bug#26107: Inaccurate location info for unbound-variable errors

[Ludovic Courtès]

Hi!

In 2.1.8/2.2.0, walking up the stack to find the origin of an unbound
variable error yields inaccurate location info.  Consider this:

--8<---------------cut here---------------start------------->8---
(list 'a
      (symbol->string 'b)
      (symbol->string 'c)  ;← unbound variable error reported here
      (FOO 'bar)           ;… instead of here
      3
      4)
--8<---------------cut here---------------end--------------->8---

We get:

--8<---------------cut here---------------start------------->8---
$ guild compile -t value t.scm
Backtrace:
          18 (apply-smob/1 #<catch-closure 1312aa0>)
In ice-9/boot-9.scm:
    710:2 17 (call-with-prompt _ _ #<procedure default-prompt-handler (k proc)>)
In ice-9/eval.scm:
    619:8 16 (_ #(#(#<directory (guile-user) 13ba1e0>)))
In /gnu/store/4xir0dlzxl7h5v7va9rz8x3mvarlv2fd-profile/bin/guild:
    72:17 15 (main _)
In srfi/srfi-1.scm:
    640:9 14 (for-each #<procedure 128be40 at scripts/compile.scm:248:14 
(file)> ("t.scm"))
In scripts/compile.scm:
   251:26 13 (_ _)
In system/base/target.scm:
     57:6 12 (with-target _ _)
In system/base/compile.scm:
    152:6 11 (compile-file "t.scm" #:output-file _ #:from _ #:to _ #:env _ 
#:opts _ #:canonicalization _)
     43:4 10 (call-once _)
In ice-9/boot-9.scm:
    846:4  9 (with-throw-handler _ _ _)
In system/base/compile.scm:
    59:11  8 (_)
   155:11  7 (_ #<closed: file 160a0e0>)
   224:14  6 (read-and-compile _ #:from _ #:to _ #:env _ #:opts _)
    255:6  5 (compile _ #:from _ #:to _ #:env _ #:opts _)
   183:32  4 (compile-fold _ #<tree-il (call (toplevel list) (const a) (call 
(toplevel symbol->string) (const b)) (call (toplevel symbol->string) (const c)) 
(call (toplevel FO…> …)
In ice-9/boot-9.scm:
   2309:4  3 (save-module-excursion #<procedure 1345d60 at 
language/bytecode/spec.scm:33:9 ()>)
In language/bytecode/spec.scm:
    35:19  2 (_)
In t.scm:
      3:6  1 (_)       ;← incorrect line number! (expected line 4)
In ice-9/boot-9.scm:
   757:25  0 (dispatch-exception _ _ _)

ice-9/boot-9.scm:757:25: In procedure dispatch-exception:
ice-9/boot-9.scm:757:25: In procedure module-lookup: Unbound variable: FOO
--8<---------------cut here---------------end--------------->8---

It Would Be Nice if we could get the right location info here, though I
understand this is a corner case.

Thoughts?

Ludo’.