bugfix: unbound variables were not raising error

Since we switched error trace semantics from a designated label to a designated
depth (commit 9831a8cef9 on May 19).

editor/mu.vim

@@ -49,7 +49,7 @@ highlight link muError Error
 " sources of action at a distance
 syntax match muAssign "<-"
 highlight link muAssign SpecialChar
-syntax keyword muAssign error
+syntax keyword muAssign error error-stream
 highlight link muAssign Special
 
 " common keywords

shell/evaluate.mu

@@ -819,7 +819,7 @@ fn lookup-symbol sym: (addr cell), out: (addr handle cell), env-h: (handle cell)
   trace-lower trace
   var _env/eax: (addr cell) <- lookup env-h
   var env/ebx: (addr cell) <- copy _env
-  # if env is not a list, abort
+  # if env is not a list, error
   {
     var env-type/ecx: (addr int) <- get env, type
     compare *env-type, 0/pair

shell/global.mu

@@ -395,7 +395,7 @@ fn lookup-symbol-in-globals _sym: (addr cell), out: (addr handle cell), _globals
   write stream, "unbound symbol: "
   rewind-stream sym-name
   write-stream stream, sym-name
-  trace trace, "error", stream
+  error-stream trace, stream
 }
 
 fn maybe-lookup-symbol-in-globals _sym: (addr cell), out: (addr handle cell), _globals: (addr global-table), trace: (addr trace) {
@@ -506,7 +506,7 @@ fn mutate-binding-in-globals name: (addr stream byte), val: (addr handle cell),
   write stream, "unbound symbol: "
   rewind-stream name
   write-stream stream, name
-  trace trace, "error", stream
+  error-stream trace, stream
 }
 
 # a little strange; goes from value to name and selects primitive based on name

shell/parse.mu

@@ -192,7 +192,7 @@ fn parse-sexpression tokens: (addr stream cell), _out: (addr handle cell), trace
     var curr-token-data/eax: (addr stream byte) <- lookup *curr-token-data-ah
     rewind-stream curr-token-data
     write-stream stream, curr-token-data
-    trace trace, "error", stream
+    error-stream trace, stream
   }
   trace-higher trace
   return 0/false, 0/false

shell/sandbox.mu

@@ -720,9 +720,27 @@ fn test-run-error-invalid-integer {
   #
   render-sandbox screen, sandbox, 0/x, 0/y, 0x80/width, 0x10/height
   # skip one line of padding
-  check-screen-row screen, 1/y, " 1a             ", "F - test-run-error-invalid-integer/0"
-  check-screen-row screen, 2/y, " ...            ", "F - test-run-error-invalid-integer/0"
-  check-screen-row screen, 3/y, " invalid number ", "F - test-run-error-invalid-integer/2"
+  check-screen-row            screen,               1/y, " 1a             ", "F - test-run-error-invalid-integer/0"
+  check-screen-row            screen,               2/y, " ...            ", "F - test-run-error-invalid-integer/1"
+  check-screen-row-in-color   screen, 0xc/fg=error, 3/y, " invalid number ", "F - test-run-error-invalid-integer/2"
+}
+
+fn test-run-error-unknown-symbol {
+  var sandbox-storage: sandbox
+  var sandbox/esi: (addr sandbox) <- address sandbox-storage
+  initialize-sandbox-with sandbox, "a"
+  # eval
+  edit-sandbox sandbox, 0x13/ctrl-s, 0/no-globals, 0/no-disk, 0/no-screen, 0/no-tweak-screen
+  # setup: screen
+  var screen-on-stack: screen
+  var screen/edi: (addr screen) <- address screen-on-stack
+  initialize-screen screen, 0x80/width, 0x10/height, 0/no-pixel-graphics
+  #
+  render-sandbox screen, sandbox, 0/x, 0/y, 0x80/width, 0x10/height
+  # skip one line of padding
+  check-screen-row            screen,               1/y, " a                  ", "F - test-run-error-unknown-symbol/0"
+  check-screen-row            screen,               2/y, " ...                ", "F - test-run-error-unknown-symbol/1"
+  check-screen-row-in-color   screen, 0xc/fg=error, 3/y, " unbound symbol: a  ", "F - test-run-error-unknown-symbol/2"
 }
 
 fn test-run-with-spaces {

shell/trace.mu

@@ -195,6 +195,20 @@ fn error _self: (addr trace), message: (addr array byte) {
   copy-to *curr-depth-a, save-depth
 }
 
+fn error-stream _self: (addr trace), message: (addr stream byte) {
+  var self/esi: (addr trace) <- copy _self
+  compare self, 0
+  {
+    break-if-!=
+    abort "null trace"
+  }
+  var curr-depth-a/eax: (addr int) <- get self, curr-depth
+  var save-depth/ecx: int <- copy *curr-depth-a
+  copy-to *curr-depth-a, 0/error
+  trace self, "error", message
+  copy-to *curr-depth-a, save-depth
+}
+
 fn initialize-trace-line depth: int, label: (addr array byte), data: (addr stream byte), _out: (addr trace-line) {
   var out/edi: (addr trace-line) <- copy _out
   # depth