vim-patch:8.1.0401: can't get swap name of another buffer

Problem:    Can't get swap name of another buffer.
Solution:   Add swapname(). (Ozaki Kiichi, closes vim/vim#3441)
110bd60985

runtime/doc/eval.txt

@@ -2322,6 +2322,7 @@ submatch({nr} [, {list}])	String or List
 substitute({expr}, {pat}, {sub}, {flags})
 				String	all {pat} in {expr} replaced with {sub}
 swapinfo({fname})		Dict	information about swap file {fname}
+swapname({expr})		String	swap file of buffer {expr}
 synID({lnum}, {col}, {trans})	Number	syntax ID at {lnum} and {col}
 synIDattr({synID}, {what} [, {mode}])
 				String	attribute {what} of syntax ID {synID}
@@ -7777,6 +7778,13 @@ swapinfo({fname})					swapinfo()
 			Not a swap file: does not contain correct block ID
 			Magic number mismatch: Info in first block is invalid
 
+swapname({expr})					*swapname()*
+		The result is the swap file path of the buffer {expr}.
+		For the use of {expr}, see |bufname()| above.
+		If buffer {expr} is the current buffer, the result is equal to
+		|:swapname| (unless no swap file).
+		If buffer {expr} has no swap file, returns an empty string.
+
 synID({lnum}, {col}, {trans})				*synID()*
 		The result is a Number, which is the syntax ID at the position
 		{lnum} and {col} in the current window.

src/nvim/eval.c

@@ -16430,6 +16430,20 @@ static void f_swapinfo(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   get_b0_dict(tv_get_string(argvars), rettv->vval.v_dict);
 }
 
+/// "swapname(expr)" function
+static void f_swapname(typval_T *argvars, typval_T *rettv, FunPtr fptr)
+{
+  rettv->v_type = VAR_STRING;
+  buf_T *buf = tv_get_buf(&argvars[0], false);
+  if (buf == NULL
+      || buf->b_ml.ml_mfp == NULL
+      || buf->b_ml.ml_mfp->mf_fname == NULL) {
+    rettv->vval.v_string = NULL;
+  } else {
+    rettv->vval.v_string = vim_strsave(buf->b_ml.ml_mfp->mf_fname);
+  }
+}
+
 /// "synID(lnum, col, trans)" function
 static void f_synID(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 {

src/nvim/eval.lua

@@ -302,6 +302,7 @@ return {
     submatch={args={1, 2}},
     substitute={args=4},
     swapinfo={args={1}},
+    swapname={args={1}},
     synID={args=3},
     synIDattr={args={2, 3}},
     synIDtrans={args=1},

src/nvim/testdir/test_swap.vim

@@ -1,5 +1,9 @@
 " Tests for the swap feature
 
+func s:swapname()
+  return trim(execute('swapname'))
+endfunc
+
 " Tests for 'directory' option.
 func Test_swap_directory()
   if !has("unix")
@@ -17,7 +21,7 @@ func Test_swap_directory()
   " Verify that the swap file doesn't exist in the current directory
   call assert_equal([], glob(".Xtest1*.swp", 1, 1, 1))
   edit Xtest1
-  let swfname = split(execute("swapname"))[0]
+  let swfname = s:swapname()
   call assert_equal([swfname], glob(swfname, 1, 1, 1))
 
   " './dir', swap file in a directory relative to the file
@@ -27,7 +31,7 @@ func Test_swap_directory()
   edit Xtest1
   call assert_equal([], glob(swfname, 1, 1, 1))
   let swfname = "Xtest2/Xtest1.swp"
-  call assert_equal(swfname, split(execute("swapname"))[0])
+  call assert_equal(swfname, s:swapname())
   call assert_equal([swfname], glob("Xtest2/*", 1, 1, 1))
 
   " 'dir', swap file in directory relative to the current dir
@@ -38,7 +42,7 @@ func Test_swap_directory()
   edit Xtest2/Xtest3
   call assert_equal(["Xtest2/Xtest3"], glob("Xtest2/*", 1, 1, 1))
   let swfname = "Xtest.je/Xtest3.swp"
-  call assert_equal(swfname, split(execute("swapname"))[0])
+  call assert_equal(swfname, s:swapname())
   call assert_equal([swfname], glob("Xtest.je/*", 1, 1, 1))
 
   set dir&
@@ -66,7 +70,7 @@ func Test_swapinfo()
   new Xswapinfo
   call setline(1, ['one', 'two', 'three'])
   w
-  let fname = trim(execute('swapname'))
+  let fname = s:swapname()
   call assert_match('Xswapinfo', fname)
   let info = swapinfo(fname)
 
@@ -100,3 +104,24 @@ func Test_swapinfo()
   call assert_equal('Not a swap file', info.error)
   call delete('Xnotaswapfile')
 endfunc
+
+func Test_swapname()
+  edit Xtest1
+  let expected = s:swapname()
+  call assert_equal(expected, swapname('%'))
+
+  new Xtest2
+  let buf = bufnr('%')
+  let expected = s:swapname()
+  wincmd p
+  call assert_equal(expected, swapname(buf))
+
+  new Xtest3
+  setlocal noswapfile
+  call assert_equal('', swapname('%'))
+
+  bwipe!
+  call delete('Xtest1')
+  call delete('Xtest2')
+  call delete('Xtest3')
+endfunc