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>

3 files changed, 89 insertions, 30 deletions

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"));