Change the default stdlib location for Linux packaging (#21328)

* Correctly evaluate the Nim prefix on Posix

* Document new packaging layout

compiler/options.nim

@@ -717,22 +717,24 @@ proc getPrefixDir*(conf: ConfigRef): AbsoluteDir =
   ##  clone or using installed nim, so that these exist: `result/doc/advopt.txt`
   ## and `result/lib/system.nim`
   if not conf.prefixDir.isEmpty: result = conf.prefixDir
-  else: result = AbsoluteDir splitPath(getAppDir()).head
+  else:
+    let binParent = AbsoluteDir splitPath(getAppDir()).head
+    when defined(posix):
+      if binParent == AbsoluteDir"/usr":
+        result = AbsoluteDir"/usr/lib/nim"
+      elif binParent == AbsoluteDir"/usr/local":
+        result = AbsoluteDir"/usr/local/lib/nim"
+      else:
+        result = binParent
+    else:
+      result = binParent
 
 proc setDefaultLibpath*(conf: ConfigRef) =
   # set default value (can be overwritten):
   if conf.libpath.isEmpty:
     # choose default libpath:
     var prefix = getPrefixDir(conf)
-    when defined(posix):
-      if prefix == AbsoluteDir"/usr":
-        conf.libpath = AbsoluteDir"/usr/lib/nim"
-      elif prefix == AbsoluteDir"/usr/local":
-        conf.libpath = AbsoluteDir"/usr/local/lib/nim"
-      else:
-        conf.libpath = prefix / RelativeDir"lib"
-    else:
-      conf.libpath = prefix / RelativeDir"lib"
+    conf.libpath = prefix / RelativeDir"lib"
 
     # Special rule to support other tools (nimble) which import the compiler
     # modules and make use of them.

doc/packaging.md

@@ -69,8 +69,11 @@ Hints on the build process:
 
 What to install:
 
-- The expected stdlib location is /usr/lib/nim
-- Global configuration files under /etc/nim
+- The expected stdlib location is `/usr/lib/nim/lib`, previously it was just `/usr/lib/nim`
+- `nimdoc.css` and `nimdoc.cls` from the `doc` folder should go into `/usr/lib/nim/doc/`
+- `tools/nim-gdb.py` should go into `/usr/lib/nim/tools/`
+- `tools/dochack/dochack.js` should be installed to `/usr/lib/nim/tools/dochack/`
+- Global configuration files under `/etc/nim`
 - Optionally: manpages, documentation, shell completion
 - When installing documentation, .idx files are not required
 - The "compiler" directory contains compiler sources and should not be part of the compiler binary package