5 files changed, 82 insertions, 18 deletions
diff --git a/lib/nimbase.h b/lib/nimbase.h
index eb750864a..5b0dcc2c8 100644
--- a/lib/nimbase.h
+++ b/lib/nimbase.h
@@ -489,6 +489,7 @@ struct TFrame_ {
   NCSTRING filename;
   NI16 len;
   NI16 calldepth;
+  NI frameMsgLen;
 };
 
 #define NIM_POSIX_INIT  __attribute__((constructor))
diff --git a/lib/std/stackframes.nim b/lib/std/stackframes.nim
new file mode 100644
index 000000000..dbd866536
--- /dev/null
+++ b/lib/std/stackframes.nim
@@ -0,0 +1,30 @@
+const NimStackTrace = compileOption("stacktrace")
+const NimStackTraceMsgs = compileOption("stacktraceMsgs")
+
+template procName*(): string =
+  ## returns current C/C++ function name
+  when defined(c) or defined(cpp):
+    var name {.inject.}: cstring
+    {.emit: "`name` = __func__;".}
+    $name
+
+template getPFrame*(): PFrame =
+  ## avoids a function call (unlike `getFrame()`)
+  block:
+    when NimStackTrace:
+      var framePtr {.inject.}: PFrame
+      {.emit: "`framePtr` = &FR_;".}
+      framePtr
+
+template setFrameMsg*(msg: string, prefix = " ") =
+  ## attach a msg to current `PFrame`. This can be called multiple times
+  ## in a given PFrame. Noop unless passing --stacktraceMsgs and --stacktrace
+  when NimStackTrace and NimStackTraceMsgs:
+    block:
+      var fr {.inject.}: PFrame
+      {.emit: "`fr` = &FR_;".}
+      # consider setting a custom upper limit on size (analog to stack overflow)
+      frameMsgBuf.setLen fr.frameMsgLen
+      frameMsgBuf.add prefix
+      frameMsgBuf.add msg
+      fr.frameMsgLen += prefix.len + msg.len
diff --git a/lib/system.nim b/lib/system.nim
index cd941b19f..469821fab 100644
--- a/lib/system.nim
+++ b/lib/system.nim
@@ -54,6 +54,23 @@ type
 
 include "system/basic_types"
 
+
+proc compileOption*(option: string): bool {.
+  magic: "CompileOption", noSideEffect.}
+  ## Can be used to determine an `on|off` compile-time option. Example:
+  ##
+  ## .. code-block:: Nim
+  ##   when compileOption("floatchecks"):
+  ##     echo "compiled with floating point NaN and Inf checks"
+
+proc compileOption*(option, arg: string): bool {.
+  magic: "CompileOptionArg", noSideEffect.}
+  ## Can be used to determine an enum compile-time option. Example:
+  ##
+  ## .. code-block:: Nim
+  ##   when compileOption("opt", "size") and compileOption("gc", "boehm"):
+  ##     echo "compiled with optimization for size and uses Boehm's GC"
+
 {.push warning[GcMem]: off, warning[Uninit]: off.}
 {.push hints: off.}
 
@@ -1040,22 +1057,6 @@ const
   # emit this flag
   # for string literals, it allows for some optimizations.
 
-proc compileOption*(option: string): bool {.
-  magic: "CompileOption", noSideEffect.}
-  ## Can be used to determine an `on|off` compile-time option. Example:
-  ##
-  ## .. code-block:: Nim
-  ##   when compileOption("floatchecks"):
-  ##     echo "compiled with floating point NaN and Inf checks"
-
-proc compileOption*(option, arg: string): bool {.
-  magic: "CompileOptionArg", noSideEffect.}
-  ## Can be used to determine an enum compile-time option. Example:
-  ##
-  ## .. code-block:: Nim
-  ##   when compileOption("opt", "size") and compileOption("gc", "boehm"):
-  ##     echo "compiled with optimization for size and uses Boehm's GC"
-
 const
   hasThreadSupport = compileOption("threads") and not defined(nimscript)
   hasSharedHeap = defined(boehmgc) or defined(gogc) # don't share heaps; every thread has its own
@@ -1891,6 +1892,7 @@ var
 type
   PFrame* = ptr TFrame  ## Represents a runtime frame of the call stack;
                         ## part of the debugger API.
+  # keep in sync with nimbase.h `struct TFrame_`
   TFrame* {.importc, nodecl, final.} = object ## The frame itself.
     prev*: PFrame       ## Previous frame; used for chaining the call stack.
     procname*: cstring  ## Name of the proc that is currently executing.
@@ -1898,6 +1900,8 @@ type
     filename*: cstring  ## Filename of the proc that is currently executing.
     len*: int16         ## Length of the inspectable slots.
     calldepth*: int16   ## Used for max call depth checking.
+    when NimStackTraceMsgs:
+      frameMsgLen*: int   ## end position in frameMsgBuf for this frame.
 
 when defined(js):
   proc add*(x: var string, y: cstring) {.asmNoStackFrame.} =
diff --git a/lib/system/exceptions.nim b/lib/system/exceptions.nim
index 3979fb66e..516de8252 100644
--- a/lib/system/exceptions.nim
+++ b/lib/system/exceptions.nim
@@ -1,3 +1,7 @@
+const NimStackTraceMsgs =
+  when defined(nimHasStacktraceMsgs): compileOption("stacktraceMsgs")
+  else: false
+
 type
   RootEffect* {.compilerproc.} = object of RootObj ## \
     ## Base effect class.
@@ -16,6 +20,11 @@ type
     procname*: cstring      ## Name of the proc that is currently executing.
     line*: int              ## Line number of the proc that is currently executing.
     filename*: cstring      ## Filename of the proc that is currently executing.
+    when NimStackTraceMsgs:
+      frameMsg*: string     ## When a stacktrace is generated in a given frame and
+                            ## rendered at a later time, we should ensure the stacktrace
+                            ## data isn't invalidated; any pointer into PFrame is
+                            ## subject to being invalidated so shouldn't be stored.
 
   Exception* {.compilerproc, magic: "Exception".} = object of RootObj ## \
     ## Base exception class.
diff --git a/lib/system/excpt.nim b/lib/system/excpt.nim
index 8ac47d26b..76d188ea6 100644
--- a/lib/system/excpt.nim
+++ b/lib/system/excpt.nim
@@ -53,6 +53,8 @@ type
     len: int
     prev: ptr GcFrameHeader
 
+when NimStackTraceMsgs:
+  var frameMsgBuf* {.threadvar.}: string
 var
   framePtr {.threadvar.}: PFrame
   excHandler {.threadvar.}: PSafePoint
@@ -224,10 +226,17 @@ proc auxWriteStackTrace(f: PFrame; s: var seq[StackTraceEntry]) =
     s[last] = StackTraceEntry(procname: it.procname,
                               line: it.line,
                               filename: it.filename)
+    when NimStackTraceMsgs:
+      let first = if it.prev == nil: 0 else: it.prev.frameMsgLen
+      if it.frameMsgLen > first:
+        s[last].frameMsg.setLen(it.frameMsgLen - first)
+        # somehow string slicing not available here
+        for i in first .. it.frameMsgLen-1:
+          s[last].frameMsg[i-first] = frameMsgBuf[i]
     it = it.prev
     dec last
 
-template addFrameEntry(s, f: untyped) =
+template addFrameEntry(s: var string, f: StackTraceEntry|PFrame) =
   var oldLen = s.len
   add(s, f.filename)
   if f.line > 0:
@@ -236,6 +245,12 @@ template addFrameEntry(s, f: untyped) =
     add(s, ')')
   for k in 1..max(1, 25-(s.len-oldLen)): add(s, ' ')
   add(s, f.procname)
+  when NimStackTraceMsgs:
+    when type(f) is StackTraceEntry:
+      add(s, f.frameMsg)
+    else:
+      var first = if f.prev == nil: 0 else: f.prev.frameMsgLen
+      for i in first..<f.frameMsgLen: add(s, frameMsgBuf[i])
   add(s, "\n")
 
 proc `$`(s: seq[StackTraceEntry]): string =
@@ -519,7 +534,12 @@ proc callDepthLimitReached() {.noinline.} =
   quit(1)
 
 proc nimFrame(s: PFrame) {.compilerRtl, inl, raises: [].} =
-  s.calldepth = if framePtr == nil: 0 else: framePtr.calldepth+1
+  if framePtr == nil:
+    s.calldepth = 0
+    when NimStackTraceMsgs: s.frameMsgLen = 0
+  else:
+    s.calldepth = framePtr.calldepth+1
+    when NimStackTraceMsgs: s.frameMsgLen = framePtr.frameMsgLen
   s.prev = framePtr
   framePtr = s
   if s.calldepth == nimCallDepthLimit: callDepthLimitReached()