vim-patch:8.0.1832: cannot use :unlet for an environment variable

Problem:    Cannot use :unlet for an environment variable.
Solution:   Make it work.  Use unsetenv() if available.
            (Yasuhiro Matsumoto, closes vim/vim#2855)
137374fd65

runtime/doc/eval.txt

@@ -9136,6 +9136,14 @@ This does NOT work: >
 			variables are automatically deleted when the function
 			ends.
 
+:unl[et] ${env-name} ...			*:unlet-environment* *:unlet-$*
+			Remove environment variable {env-name}.
+			Can mix {name} and ${env-name} in one :unlet command.
+			No error message is given for a non-existing
+			variable, also without !.
+			If the system does not support deleting an environment
+			variable, it is made emtpy.
+
 :lockv[ar][!] [depth] {name} ...			*:lockvar* *:lockv*
 			Lock the internal variable {name}.  Locking means that
 			it can no longer be changed (until it is unlocked).

src/nvim/eval.c

@@ -2818,6 +2818,18 @@ static void ex_unletlock(exarg_T *eap, char_u *argstart, int deep)
   lval_T lv;
 
   do {
+    if (*arg == '$') {
+      const char *name = (char *)++arg;
+
+      if (get_env_len((const char_u **)&arg) == 0) {
+        EMSG2(_(e_invarg2), name - 1);
+        return;
+      }
+      os_unsetenv(name);
+      arg = skipwhite(arg);
+      continue;
+    }
+
     // Parse the name and find the end.
     char_u *const name_end = (char_u *)get_lval(arg, NULL, &lv, true,
                                                 eap->skip || error,

src/nvim/testdir/test_unlet.vim

@@ -28,3 +28,27 @@ endfunc
 func Test_unlet_fails()
   call assert_fails('unlet v:["count"]', 'E46:')
 endfunc
+
+func Test_unlet_env()
+  let envcmd = has('win32') ? 'set' : 'env'
+
+  let $FOOBAR = 'test'
+  let found = 0
+  for kv in split(system(envcmd), "\r*\n")
+    if kv == 'FOOBAR=test'
+      let found = 1
+    endif
+  endfor
+  call assert_equal(1, found)
+
+  unlet $FOOBAR
+  let found = 0
+  for kv in split(system(envcmd), "\r*\n")
+    if kv == 'FOOBAR=test'
+      let found = 1
+    endif
+  endfor
+  call assert_equal(0, found)
+
+  unlet $MUST_NOT_BE_AN_ERROR
+endfunc