From e8bde450d05908f70ab2c82d9d24f0807c56a94a Mon Sep 17 00:00:00 2001
From: Emilio Jesus Gallego Arias
Date: Wed, 4 Mar 2020 21:39:42 -0500
Subject: [exn] [tactics] improve backtraces on monadic errors

Current backtraces for tactics leave a bit to desire, for example
given the program:

```coq
Lemma u n : n + 0 = n.
rewrite plus_O_n.
```

the backtrace stops at:

```
Found no subterm matching "0 + ?M160" in the current goal.
Called from file "proofs/proof.ml", line 381, characters 4-42
Called from file "tactics/pfedit.ml", line 102, characters 31-58
Called from file "plugins/ltac/g_ltac.mlg", line 378, characters 8-84
```

Backtrace information `?info` is as of today optional in some tactics,
such as `tclZERO`, it doesn't cost a lot  however to reify backtrace
information indeed in `tclZERO` and provide backtraces for all tactic
errors. The cost should be small if we are not in debug mode.

The backtrace for the failed rewrite is now:

```
Found no subterm matching "0 + ?M160" in the current goal.
Raised at file "pretyping/unification.ml", line 1827, characters 14-73
Called from file "pretyping/unification.ml", line 1929, characters 17-53
Called from file "pretyping/unification.ml", line 1948, characters 22-72
Called from file "pretyping/unification.ml", line 2020, characters 14-56
Re-raised at file "pretyping/unification.ml", line 2021, characters 66-73
Called from file "proofs/clenv.ml", line 254, characters 12-58
Called from file "proofs/clenvtac.ml", line 95, characters 16-53
Called from file "engine/proofview.ml", line 1110, characters 40-46
Called from file "engine/proofview.ml", line 1115, characters 10-34
Re-raised at file "clib/exninfo.ml", line 82, characters 4-38
Called from file "proofs/proof.ml", line 381, characters 4-42
Called from file "tactics/pfedit.ml", line 102, characters 31-58
Called from file "plugins/ltac/g_ltac.mlg", line 378, characters 8-84
```

which IMO is much better.
---
 clib/exninfo.ml  | 7 +++++++
 clib/exninfo.mli | 2 ++
 2 files changed, 9 insertions(+)

(limited to 'clib')

diff --git a/clib/exninfo.ml b/clib/exninfo.ml
index 621f7e615f..07b7f47529 100644
--- a/clib/exninfo.ml
+++ b/clib/exninfo.ml
@@ -117,3 +117,10 @@ let capture e =
     e, add info backtrace_info bt
   else
     e, info e
+
+let reify () =
+  if !is_recording then
+    let bt = Printexc.get_callstack 50 in
+    add null backtrace_info bt
+  else
+    null
diff --git a/clib/exninfo.mli b/clib/exninfo.mli
index 55f0662431..08395f59f4 100644
--- a/clib/exninfo.mli
+++ b/clib/exninfo.mli
@@ -79,3 +79,5 @@ val capture : exn -> iexn
 
 val iraise : iexn -> 'a
 (** Raise the given enriched exception. *)
+
+val reify : unit -> info
-- 
cgit v1.2.3