allow += modification of my_ variables

Fixes: neomutt#2937

- myvar: add myvar_append function
- command_parse: use it in parse_set for 'set my_var += text'
- command_parse: diagnose 'set my_var-=text' instead of silently
  treating it like =
- docs: tweak docs to match

command_parse.c

@@ -29,6 +29,7 @@
  */
 
 #include "config.h"
+#include <assert.h>
 #include <errno.h>
 #include <limits.h>
 #include <stdbool.h>
@@ -789,6 +790,11 @@ enum CommandResult parse_set(struct Buffer *buf, struct Buffer *s,
       else
         decrement = true;
 
+      if (my && decrement)
+      {
+        mutt_buffer_printf(err, _("Can't decrement a my_ variable"), set_commands[data]);
+        return MUTT_CMD_WARNING;
+      }
       s->dptr++;
       if (*s->dptr == '=')
       {
@@ -894,7 +900,15 @@ enum CommandResult parse_set(struct Buffer *buf, struct Buffer *s,
         mutt_extract_token(buf, s, MUTT_TOKEN_BACKTICK_VARS);
         if (my)
         {
-          myvar_set(name, buf->data);
+          assert(!decrement);
+          if (increment)
+          {
+            myvar_append(name, buf->data);
+          }
+          else
+          {
+            myvar_set(name, buf->data);
+          }
           FREE(&name);
         }
         else

docs/manual.xml.head

@@ -6829,15 +6829,15 @@ set spam_separator=", "
         <para>
           This command is used to set (and unset)
           <link linkend="variables">configuration variables</link>. There are
-          four basic types of variables: boolean, number, string, string list
+          several basic types of variables: boolean, number, string, string list
           and quadoption. <emphasis>boolean</emphasis> variables can be
           <emphasis>set</emphasis> (true) or <emphasis>unset</emphasis>
           (false). <emphasis>number</emphasis> variables can be assigned
-          a positive integer value. Value of number variables can be
+          a positive integer value. The value of numeric variables can be
           incremented <emphasis>+=</emphasis> and decremented
           <emphasis>-=</emphasis>. <emphasis>String list</emphasis> variables
-          use <emphasis>+=</emphasis> for appending increment to the string list
-          and <emphasis>-=</emphasis> for removal decrement from the string list.
+          use <emphasis>+=</emphasis> for appending to the string list
+          and <emphasis>-=</emphasis> for removal from the string list.
           <emphasis>string</emphasis> variables
           consist of any number of printable characters and must be enclosed in
           quotes if they contain spaces or tabs. You may also use the escape
@@ -6911,9 +6911,16 @@ set spam_separator=", "
           </para>
           <para>
             The <command>set</command> command either creates a custom
-            <literal>my_</literal> variable or changes its value if it does
-            exist already. The <command>unset</command> and
-            <command>reset</command> commands remove the variable entirely.
+            <literal>my_</literal> variable or changes its value if it
+            exists already. Use of <emphasis>+=</emphasis> will adjust
+            a custom variable using the same behavior as a string
+            variable, by appending additional characters (this is true
+            even if the current contents of the variable resemble an
+            integer, which is different than the behavior of
+            <emphasis>+=</emphasis> on built-in numeric
+            variables). The <command>unset</command> and
+            <command>reset</command> commands remove the variable
+            entirely.
           </para>
           <para>
             Since user-defined variables are expanded in the same way that

myvar.c

@@ -120,6 +120,25 @@ void myvar_set(const char *var, const char *val)
   TAILQ_INSERT_TAIL(&MyVars, myv, entries);
 }
 
+/**
+ * myvar_append - Append to the value of a "my_" variable
+ * @param var Variable name
+ * @param val Value to append
+ */
+void myvar_append(const char *var, const char *val)
+{
+  struct MyVar *myv = myvar_find(var);
+
+  if (myv)
+  {
+    mutt_str_append_item(&myv->value, val, '\0');
+    return;
+  }
+
+  myv = myvar_new(var, val);
+  TAILQ_INSERT_TAIL(&MyVars, myv, entries);
+}
+
 /**
  * myvar_del - Unset the value of a "my_" variable
  * @param var Variable name

myvar.h

@@ -41,6 +41,7 @@ extern struct MyVarList MyVars; ///< List of all the user's custom config variab
 void        myvar_del(const char *var);
 const char *myvar_get(const char *var);
 void        myvar_set(const char *var, const char *val);
+void        myvar_append(const char *var, const char *val);
 
 void myvarlist_free(struct MyVarList *list);