vim-patch:8.2.4883: string interpolation only works in heredoc

Problem:    String interpolation only works in heredoc.
Solution:   Support interpolated strings.  Use syntax for heredoc consistent
            with strings, similar to C#. (closes vim/vim#10327)

https://github.com/vim/vim/commit/2eaef106e4a7fc9dc74a7e672b5f550ec1f9786e

Cherry-pick Test_Debugger_breakadd_expr() from Vim.

Co-authored-by: LemonBoy <thatlemon@gmail.com>

7 files changed, 233 insertions, 54 deletions

diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index c8eea03f5f..518a190d3c 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -1407,6 +1407,26 @@ to be doubled.  These two commands are equivalent: >
 
 
 ------------------------------------------------------------------------------
+interpolated-string					*interp-string*
+
+$"string"		interpolated string constant		*expr-$quote*
+$'string'		interpolated literal string constant	*expr-$'*
+
+Interpolated strings are an extension of the |string| and |literal-string|,
+allowing the inclusion of Vim script expressions (see |expr1|).  Any
+expression returning a value can be enclosed between curly braces.  The value
+is converted to a string.  All the text and results of the expressions
+are concatenated to make a new string.
+
+To include an opening brace '{' or closing brace '}' in the string content
+double it.
+
+Examples: >
+	let your_name = input("What's your name? ")
+	echo $"Hello, {your_name}!"
+	echo $"The square root of 9 is {sqrt(9)}"
+
+------------------------------------------------------------------------------
 option						*expr-option* *E112* *E113*
 
 &option			option value, local value if possible
diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index b240c36977..f8a9326703 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -3100,8 +3100,13 @@ static int eval7(char **arg, typval_T *rettv, evalarg_T *const evalarg, bool wan
     ret = eval_option((const char **)arg, rettv, evaluate);
     break;
   // Environment variable: $VAR.
+  // Interpolated string: $"string" or $'string'.
   case '$':
-    ret = eval_env_var(arg, rettv, evaluate);
+    if ((*arg)[1] == '"' || (*arg)[1] == '\'') {
+      ret = eval_interp_string(arg, rettv, evaluate);
+    } else {
+      ret = eval_env_var(arg, rettv, evaluate);
+    }
     break;
 
   // Register contents: @r.
@@ -4053,6 +4058,32 @@ static int eval_lit_string(char **arg, typval_T *rettv, int evaluate)
   return OK;
 }
 
+int eval_interp_string(char **arg, typval_T *rettv, int evaluate)
+{
+  // *arg is on the '$' character.
+  (*arg)++;
+
+  rettv->v_type = VAR_STRING;
+
+  typval_T tv;
+  int ret;
+  if (**arg == '"') {
+    ret = eval_string(arg, &tv, evaluate);
+  } else {
+    ret = eval_lit_string(arg, &tv, evaluate);
+  }
+
+  if (ret == FAIL || !evaluate) {
+    return ret;
+  }
+
+  rettv->vval.v_string = eval_all_expr_in_str(tv.vval.v_string);
+
+  tv_clear(&tv);
+
+  return rettv->vval.v_string != NULL ? OK : FAIL;
+}
+
 /// @return  the function name of the partial.
 char *partial_name(partial_T *pt)
   FUNC_ATTR_PURE
diff --git a/src/nvim/eval/vars.c b/src/nvim/eval/vars.c
index a8d1e01152..b86c49fd98 100644
--- a/src/nvim/eval/vars.c
+++ b/src/nvim/eval/vars.c
@@ -53,50 +53,73 @@
 static const char *e_letunexp = N_("E18: Unexpected characters in :let");
 static const char *e_lock_unlock = N_("E940: Cannot lock or unlock variable %s");
 
-/// Evaluate all the Vim expressions (`=expr`) in string "str" and return the
+/// Evaluate all the Vim expressions ({expr}) in string "str" and return the
 /// resulting string.  The caller must free the returned string.
-static char *eval_all_expr_in_str(char *str)
+char *eval_all_expr_in_str(char *str)
 {
   garray_T ga;
   ga_init(&ga, 1, 80);
   char *p = str;
 
-  // Look for `=expr`, evaluate the expression and replace `=expr` with the
-  // result.
   while (*p != NUL) {
-    char *s = p;
-    while (*p != NUL && (*p != '`' || p[1] != '=')) {
-      p++;
+    bool escaped_brace = false;
+
+    // Look for a block start.
+    char *lit_start = p;
+    while (*p != '{' && *p != '}' && *p != NUL) {
+      ++p;
+    }
+
+    if (*p != NUL && *p == p[1]) {
+      // Escaped brace, unescape and continue.
+      // Include the brace in the literal string.
+      ++p;
+      escaped_brace = true;
+    } else if (*p == '}') {
+      semsg(_(e_stray_closing_curly_str), str);
+      ga_clear(&ga);
+      return NULL;
     }
-    ga_concat_len(&ga, s, (size_t)(p - s));
+
+    // Append the literal part.
+    ga_concat_len(&ga, lit_start, (size_t)(p - lit_start));
+
     if (*p == NUL) {
-      break;              // no backtick expression found
+      break;
     }
-    s = p;
-    p += 2;         // skip `=
 
-    int status = *p == NUL ? OK : skip_expr(&p, NULL);
-    if (status == FAIL || *p != '`') {
-      // invalid expression or missing ending backtick
-      if (status != FAIL) {
-        emsg(_("E1083: Missing backtick"));
-      }
-      xfree(ga.ga_data);
+    if (escaped_brace) {
+      // Skip the second brace.
+      ++p;
+      continue;
+    }
+
+    // Skip the opening {.
+    char *block_start = ++p;
+    char *block_end = block_start;
+    if (*block_start != NUL && skip_expr(&block_end, NULL) == FAIL) {
+      ga_clear(&ga);
       return NULL;
     }
-    s += 2;         // skip `=
-    char save_c = *p;
-    *p = NUL;
-    char *exprval = eval_to_string(s, true);
-    *p = save_c;
-    p++;
-    if (exprval == NULL) {
-      // expression evaluation failed
-      xfree(ga.ga_data);
+    block_end = skipwhite(block_end);
+    // The block must be closed by a }.
+    if (*block_end != '}') {
+      semsg(_(e_missing_close_curly_str), str);
+      ga_clear(&ga);
       return NULL;
     }
-    ga_concat(&ga, exprval);
-    xfree(exprval);
+    char save_c = *block_end;
+    *block_end = NUL;
+    char *expr_val = eval_to_string(block_start, true);
+    *block_end = save_c;
+    if (expr_val == NULL) {
+      ga_clear(&ga);
+      return NULL;
+    }
+    ga_concat(&ga, expr_val);
+    xfree(expr_val);
+
+    p = block_end + 1;
   }
   ga_append(&ga, NUL);
 
diff --git a/src/nvim/globals.h b/src/nvim/globals.h
index e406d93494..11888a5df8 100644
--- a/src/nvim/globals.h
+++ b/src/nvim/globals.h
@@ -1024,6 +1024,11 @@ EXTERN const char e_highlight_group_name_too_long[] INIT(= N_("E1249: Highlight
 
 EXTERN const char e_invalid_line_number_nr[] INIT(= N_("E966: Invalid line number: %ld"));
 
+EXTERN char e_stray_closing_curly_str[]
+INIT(= N_("E1278: Stray '}' without a matching '{': %s"));
+EXTERN char e_missing_close_curly_str[]
+INIT(= N_("E1279: Missing '}': %s"));
+
 EXTERN const char e_undobang_cannot_redo_or_move_branch[]
 INIT(= N_("E5767: Cannot use :undo! to redo or move to a different undo branch"));
 
diff --git a/test/old/testdir/test_debugger.vim b/test/old/testdir/test_debugger.vim
index f5177c8fb2..18616e8717 100644
--- a/test/old/testdir/test_debugger.vim
+++ b/test/old/testdir/test_debugger.vim
@@ -348,6 +348,39 @@ func Test_Debugger_breakadd()
   call assert_fails('breakadd file Xtest.vim /\)/', 'E55:')
 endfunc
 
+" Test for expression breakpoint set using ":breakadd expr <expr>"
+func Test_Debugger_breakadd_expr()
+  CheckRunVimInTerminal
+  let lines =<< trim END
+    let g:Xtest_var += 1
+  END
+  call writefile(lines, 'Xtest.vim')
+
+  " Start Vim in a terminal
+  let buf = RunVimInTerminal('Xtest.vim', {})
+  call RunDbgCmd(buf, ':let g:Xtest_var = 10')
+  call RunDbgCmd(buf, ':breakadd expr g:Xtest_var')
+  call RunDbgCmd(buf, ':source %')
+  let expected =<< eval trim END
+    Oldval = "10"
+    Newval = "11"
+    {fnamemodify('Xtest.vim', ':p')}
+    line 1: let g:Xtest_var += 1
+  END
+  call RunDbgCmd(buf, ':source %', expected)
+  call RunDbgCmd(buf, 'cont')
+  let expected =<< eval trim END
+    Oldval = "11"
+    Newval = "12"
+    {fnamemodify('Xtest.vim', ':p')}
+    line 1: let g:Xtest_var += 1
+  END
+  call RunDbgCmd(buf, ':source %', expected)
+
+  call StopVimInTerminal(buf)
+  call delete('Xtest.vim')
+endfunc
+
 func Test_Backtrace_Through_Source()
   CheckRunVimInTerminal
   CheckCWD
diff --git a/test/old/testdir/test_expr.vim b/test/old/testdir/test_expr.vim
index c8b7fde100..86e720a1ae 100644
--- a/test/old/testdir/test_expr.vim
+++ b/test/old/testdir/test_expr.vim
@@ -848,4 +848,60 @@ func Test_float_compare()
   call CheckLegacyAndVim9Success(lines)
 endfunc
 
+func Test_string_interp()
+  let lines =<< trim END
+    call assert_equal('', $"")
+    call assert_equal('foobar', $"foobar")
+    #" Escaping rules.
+    call assert_equal('"foo"{bar}', $"\"foo\"{{bar}}")
+    call assert_equal('"foo"{bar}', $'"foo"{{bar}}')
+    call assert_equal('foobar', $"{\"foo\"}" .. $'{''bar''}')
+    #" Whitespace before/after the expression.
+    call assert_equal('3', $"{ 1 + 2 }")
+    #" String conversion.
+    call assert_equal('hello from ' .. v:version, $"hello from {v:version}")
+    call assert_equal('hello from ' .. v:version, $'hello from {v:version}')
+    #" Paper over a small difference between VimScript behaviour.
+    call assert_equal(string(v:true), $"{v:true}")
+    call assert_equal('(1+1=2)', $"(1+1={1 + 1})")
+    #" Hex-escaped opening brace: char2nr('{') == 0x7b
+    call assert_equal('esc123ape', $"esc\x7b123}ape")
+    call assert_equal('me{}me', $"me{\x7b}\x7dme")
+    VAR var1 = "sun"
+    VAR var2 = "shine"
+    call assert_equal('sunshine', $"{var1}{var2}")
+    call assert_equal('sunsunsun', $"{var1->repeat(3)}")
+    #" Multibyte strings.
+    call assert_equal('say ハロー・ワールド', $"say {'ハロー・ワールド'}")
+    #" Nested.
+    call assert_equal('foobarbaz', $"foo{$\"{'bar'}\"}baz")
+    #" Do not evaluate blocks when the expr is skipped.
+    VAR tmp = 0
+    if v:false
+      echo "${ LET tmp += 1 }"
+    endif
+    call assert_equal(0, tmp)
+
+    #" Stray closing brace.
+    call assert_fails('echo $"moo}"', 'E1278:')
+    #" Undefined variable in expansion.
+    call assert_fails('echo $"{moo}"', 'E121:')
+    #" Empty blocks are rejected.
+    call assert_fails('echo $"{}"', 'E15:')
+    call assert_fails('echo $"{   }"', 'E15:')
+  END
+  call CheckLegacyAndVim9Success(lines)
+
+  let lines =<< trim END
+    call assert_equal('5', $"{({x -> x + 1})(4)}")
+  END
+  call CheckLegacySuccess(lines)
+
+  let lines =<< trim END
+    call assert_equal('5', $"{((x) => x + 1)(4)}")
+    call assert_fails('echo $"{ # foo }"', 'E1279:')
+  END
+  call CheckDefAndScriptSuccess(lines)
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab
diff --git a/test/old/testdir/test_let.vim b/test/old/testdir/test_let.vim
index 915bba2314..8f7121935e 100644
--- a/test/old/testdir/test_let.vim
+++ b/test/old/testdir/test_let.vim
@@ -387,6 +387,17 @@ END
   call assert_equal(['Text', 'with', 'indent'], text)
 endfunc
 
+func Test_let_interpolated()
+  call assert_equal('{text}', $'{{text}}')
+  call assert_equal('{{text}}', $'{{{{text}}}}')
+  let text = 'text'
+  call assert_equal('text{{', $'{text .. "{{"}')
+  call assert_equal('text{{', $"{text .. '{{'}")
+  " FIXME: should not need to escape quotes in the expression
+  call assert_equal('text{{', $'{text .. ''{{''}')
+  call assert_equal('text{{', $"{text .. \"{{\"}")
+endfunc
+
 " Test for the setting a variable using the heredoc syntax.
 " Keep near the end, this messes up highlighting.
 func Test_let_heredoc()
@@ -496,72 +507,72 @@ END
   call assert_equal(['     x', '     \y', '     z'], [a, b, c])
 endfunc
 
-" Test for evaluating Vim expressions in a heredoc using `=expr`
+" Test for evaluating Vim expressions in a heredoc using {expr}
 " Keep near the end, this messes up highlighting.
 func Test_let_heredoc_eval()
   let str = ''
   let code =<< trim eval END
-    let a = `=5 + 10`
-    let b = `=min([10, 6])` + `=max([4, 6])`
-    `=str`
-    let c = "abc`=str`d"
+    let a = {5 + 10}
+    let b = {min([10, 6])} + {max([4, 6])}
+    {str}
+    let c = "abc{str}d"
   END
   call assert_equal(['let a = 15', 'let b = 6 + 6', '', 'let c = "abcd"'], code)
 
   let $TESTVAR = "Hello"
   let code =<< eval trim END
-    let s = "`=$TESTVAR`"
+    let s = "{$TESTVAR}"
   END
   call assert_equal(['let s = "Hello"'], code)
 
   let code =<< eval END
-    let s = "`=$TESTVAR`"
+    let s = "{$TESTVAR}"
 END
   call assert_equal(['    let s = "Hello"'], code)
 
   let a = 10
   let data =<< eval END
-`=a`
+{a}
 END
   call assert_equal(['10'], data)
 
   let x = 'X'
   let code =<< eval trim END
-    let a = `abc`
-    let b = `=x`
-    let c = `
+    let a = {{abc}}
+    let b = {x}
+    let c = {{
   END
-  call assert_equal(['let a = `abc`', 'let b = X', 'let c = `'], code)
+  call assert_equal(['let a = {abc}', 'let b = X', 'let c = {'], code)
 
   let code = 'xxx'
   let code =<< eval trim END
-    let n = `=5 +
-    6`
+    let n = {5 +
+    6}
   END
   call assert_equal('xxx', code)
 
   let code =<< eval trim END
-     let n = `=min([1, 2]` + `=max([3, 4])`
+     let n = {min([1, 2]} + {max([3, 4])}
   END
   call assert_equal('xxx', code)
 
   let lines =<< trim LINES
       let text =<< eval trim END
-        let b = `=
+        let b = {
       END
   LINES
-  call CheckScriptFailure(lines, 'E1083:')
+  call CheckScriptFailure(lines, 'E1279:')
 
   let lines =<< trim LINES
       let text =<< eval trim END
-        let b = `=abc
+        let b = {abc
       END
   LINES
-  call CheckScriptFailure(lines, 'E1083:')
+  call CheckScriptFailure(lines, 'E1279:')
 
   let lines =<< trim LINES
       let text =<< eval trim END
-        let b = `=`
+        let b = {}
       END
   LINES
   call CheckScriptFailure(lines, 'E15:')
@@ -569,7 +580,7 @@ END
   " skipped heredoc
   if 0
     let msg =<< trim eval END
-        n is: `=n`
+        n is: {n}
     END
   endif
 
@@ -581,7 +592,7 @@ END
   let lines =<< trim END
     let Xvar =<< eval CODE
     let a = 1
-    let b = `=5+`
+    let b = {5+}
     let c = 2
     CODE
     let g:Count += 1
@@ -590,10 +601,10 @@ END
   let g:Count = 0
   call assert_fails('source', 'E15:')
   call assert_equal(1, g:Count)
-  call setline(3, 'let b = `=abc`')
+  call setline(3, 'let b = {abc}')
   call assert_fails('source', 'E121:')
   call assert_equal(2, g:Count)
-  call setline(3, 'let b = `=abc` + `=min([9, 4])` + 2')
+  call setline(3, 'let b = {abc} + {min([9, 4])} + 2')
   call assert_fails('source', 'E121:')
   call assert_equal(3, g:Count)
   call assert_equal('test', g:Xvar)