viml/parser/expressions: Add support for parsing assignments

8 files changed, 327 insertions, 60 deletions

diff --git a/src/nvim/api/vim.c b/src/nvim/api/vim.c
index bac19ef363..ce554d351c 100644
--- a/src/nvim/api/vim.c
+++ b/src/nvim/api/vim.c
@@ -900,14 +900,21 @@ typedef kvec_withinit_t(ExprASTConvStackItem, 16) ExprASTConvStack;
 /// Parse a VimL expression
 ///
 /// @param[in]  expr  Expression to parse. Is always treated as a single line.
-/// @param[in]  flags  Flags: "m" if multiple expressions in a row are allowed
-///                    (only the first one will be parsed), "E" if EOC tokens
-///                    are not allowed (determines whether they will stop
-///                    parsing process or be recognized as an operator/space,
-///                    though also yielding an error).
-///
-///                    Use only "m" to parse like for "<C-r>=", only "E" to
-///                    parse like for ":echo", empty string for ":let".
+/// @param[in]  flags  Flags:
+///
+///                    - "m" if multiple expressions in a row are allowed (only
+///                      the first one will be parsed),
+///                    - "E" if EOC tokens are not allowed (determines whether
+///                      they will stop parsing process or be recognized as an
+///                      operator/space, though also yielding an error).
+///                    - "l" when needing to start parsing with lvalues for
+///                      ":let" or ":for".
+///
+///                    Common flag sets:
+///                    - "m" to parse like for ":echo".
+///                    - "E" to parse like for "<C-r>=".
+///                    - empty string for ":call".
+///                    - "lm" to parse for ":let".
 /// @param[in]  highlight  If true, return value will also include "highlight"
 ///                        key containing array of 4-tuples (arrays) (Integer,
 ///                        Integer, Integer, String), where first three numbers
@@ -964,6 +971,9 @@ typedef kvec_withinit_t(ExprASTConvStackItem, 16) ExprASTConvStack;
 ///                           value names from ExprCaseCompareStrategy,
 ///                           stringified without "kCCStrategy" prefix. Only
 ///                           present for "Comparison" nodes.
+///           "augmentation": String, augmentation type for "Assignment" nodes.
+///                           Is either an empty string, "Add", "Subtract" or
+///                           "Concat" for "=", "+=", "-=" or ".=" respectively.
 ///           "invert": Boolean, true if result of comparison needs to be
 ///                     inverted. Only present for "Comparison" nodes.
 ///           "ivalue": Integer, integer value for "Integer" nodes.
@@ -979,6 +989,7 @@ Dictionary nvim_parse_expression(String expr, String flags, Boolean highlight,
     switch (flags.data[i]) {
       case 'm': { pflags |= kExprFlagsMulti; break; }
       case 'E': { pflags |= kExprFlagsDisallowEOC; break; }
+      case 'l': { pflags |= kExprFlagsParseLet; break; }
       case NUL: {
         api_set_error(err, kErrorTypeValidation, "Invalid flag: '\\0' (%u)",
                       (unsigned)flags.data[i]);
@@ -1114,6 +1125,7 @@ Dictionary nvim_parse_expression(String expr, String flags, Boolean highlight,
             + (node->type == kExprNodeFloat)  // "fvalue"
             + (node->type == kExprNodeDoubleQuotedString
                || node->type == kExprNodeSingleQuotedString)  // "svalue"
+            + (node->type == kExprNodeAssignment)  // "augmentation"
             + 0);
         Dictionary ret_node = {
           .items = xmalloc(ret_node_items_size * sizeof(ret_node.items[0])),
@@ -1272,6 +1284,17 @@ Dictionary nvim_parse_expression(String expr, String flags, Boolean highlight,
             };
             break;
           }
+          case kExprNodeAssignment: {
+            const ExprAssignmentType asgn_type = node->data.ass.type;
+            ret_node->items[ret_node->size++] = (KeyValuePair) {
+              .key = STATIC_CSTR_TO_STRING("augmentation"),
+              .value = STRING_OBJ(
+                  asgn_type == kExprAsgnPlain
+                  ? (String)STRING_INIT
+                  : cstr_to_string(expr_asgn_type_tab[asgn_type])),
+            };
+            break;
+          }
           case kExprNodeMissing:
           case kExprNodeOpMissing:
           case kExprNodeTernary:
diff --git a/src/nvim/syntax.c b/src/nvim/syntax.c
index e0bf74567d..9f98b26905 100644
--- a/src/nvim/syntax.c
+++ b/src/nvim/syntax.c
@@ -6021,11 +6021,21 @@ static const char *highlight_init_dark[] = {
 };
 
 static const char *highlight_init_cmdline[] = {
+  // XXX When modifying a list modify it in both valid and invalid halfs.
+  // TODO(ZyX-I): merge valid and invalid groups via a macros.
+
   // NVimInternalError should appear only when highlighter has a bug.
   "NVimInternalError ctermfg=Red ctermbg=Red guifg=Red guibg=Red",
 
   // Highlight groups (links) used by parser:
 
+  "default link NVimAssignment Operator",
+  "default link NVimPlainAssignment NVimAssignment",
+  "default link NVimAugmentedAssignment NVimAssignment",
+  "default link NVimAssignmentWithAddition NVimAugmentedAssignment",
+  "default link NVimAssignmentWithSubtraction NVimAugmentedAssignment",
+  "default link NVimAssignmentWithConcatenation NVimAugmentedAssignment",
+
   "default link NVimOperator Operator",
 
   "default link NVimUnaryOperator NVimOperator",
@@ -6113,6 +6123,16 @@ static const char *highlight_init_cmdline[] = {
 
   "default link NVimInvalid Error",
 
+  "default link NVimInvalidAssignment NVimInvalid",
+  "default link NVimInvalidPlainAssignment NVimInvalidAssignment",
+  "default link NVimInvalidAugmentedAssignment NVimInvalidAssignment",
+  "default link NVimInvalidAssignmentWithAddition "
+      "NVimInvalidAugmentedAssignment",
+  "default link NVimInvalidAssignmentWithSubtraction "
+      "NVimInvalidAugmentedAssignment",
+  "default link NVimInvalidAssignmentWithConcatenation "
+      "NVimInvalidAugmentedAssignment",
+
   "default link NVimInvalidOperator NVimInvalid",
 
   "default link NVimInvalidUnaryOperator NVimInvalidOperator",
diff --git a/src/nvim/viml/parser/expressions.c b/src/nvim/viml/parser/expressions.c
index e23c58bfd1..13f7131744 100644
--- a/src/nvim/viml/parser/expressions.c
+++ b/src/nvim/viml/parser/expressions.c
@@ -84,6 +84,10 @@ typedef enum {
   /// Just like parsing function arguments, but it is valid to be ended with an
   /// arrow only.
   kEPTLambdaArguments,
+  /// Assignment: parsing for :let
+  kEPTAssignment,
+  /// Single assignment: used when lists are not allowed (i.e. when nesting)
+  kEPTSingleAssignment,
 } ExprASTParseType;
 
 typedef kvec_withinit_t(ExprASTParseType, 4) ExprASTParseTypeStack;
@@ -93,6 +97,7 @@ typedef enum {
   kEOpLvlInvalid = 0,
   kEOpLvlComplexIdentifier,
   kEOpLvlParens,
+  kEOpLvlAssignment,
   kEOpLvlArrow,
   kEOpLvlComma,
   kEOpLvlColon,
@@ -217,8 +222,6 @@ LexExprToken viml_pexpr_next_token(ParserState *const pstate, const int flags)
     }
     CHAR(kExprLexQuestion, '?')
     CHAR(kExprLexColon, ':')
-    CHAR(kExprLexDot, '.')
-    CHAR(kExprLexPlus, '+')
     CHAR(kExprLexComma, ',')
 #undef CHAR
 
@@ -532,12 +535,8 @@ LexExprToken viml_pexpr_next_token(ParserState *const pstate, const int flags)
     case '!':
     case '=': {
       if (pline.size == 1) {
-viml_pexpr_next_token_invalid_comparison:
-        ret.type = (schar == '!' ? kExprLexNot : kExprLexInvalid);
-        if (ret.type == kExprLexInvalid) {
-          ret.data.err.msg = _("E15: Expected == or =~: %.*s");
-          ret.data.err.type = kExprLexComparison;
-        }
+        ret.type = (schar == '!' ? kExprLexNot : kExprLexAssignment);
+        ret.data.ass.type = kExprAsgnPlain;
         break;
       }
       ret.type = kExprLexComparison;
@@ -548,8 +547,11 @@ viml_pexpr_next_token_invalid_comparison:
       } else if (pline.data[1] == '~') {
         ret.data.cmp.type = kExprCmpMatches;
         ret.len++;
+      } else if (schar == '!') {
+        ret.type = kExprLexNot;
       } else {
-        goto viml_pexpr_next_token_invalid_comparison;
+        ret.type = kExprLexAssignment;
+        ret.data.ass.type = kExprAsgnPlain;
       }
       GET_CCS(ret, pline);
       break;
@@ -571,17 +573,37 @@ viml_pexpr_next_token_invalid_comparison:
       break;
     }
 
-    // Minus sign or arrow from lambdas.
+    // Minus sign, arrow from lambdas or augmented assignment.
     case '-': {
       if (pline.size > 1 && pline.data[1] == '>') {
         ret.len++;
         ret.type = kExprLexArrow;
+      } else if (pline.size > 1 && pline.data[1] == '=') {
+        ret.len++;
+        ret.type = kExprLexAssignment;
+        ret.data.ass.type = kExprAsgnSubtract;
       } else {
         ret.type = kExprLexMinus;
       }
       break;
     }
 
+    // Sign or augmented assignment.
+#define CHAR_OR_ASSIGN(ch, ch_type, ass_type) \
+    case ch: { \
+      if (pline.size > 1 && pline.data[1] == '=') { \
+        ret.len++; \
+        ret.type = kExprLexAssignment; \
+        ret.data.ass.type = ass_type; \
+      } else { \
+        ret.type = ch_type; \
+      } \
+      break; \
+    }
+    CHAR_OR_ASSIGN('+', kExprLexPlus, kExprAsgnAdd)
+    CHAR_OR_ASSIGN('.', kExprLexDot, kExprAsgnConcat)
+#undef CHAR_OR_ASSIGN
+
     // Expression end because Ex command ended.
     case NUL:
     case NL: {
@@ -661,6 +683,7 @@ static const char *const eltkn_type_tab[] = {
   [kExprLexParenthesis] = "Parenthesis",
   [kExprLexComma] = "Comma",
   [kExprLexArrow] = "Arrow",
+  [kExprLexAssignment] = "Assignment",
 };
 
 const char *const eltkn_cmp_type_tab[] = {
@@ -671,6 +694,13 @@ const char *const eltkn_cmp_type_tab[] = {
   [kExprCmpIdentical] = "Identical",
 };
 
+const char *const expr_asgn_type_tab[] = {
+  [kExprAsgnPlain] = "Plain",
+  [kExprAsgnAdd] = "Add",
+  [kExprAsgnSubtract] = "Subtract",
+  [kExprAsgnConcat] = "Concat",
+};
+
 const char *const ccs_tab[] = {
   [kCCStrategyUseOption] = "UseOption",
   [kCCStrategyMatchCase] = "MatchCase",
@@ -732,6 +762,8 @@ const char *viml_pexpr_repr_token(const ParserState *const pstate,
             (int)token.data.cmp.inv)
     TKNARGS(kExprLexMultiplication, "(type=%s)",
             eltkn_mul_type_tab[token.data.mul.type])
+    TKNARGS(kExprLexAssignment, "(type=%s)",
+            expr_asgn_type_tab[token.data.ass.type])
     TKNARGS(kExprLexRegister, "(name=%s)", intchar2str(token.data.reg.name))
     case kExprLexDoubleQuotedString:
     TKNARGS(kExprLexSingleQuotedString, "(closed=%i)",
@@ -811,6 +843,7 @@ const char *const east_node_type_tab[] = {
   [kExprNodeMod] = "Mod",
   [kExprNodeOption] = "Option",
   [kExprNodeEnvironment] = "Environment",
+  [kExprNodeAssignment] = "Assignment",
 };
 
 /// Represent `int` character as a string
@@ -933,6 +966,7 @@ const uint8_t node_maxchildren[] = {
   [kExprNodeMod] = 2,
   [kExprNodeOption] = 0,
   [kExprNodeEnvironment] = 0,
+  [kExprNodeAssignment] = 2,
 };
 
 /// Free memory occupied by AST
@@ -993,6 +1027,7 @@ void viml_pexpr_free_ast(ExprAST ast)
         case kExprNodeLambda:
         case kExprNodeDictLiteral:
         case kExprNodeCurlyBracesIdentifier:
+        case kExprNodeAssignment:
         case kExprNodeComma:
         case kExprNodeColon:
         case kExprNodeArrow:
@@ -1111,6 +1146,8 @@ static struct {
 
   [kExprNodeCurlyBracesIdentifier] = { kEOpLvlComplexIdentifier, kEOpAssLeft },
 
+  [kExprNodeAssignment] = { kEOpLvlAssignment, kEOpAssLeft },
+
   [kExprNodeComplexIdentifier] = { kEOpLvlValue, kEOpAssLeft },
 
   [kExprNodePlainIdentifier] = { kEOpLvlValue, kEOpAssNo },
@@ -1478,6 +1515,17 @@ static inline void east_set_error(const ParserState *const pstate,
       } \
     } while (0)
 
+/// Determine whether given parse type is an assignment
+///
+/// @param[in]  pt  Checked parse type.
+///
+/// @return true if parsing an assignment, false otherwise.
+static inline bool pt_is_assignment(const ExprASTParseType pt)
+  FUNC_ATTR_ALWAYS_INLINE FUNC_ATTR_CONST FUNC_ATTR_WARN_UNUSED_RESULT
+{
+  return (pt == kEPTAssignment || pt == kEPTSingleAssignment);
+}
+
 /// Structure used to define “string shifts” necessary to map string
 /// highlighting to actual strings.
 typedef struct {
@@ -1839,6 +1887,9 @@ ExprAST viml_pexpr_parse(ParserState *const pstate, const int flags)
   ExprASTParseTypeStack pt_stack;
   kvi_init(pt_stack);
   kvi_push(pt_stack, kEPTExpr);
+  if (flags & kExprFlagsParseLet) {
+    kvi_push(pt_stack, kEPTAssignment);
+  }
   LexExprToken prev_token = { .type = kExprLexMissing };
   bool highlighted_prev_spacing = false;
   // Lambda node, valid when parsing lambda arguments only.
@@ -1938,33 +1989,83 @@ viml_pexpr_parse_process_token:
       // circumstances, and in any case runtime and not parse time errors.
       (*kv_Z(ast_stack, 1))->type = kExprNodeConcat;
     }
-    if (kv_last(pt_stack) == kEPTLambdaArguments
-        && ((want_node == kENodeOperator
+    // Pop some stack pt_stack items in case of misplaced nodes.
+    const bool is_single_assignment = kv_last(pt_stack) == kEPTSingleAssignment;
+    switch (kv_last(pt_stack)) {
+      case kEPTExpr: {
+        break;
+      }
+      case kEPTLambdaArguments: {
+        if ((want_node == kENodeOperator
              && tok_type != kExprLexComma
              && tok_type != kExprLexArrow)
             || (want_node == kENodeValue
                 && !(cur_token.type == kExprLexPlainIdentifier
                      && cur_token.data.var.scope == kExprVarScopeMissing
                      && !cur_token.data.var.autoload)
-                && tok_type != kExprLexArrow))) {
-      lambda_node->data.fig.type_guesses.allow_lambda = false;
-      if (lambda_node->children != NULL
-          && lambda_node->children->type == kExprNodeComma) {
-        // If lambda has comma child this means that parser has already seen at
-        // least "{arg1,", so node cannot possibly be anything, but lambda.
-
-        // Vim may give E121 or E720 in this case, but it does not look right to
-        // have either because both are results of reevaluation possibly-lambda
-        // node as a dictionary and here this is not going to happen.
-        ERROR_FROM_TOKEN_AND_MSG(
-            cur_token, _("E15: Expected lambda arguments list or arrow: %.*s"));
-      } else {
-        // Else it may appear that possibly-lambda node is actually a dictionary
-        // or curly-braces-name identifier.
-        lambda_node = NULL;
-        kv_drop(pt_stack, 1);
+                && tok_type != kExprLexArrow)) {
+          lambda_node->data.fig.type_guesses.allow_lambda = false;
+          if (lambda_node->children != NULL
+              && lambda_node->children->type == kExprNodeComma) {
+            // If lambda has comma child this means that parser has already seen at
+            // least "{arg1,", so node cannot possibly be anything, but lambda.
+
+            // Vim may give E121 or E720 in this case, but it does not look right to
+            // have either because both are results of reevaluation possibly-lambda
+            // node as a dictionary and here this is not going to happen.
+            ERROR_FROM_TOKEN_AND_MSG(
+                cur_token, _("E15: Expected lambda arguments list or arrow: %.*s"));
+          } else {
+            // Else it may appear that possibly-lambda node is actually a dictionary
+            // or curly-braces-name identifier.
+            lambda_node = NULL;
+            kv_drop(pt_stack, 1);
+          }
+        }
+        break;
+      }
+      case kEPTSingleAssignment: {
+        if (tok_type == kExprLexBracket && !cur_token.data.brc.closing) {
+          ERROR_FROM_TOKEN_AND_MSG(
+              cur_token,
+              _("E475: Nested lists not allowed when assigning: %.*s"));
+          kv_drop(pt_stack, 2);
+          assert(kv_size(pt_stack));
+          assert(kv_last(pt_stack) == kEPTExpr);
+          break;
+        }
+        FALLTHROUGH;
+      }
+      case kEPTAssignment: {
+        if (want_node == kENodeValue
+            && tok_type != kExprLexBracket
+            && tok_type != kExprLexPlainIdentifier
+            && (tok_type != kExprLexFigureBrace || cur_token.data.brc.closing)
+            && !(node_is_key && tok_type == kExprLexNumber)
+            && tok_type != kExprLexEnv
+            && tok_type != kExprLexOption
+            && tok_type != kExprLexRegister) {
+          ERROR_FROM_TOKEN_AND_MSG(
+              cur_token,
+              _("E15: Expected value part of assignment lvalue: %.*s"));
+          kv_drop(pt_stack, 1);
+        } else if (want_node == kENodeOperator
+                   && tok_type != kExprLexBracket
+                   && (tok_type != kExprLexFigureBrace
+                       || cur_token.data.brc.closing)
+                   && tok_type != kExprLexDot
+                   && (tok_type != kExprLexComma || !is_single_assignment)
+                   && tok_type != kExprLexAssignment) {
+          ERROR_FROM_TOKEN_AND_MSG(
+              cur_token,
+              _("E15: Expected assignment operator or subscript: %.*s"));
+          kv_drop(pt_stack, 1);
+        }
+        assert(kv_size(pt_stack));
+        break;
       }
     }
+    assert(kv_size(pt_stack));
     const ExprASTParseType cur_pt = kv_last(pt_stack);
     assert(lambda_node == NULL || cur_pt == kEPTLambdaArguments);
     switch (tok_type) {
@@ -2339,21 +2440,41 @@ viml_pexpr_parse_bracket_closing_error:
           }
           kvi_push(ast_stack, new_top_node_p);
           want_node = kENodeOperator;
+          if (cur_pt == kEPTSingleAssignment) {
+            kv_drop(pt_stack, 1);
+          } else if (cur_pt == kEPTAssignment) {
+            assert(ast.err.msg);
+          } else if (cur_pt == kEPTExpr
+                     && kv_size(pt_stack) > 1
+                     && pt_is_assignment(kv_Z(pt_stack, 1))) {
+            kv_drop(pt_stack, 1);
+          }
         } else {
           if (want_node == kENodeValue) {
-            // Value means list literal.
+            // Value means list literal or list assignment.
             HL_CUR_TOKEN(List);
             NEW_NODE_WITH_CUR_POS(cur_node, kExprNodeListLiteral);
             *top_node_p = cur_node;
             kvi_push(ast_stack, &cur_node->children);
             want_node = kENodeValue;
+            if (cur_pt == kEPTAssignment) {
+              // Additional assignment parse type allows to easily forbid nested
+              // lists.
+              kvi_push(pt_stack, kEPTSingleAssignment);
+            }
           } else {
+            // Operator means subscript, also in assignment. But in assignment
+            // subscript may be pretty much any expression, so need to push
+            // kEPTExpr.
             if (prev_token.type == kExprLexSpacing) {
               OP_MISSING;
             }
             NEW_NODE_WITH_CUR_POS(cur_node, kExprNodeSubscript);
             ADD_OP_NODE(cur_node);
             HL_CUR_TOKEN(SubscriptBracket);
+            if (pt_is_assignment(cur_pt)) {
+              kvi_push(pt_stack, kEPTExpr);
+            }
           }
         }
         break;
@@ -2458,15 +2579,31 @@ viml_pexpr_parse_figure_brace_closing_error:
           }
           kvi_push(ast_stack, new_top_node_p);
           want_node = kENodeOperator;
+          if (cur_pt == kEPTExpr
+              && kv_size(pt_stack) > 1
+              && pt_is_assignment(kv_Z(pt_stack, 1))) {
+            kv_drop(pt_stack, 1);
+          }
         } else {
           if (want_node == kENodeValue) {
             HL_CUR_TOKEN(FigureBrace);
             // Value: may be any of lambda, dictionary literal and curly braces
             // name.
-            NEW_NODE_WITH_CUR_POS(cur_node, kExprNodeUnknownFigure);
-            cur_node->data.fig.type_guesses.allow_lambda = true;
-            cur_node->data.fig.type_guesses.allow_dict = true;
-            cur_node->data.fig.type_guesses.allow_ident = true;
+
+            // Though if we are in an assignment this may only be a curly braces
+            // name.
+            if (pt_is_assignment(cur_pt)) {
+              NEW_NODE_WITH_CUR_POS(cur_node, kExprNodeCurlyBracesIdentifier);
+              cur_node->data.fig.type_guesses.allow_lambda = false;
+              cur_node->data.fig.type_guesses.allow_dict = false;
+              cur_node->data.fig.type_guesses.allow_ident = true;
+              kvi_push(pt_stack, kEPTExpr);
+            } else {
+              NEW_NODE_WITH_CUR_POS(cur_node, kExprNodeUnknownFigure);
+              cur_node->data.fig.type_guesses.allow_lambda = true;
+              cur_node->data.fig.type_guesses.allow_dict = true;
+              cur_node->data.fig.type_guesses.allow_ident = true;
+            }
             if (pstate->colors) {
               cur_node->data.fig.opening_hl_idx = kv_size(*pstate->colors) - 1;
             }
@@ -2484,6 +2621,9 @@ viml_pexpr_parse_figure_brace_closing_error:
                   cur_node->data.fig.type_guesses.allow_dict = false;
                   cur_node->data.fig.type_guesses.allow_ident = true;
                   kvi_push(ast_stack, &cur_node->children);
+                  if (pt_is_assignment(cur_pt)) {
+                    kvi_push(pt_stack, kEPTExpr);
+                  }
                   want_node = kENodeValue;
                 } while (0),
                 Curly);
@@ -2746,6 +2886,36 @@ viml_pexpr_parse_no_paren_closing_error: {}
         want_node = kENodeOperator;
         break;
       }
+      case kExprLexAssignment: {
+        if (cur_pt == kEPTAssignment) {
+          kv_drop(pt_stack, 1);
+        } else if (cur_pt == kEPTSingleAssignment) {
+          kv_drop(pt_stack, 2);
+          ERROR_FROM_TOKEN_AND_MSG(
+              cur_token,
+              _("E475: Expected closing bracket to end list assignment "
+                "lvalue: %.*s"));
+        } else {
+          ERROR_FROM_TOKEN_AND_MSG(
+              cur_token, _("E15: Misplaced assignment: %.*s"));
+        }
+        assert(kv_size(pt_stack));
+        assert(kv_last(pt_stack) == kEPTExpr);
+        ADD_VALUE_IF_MISSING(_("E15: Unexpected assignment: %.*s"));
+        NEW_NODE_WITH_CUR_POS(cur_node, kExprNodeAssignment);
+        cur_node->data.ass.type = cur_token.data.ass.type;
+        switch (cur_token.data.ass.type) {
+#define HL_ASGN(asgn, hl) \
+          case kExprAsgn##asgn: { HL_CUR_TOKEN(hl); break; }
+          HL_ASGN(Plain, PlainAssignment)
+          HL_ASGN(Add, AssignmentWithAddition)
+          HL_ASGN(Subtract, AssignmentWithSubtraction)
+          HL_ASGN(Concat, AssignmentWithConcatenation)
+#undef HL_ASGN
+        }
+        ADD_OP_NODE(cur_node);
+        break;
+      }
     }
 viml_pexpr_parse_cycle_end:
     prev_token = cur_token;
@@ -2862,6 +3032,7 @@ viml_pexpr_parse_end:
           // FIXME: Investigate whether above are OK to be present in the stack.
           break;
         }
+        case kExprNodeAssignment:
         case kExprNodeMod:
         case kExprNodeDivision:
         case kExprNodeMultiplication:
diff --git a/src/nvim/viml/parser/expressions.h b/src/nvim/viml/parser/expressions.h
index 648f8cbc1f..23e172da75 100644
--- a/src/nvim/viml/parser/expressions.h
+++ b/src/nvim/viml/parser/expressions.h
@@ -51,6 +51,9 @@ typedef enum {
   kExprLexParenthesis,  ///< Parenthesis, either opening or closing.
   kExprLexComma,  ///< Comma.
   kExprLexArrow,  ///< Arrow, like from lambda expressions.
+  kExprLexAssignment,  ///< Assignment: `=` or `{op}=`.
+  // XXX When modifying this enum you need to also modify eltkn_type_tab in
+  //     expressions.c and tests and, possibly, viml_pexpr_repr_token.
 } LexExprTokenType;
 
 typedef enum {
@@ -68,6 +71,14 @@ typedef enum {
   kExprOptScopeLocal = 'l',
 } ExprOptScope;
 
+/// All possible assignment types: `=` and `{op}=`.
+typedef enum {
+  kExprAsgnPlain = 0,  ///< Plain assignment: `=`.
+  kExprAsgnAdd,  ///< Assignment augmented with addition: `+=`.
+  kExprAsgnSubtract,  ///< Assignment augmented with subtraction: `-=`.
+  kExprAsgnConcat,  ///< Assignment augmented with concatenation: `.=`.
+} ExprAssignmentType;
+
 #define EXPR_OPT_SCOPE_LIST \
     ((char[]){ kExprOptScopeGlobal, kExprOptScopeLocal })
 
@@ -147,6 +158,10 @@ typedef struct {
       uint8_t base;  ///< Base: 2, 8, 10 or 16.
       bool is_float;  ///< True if number is a floating-point.
     } num;  ///< For kExprLexNumber
+
+    struct {
+      ExprAssignmentType type;
+    } ass;  ///< For kExprLexAssignment
   } data;  ///< Additional data, if needed.
 } LexExprToken;
 
@@ -170,8 +185,8 @@ typedef enum {
   /// “EOC” is something like "|". It is fine with emitting EOC at the end of
   /// string still, with or without this flag set.
   kELFlagForbidEOC = (1 << 4),
-  // WARNING: whenever you add a new flag, alter klee_assume() statement in
-  // viml_expressions_lexer.c.
+  // XXX Whenever you add a new flag, alter klee_assume() statement in
+  //     viml_expressions_lexer.c.
 } LexExprFlags;
 
 /// Expression AST node type
@@ -233,6 +248,10 @@ typedef enum {
   kExprNodeMod,
   kExprNodeOption,
   kExprNodeEnvironment,
+  kExprNodeAssignment,
+  // XXX When modifying this list also modify east_node_type_tab both in parser
+  //     and in tests, and you most likely will also have to alter list of
+  //     highlight groups stored in highlight_init_cmdline variable.
 } ExprASTNodeType;
 
 typedef struct expr_ast_node ExprASTNode;
@@ -301,6 +320,9 @@ struct expr_ast_node {
       const char *ident;  ///< Environment variable name start.
       size_t ident_len;  ///< Environment variable name length.
     } env;  ///< For kExprNodeEnvironment.
+    struct {
+      ExprAssignmentType type;
+    } ass;  ///< For kExprNodeAssignment
   } data;
 };
 
@@ -314,8 +336,15 @@ enum {
   /// When parsing expressions input by user bar is assumed to be a binary
   /// operator and other two are spacings.
   kExprFlagsDisallowEOC = (1 << 1),
-  // WARNING: whenever you add a new flag, alter klee_assume() statement in
-  // viml_expressions_parser.c.
+  /// Parse :let argument
+  ///
+  /// That mean that top level node must be an assignment and first nodes
+  /// belong to lvalues.
+  kExprFlagsParseLet = (1 << 2),
+  // XXX whenever you add a new flag, alter klee_assume() statement in
+  //     viml_expressions_parser.c, nvim_parse_expression() flags parsing
+  //     alongside with its documentation and flag sets in check_parsing()
+  //     function in expressions parser functional and unit tests.
 } ExprParserFlags;
 
 /// AST error definition
@@ -350,6 +379,9 @@ extern const char *const eltkn_cmp_type_tab[];
 /// Array mapping ExprCaseCompareStrategy values to their stringified versions
 extern const char *const ccs_tab[];
 
+/// Array mapping ExprAssignmentType values to their stringified versions
+extern const char *const expr_asgn_type_tab[];
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "viml/parser/expressions.h.generated.h"
 #endif
diff --git a/test/symbolic/klee/viml_expressions_parser.c b/test/symbolic/klee/viml_expressions_parser.c
index 9448937430..fd75e8d355 100644
--- a/test/symbolic/klee/viml_expressions_parser.c
+++ b/test/symbolic/klee/viml_expressions_parser.c
@@ -48,7 +48,8 @@ int main(const int argc, const char *const *const argv,
   klee_make_symbolic(&shift, sizeof(shift), "shift");
   klee_make_symbolic(&flags, sizeof(flags), "flags");
   klee_assume(shift < INPUT_SIZE);
-  klee_assume(flags <= (kExprFlagsMulti|kExprFlagsDisallowEOC));
+  klee_assume(
+      flags <= (kExprFlagsMulti|kExprFlagsDisallowEOC|kExprFlagsParseLet));
 #endif
 
   ParserLine plines[] = {
diff --git a/test/unit/viml/expressions/lexer_spec.lua b/test/unit/viml/expressions/lexer_spec.lua
index 75a641c48a..1b57a24ad5 100644
--- a/test/unit/viml/expressions/lexer_spec.lua
+++ b/test/unit/viml/expressions/lexer_spec.lua
@@ -13,6 +13,7 @@ local conv_ccs = viml_helpers.conv_ccs
 local new_pstate = viml_helpers.new_pstate
 local conv_cmp_type = viml_helpers.conv_cmp_type
 local pstate_set_str = viml_helpers.pstate_set_str
+local conv_expr_asgn_type = viml_helpers.conv_expr_asgn_type
 
 local shallowcopy = global_helpers.shallowcopy
 local intchar2lua = global_helpers.intchar2lua
@@ -52,6 +53,8 @@ child_call_once(function()
     [tonumber(lib.kExprLexParenthesis)] = 'Parenthesis',
     [tonumber(lib.kExprLexComma)] = 'Comma',
     [tonumber(lib.kExprLexArrow)] = 'Arrow',
+
+    [tonumber(lib.kExprLexAssignment)] = 'Assignment',
   }
 
   eltkn_mul_type_tab = {
@@ -118,6 +121,8 @@ local function eltkn2lua(pstate, tkn)
     ret.data.val = tonumber(tkn.data.num.is_float
                             and tkn.data.num.val.floating
                             or tkn.data.num.val.integer)
+  elseif ret.type == 'Assignment' then
+    ret.data = { type = conv_expr_asgn_type(tkn.data.ass.type) }
   elseif ret.type == 'Invalid' then
     ret.data = { error = ffi.string(tkn.data.err.msg) }
   end
@@ -198,7 +203,9 @@ describe('Expressions lexer', function()
     singl_eltkn_test('Question', '?')
     singl_eltkn_test('Colon', ':')
     singl_eltkn_test('Dot', '.')
+    singl_eltkn_test('Assignment', '.=', {type='Concat'})
     singl_eltkn_test('Plus', '+')
+    singl_eltkn_test('Assignment', '+=', {type='Add'})
     singl_eltkn_test('Comma', ',')
     singl_eltkn_test('Multiplication', '*', {type='Mul'})
     singl_eltkn_test('Multiplication', '/', {type='Div'})
@@ -266,12 +273,13 @@ describe('Expressions lexer', function()
     singl_eltkn_test('DoubleQuotedString', '"x\\"', {closed=false})
     singl_eltkn_test('DoubleQuotedString', '"\\"x', {closed=false})
     singl_eltkn_test('Not', '!')
-    singl_eltkn_test('Invalid', '=', {error='E15: Expected == or =~: %.*s'})
+    singl_eltkn_test('Assignment', '=', {type='Plain'})
     comparison_test('==', '!=', 'Equal')
     comparison_test('=~', '!~', 'Matches')
     comparison_test('>', '<=', 'Greater')
     comparison_test('>=', '<', 'GreaterOrEqual')
     singl_eltkn_test('Minus', '-')
+    singl_eltkn_test('Assignment', '-=', {type='Subtract'})
     singl_eltkn_test('Arrow', '->')
     singl_eltkn_test('Invalid', '~', {error='E15: Unidentified character: %.*s'})
     simple_test({{data=nil, size=0}}, 'EOC', 0, {error='start.col >= #pstr'})
diff --git a/test/unit/viml/expressions/parser_spec.lua b/test/unit/viml/expressions/parser_spec.lua
index cfc9fe95ac..810d7bfbc6 100644
--- a/test/unit/viml/expressions/parser_spec.lua
+++ b/test/unit/viml/expressions/parser_spec.lua
@@ -18,6 +18,7 @@ local conv_ccs = viml_helpers.conv_ccs
 local new_pstate = viml_helpers.new_pstate
 local conv_cmp_type = viml_helpers.conv_cmp_type
 local pstate_set_str = viml_helpers.pstate_set_str
+local conv_expr_asgn_type = viml_helpers.conv_expr_asgn_type
 
 local mergedicts_copy = global_helpers.mergedicts_copy
 local format_string = global_helpers.format_string
@@ -109,6 +110,7 @@ make_enum_conv_tab(lib, {
   'kExprNodeMod',
   'kExprNodeOption',
   'kExprNodeEnvironment',
+  'kExprNodeAssignment',
 }, 'kExprNode', function(ret) east_node_type_tab = ret end)
 
 local function conv_east_node_type(typ)
@@ -174,6 +176,8 @@ local function eastnode2lua(pstate, eastnode, checked_nodes)
     typ = ('%s(ident=%s)'):format(
       typ,
       ffi.string(eastnode.data.env.ident, eastnode.data.env.ident_len))
+  elseif typ == 'Assignment' then
+    typ = ('%s(%s)'):format(typ, conv_expr_asgn_type(eastnode.data.ass.type))
   end
   ret_str = typ .. ':' .. ret_str
   local can_simplify = not ret.children
@@ -3976,27 +3980,20 @@ describe('Expressions parser', function()
       --           012345
       ast = {
         {
-          'Comparison(type=Equal,inv=0,ccs=UseOption):0:3:=',
+          'Assignment(Add):0:1: +=',
           children = {
-            {
-              'BinaryPlus:0:1: +',
-              children = {
-                'PlainIdentifier(scope=0,ident=a):0:0:a',
-                'Missing:0:3:',
-              },
-            },
+            'PlainIdentifier(scope=0,ident=a):0:0:a',
             'PlainIdentifier(scope=0,ident=b):0:4: b',
           },
         },
       },
       err = {
-        arg = '= b',
-        msg = 'E15: Expected == or =~: %.*s',
+        arg = '+= b',
+        msg = 'E15: Misplaced assignment: %.*s',
       },
     }, {
       hl('IdentifierName', 'a'),
-      hl('BinaryPlus', '+', 1),
-      hl('InvalidComparison', '='),
+      hl('InvalidAssignmentWithAddition', '+=', 1),
       hl('IdentifierName', 'b', 1),
     })
     check_parsing('a + b == c + d', {
@@ -7347,4 +7344,6 @@ describe('Expressions parser', function()
       },
     })
   end)
+  -- FIXME: Test assignments thoroughly
+  -- FIXME: Test that parsing assignments can be used for `:for` pre-`in` part.
 end)
diff --git a/test/unit/viml/helpers.lua b/test/unit/viml/helpers.lua
index 9d2d7b61c7..9d8102e023 100644
--- a/test/unit/viml/helpers.lua
+++ b/test/unit/viml/helpers.lua
@@ -107,6 +107,18 @@ local function conv_ccs(ccs)
   return conv_enum(ccs_tab, ccs)
 end
 
+local expr_asgn_type_tab
+make_enum_conv_tab(lib, {
+  'kExprAsgnPlain',
+  'kExprAsgnAdd',
+  'kExprAsgnSubtract',
+  'kExprAsgnConcat',
+}, 'kExprAsgn', function(ret) expr_asgn_type_tab = ret end)
+
+local function conv_expr_asgn_type(expr_asgn_type)
+  return conv_enum(expr_asgn_type_tab, expr_asgn_type)
+end
+
 return {
   conv_ccs = conv_ccs,
   pline2lua = pline2lua,
@@ -114,4 +126,5 @@ return {
   new_pstate = new_pstate,
   conv_cmp_type = conv_cmp_type,
   pstate_set_str = pstate_set_str,
+  conv_expr_asgn_type = conv_expr_asgn_type,
 }