries a header. That is deliberate but not self-evident: storing it
there would store a WSL-safe path, which the next $compile() would then
select as its user_header, and that is a host path by design because
file.exists() on a WSL-safe path fails under WSLv1.

Also records in NEWS that a dry run no longer sets $cpp_options(),
alongside the existing note about $hpp_file().

Author: jgabry

NEWS.md

@@ -58,9 +58,9 @@ pre-compiled Stan model" on a model that had compiled itself. (#1235)
 * A failed compilation no longer moves `$exe_file()` or replaces the generated
 C++ used by `$hpp_file()` and `fit$init_model_methods()`. Previously a failure
 at the C++ stage left the old executable paired with model methods generated
-from the new program. A `dry_run = TRUE` compilation likewise no longer moves
-`$hpp_file()`, which previously pointed at a temporary file it never wrote.
-(#1235)
+from the new program. A `dry_run = TRUE` compilation likewise no longer records
+`$cpp_options()` or moves `$hpp_file()`, which previously pointed at a temporary
+file it never wrote. (#1235)
 * `$compile()` now errors if the newly compiled executable cannot be installed,
 restoring the previous executable. Previously the replacement was unchecked, so
 a failure could silently leave the model with no executable at all. (#1235)

R/model.R

@@ -282,6 +282,12 @@ CmdStanModel <- R6::R6Class(
           # once; warning here as well would double up.
           warn_user_header_conflict(resolved_header$conflict)
         }
+        # Deliberately without the header: the resolver strips both spellings
+        # and only $compile() reinserts the selected one. Storing it here would
+        # mean storing a WSL-safe path, which the next $compile() would pick up
+        # as its host-path `user_header` and fail to find on WSLv1 -- the hazard
+        # the comment at the resolver call in $compile() describes. user_header_
+        # below is the single source instead.
         private$precompile_cpp_options_ <- resolved_header$cpp_options
         # Prepopulating this also keeps the first $compile() from seeing a
         # change of header and rebuilding an already-current executable.