records: Improve reporting of "invalid field specifier" errors.

Previously users would just see:

  error: invalid field specifier

without source location or hints.

* guix/records.scm (expand): Add optional 'parent-form' parameter and
pass it to 'syntax-violation' when it is true.
(make-syntactic-constructor): Pass S as a third argument to
'report-invalid-field-specifier'.
* guix/ui.scm (report-load-error): For 'syntax-error', show SUBFORM or
FORM in the message.
* tests/records.scm ("define-record-type* & wrong field specifier"): Add
a 'subform' parameter and adjust test accordingly.
("define-record-type* & wrong field specifier, identifier"): New test.
* tests/guix-system.sh: Add test.

2 files changed, 17 insertions, 7 deletions

diff --git a/guix/records.scm b/guix/records.scm
index 99507dc384..4bda5426a3 100644
--- a/guix/records.scm
+++ b/guix/records.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2018 Mark H Weaver <mhw@netris.org>
 ;;;
 ;;; This file is part of GNU Guix.
@@ -70,14 +70,22 @@ interface\" (ABI) for TYPE is equal to COOKIE."
                  "~a: record ABI mismatch; recompilation needed"
                  (list #,type) '()))))
 
-  (define (report-invalid-field-specifier name bindings)
-    "Report the first invalid binding among BINDINGS."
+  (define* (report-invalid-field-specifier name bindings
+                                           #:optional parent-form)
+    "Report the first invalid binding among BINDINGS.  PARENT-FORM is used for
+error-reporting purposes."
     (let loop ((bindings bindings))
       (syntax-case bindings ()
         (((field value) rest ...)                   ;good
          (loop #'(rest ...)))
         ((weird _ ...)                              ;weird!
-         (syntax-violation name "invalid field specifier" #'weird)))))
+         ;; WEIRD may be an identifier, thus lacking source location info, and
+         ;; BINDINGS is a list, also lacking source location info.  Hopefully
+         ;; PARENT-FORM provides source location info.
+         (apply syntax-violation name "invalid field specifier"
+                (if parent-form
+                    (list parent-form #'weird)
+                    (list #'weird)))))))
 
   (define (report-duplicate-field-specifier name ctor)
     "Report the first duplicate identifier among the bindings in CTOR."
@@ -233,7 +241,8 @@ of TYPE matches the expansion-time ABI."
             ;; Report precisely which one is faulty, instead of letting the
             ;; "source expression failed to match any pattern" error.
             (report-invalid-field-specifier 'name
-                                            #'(bindings (... ...))))))))))
+                                            #'(bindings (... ...))
+                                            s))))))))
 
 (define-syntax-rule (define-field-property-predicate predicate property)
   "Define PREDICATE as a procedure that takes a syntax object and, when passed
diff --git a/guix/ui.scm b/guix/ui.scm
index b99a9e59f5..01aeee49eb 100644
--- a/guix/ui.scm
+++ b/guix/ui.scm
@@ -372,9 +372,10 @@ ARGS is the list of arguments received by the 'throw' handler."
            (format (current-error-port) (G_ "~amissing closing parenthesis~%")
                    location))
          (apply throw args)))
-    (('syntax-error proc message properties form . rest)
+    (('syntax-error proc message properties form subform . rest)
      (let ((loc (source-properties->location properties)))
-       (report-error loc (G_ "~a~%") message)))
+       (report-error loc (G_ "~s: ~a~%")
+                     (or subform form) message)))
     (('unbound-variable _ ...)
      (report-unbound-variable-error args #:frame frame))
     (((or 'srfi-34 '%exception) obj)