Emit warning when called during precompilation

src/internals.jl

@@ -75,6 +75,28 @@ function ensureexecutable(path::String)
     path
 end
 
+function warn_if_precompiling()
+    if ccall(:jl_generating_output, Cint, ()) != 0
+        @noinline (function ()
+                       st = stacktrace(backtrace())
+                       naughtyline = findfirst(sf -> sf.file != Symbol(@__FILE__), st)
+                       naughtysf = st[something(naughtyline, 1)]
+                       # Once we've raised compat to `VERSION >= v"1.8"`, use this version:
+                       #
+                       # @warn """A base directory is being computed during precompilation.
+                       #          This is dangerous, as results depend on the live system configuration.
+                       #
+                       #          It is recommended that you invoke BaseDirs as required in
+                       #          function bodies, rather than at the top level. Calls are very
+                       #          cheap, and you can always pass the result of a live call around.
+                       #          """ _module="BaseDirs" _file=String(naughtysf.file) _line=naughtysf.line
+                       #
+                       # Until then, we can't have a multiline string :'(
+                       @warn "A base directory is being computed during precompilation.\nThis is dangerous, as results depend on the live system configuration.\n\nIt is recommended that you invoke BaseDirs as required in\nfunction bodies, rather than at the top level. Calls are very\ncheap, and you can always pass the result of a live call around." _module="BaseDirs" _file=String(naughtysf.file) _line=naughtysf.line
+                   end)()
+    end
+end
+
 function resolvedirpath(basedir::String, pathcomponents::Union{Tuple, AbstractVector}; create::Bool=false)
     create && ensurebasedir(basedir)
     if isempty(pathcomponents)
@@ -108,8 +130,10 @@ macro defaccessor(fnname::Symbol, var::Union{Symbol, Expr})
         :resolvedirpath
     end
     quote
-        $(esc(fnname))(pathcomponents...; kwargs...) =
+        function $(esc(fnname))(pathcomponents...; kwargs...)
+            $warn_if_precompiling()
             $resolver($dirvar, pathcomponents; kwargs...)
+        end
         $(esc(fnname))(project::BaseDirs.Project, pathcomponents...; kwargs...) =
             $(esc(fnname))(BaseDirs.projectpath(project, $dirvar), pathcomponents...; kwargs...)
     end