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diff --git a/src/nvim/viml/parser/expressions.h b/src/nvim/viml/parser/expressions.h
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+++ b/src/nvim/viml/parser/expressions.h
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+#ifndef NVIM_VIML_PARSER_EXPRESSIONS_H
+#define NVIM_VIML_PARSER_EXPRESSIONS_H
+
+#include <stddef.h>
+#include <stdint.h>
+#include <stdbool.h>
+
+#include "nvim/types.h"
+#include "nvim/viml/parser/parser.h"
+#include "nvim/eval/typval.h"
+
+// Defines whether to ignore case:
+//    ==   kCCStrategyUseOption
+//    ==#  kCCStrategyMatchCase
+//    ==?  kCCStrategyIgnoreCase
+typedef enum {
+  kCCStrategyUseOption = 0,  // 0 for xcalloc
+  kCCStrategyMatchCase = '#',
+  kCCStrategyIgnoreCase = '?',
+} ExprCaseCompareStrategy;
+
+/// Lexer token type
+typedef enum {
+  kExprLexInvalid = 0,  ///< Invalid token, indicaten an error.
+  kExprLexMissing,  ///< Missing token, for use in parser.
+  kExprLexSpacing,  ///< Spaces, tabs, newlines, etc.
+  kExprLexEOC,  ///< End of command character: NL, |, just end of stream.
+
+  kExprLexQuestion,  ///< Question mark, for use in ternary.
+  kExprLexColon,  ///< Colon, for use in ternary.
+  kExprLexOr,  ///< Logical or operator.
+  kExprLexAnd,  ///< Logical and operator.
+  kExprLexComparison,  ///< One of the comparison operators.
+  kExprLexPlus,  ///< Plus sign.
+  kExprLexMinus,  ///< Minus sign.
+  kExprLexDot,  ///< Dot: either concat or subscript, also part of the float.
+  kExprLexMultiplication,  ///< Multiplication, division or modulo operator.
+
+  kExprLexNot,  ///< Not: !.
+
+  kExprLexNumber,  ///< Integer number literal, or part of a float.
+  kExprLexSingleQuotedString,  ///< Single quoted string literal.
+  kExprLexDoubleQuotedString,  ///< Double quoted string literal.
+  kExprLexOption,  ///< &optionname option value.
+  kExprLexRegister,  ///< @r register value.
+  kExprLexEnv,  ///< Environment $variable value.
+  kExprLexPlainIdentifier,  ///< Identifier without scope: `abc`, `foo#bar`.
+
+  kExprLexBracket,  ///< Bracket, either opening or closing.
+  kExprLexFigureBrace,  ///< Figure brace, either opening or closing.
+  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 {
+  kExprCmpEqual,  ///< Equality, unequality.
+  kExprCmpMatches,  ///< Matches regex, not matches regex.
+  kExprCmpGreater,  ///< `>` or `<=`
+  kExprCmpGreaterOrEqual,  ///< `>=` or `<`.
+  kExprCmpIdentical,  ///< `is` or `isnot`
+} ExprComparisonType;
+
+/// All possible option scopes
+typedef enum {
+  kExprOptScopeUnspecified = 0,
+  kExprOptScopeGlobal = 'g',
+  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 })
+
+/// All possible variable scopes
+typedef enum {
+  kExprVarScopeMissing = 0,
+  kExprVarScopeScript = 's',
+  kExprVarScopeGlobal = 'g',
+  kExprVarScopeVim = 'v',
+  kExprVarScopeBuffer = 'b',
+  kExprVarScopeWindow = 'w',
+  kExprVarScopeTabpage = 't',
+  kExprVarScopeLocal = 'l',
+  kExprVarScopeArguments = 'a',
+} ExprVarScope;
+
+#define EXPR_VAR_SCOPE_LIST \
+    ((char[]) { \
+        kExprVarScopeScript, kExprVarScopeGlobal, kExprVarScopeVim, \
+        kExprVarScopeBuffer, kExprVarScopeWindow, kExprVarScopeTabpage, \
+        kExprVarScopeLocal, kExprVarScopeBuffer, kExprVarScopeArguments, \
+    })
+
+/// Lexer token
+typedef struct {
+  ParserPosition start;
+  size_t len;
+  LexExprTokenType type;
+  union {
+    struct {
+      ExprComparisonType type;  ///< Comparison type.
+      ExprCaseCompareStrategy ccs;  ///< Case comparison strategy.
+      bool inv;  ///< True if comparison is to be inverted.
+    } cmp;  ///< For kExprLexComparison.
+
+    struct {
+      enum {
+        kExprLexMulMul,  ///< Real multiplication.
+        kExprLexMulDiv,  ///< Division.
+        kExprLexMulMod,  ///< Modulo.
+      } type;  ///< Multiplication type.
+    } mul;  ///< For kExprLexMultiplication.
+
+    struct {
+      bool closing;  ///< True if bracket/etc is a closing one.
+    } brc;  ///< For brackets/braces/parenthesis.
+
+    struct {
+      int name;  ///< Register name, may be -1 if name not present.
+    } reg;  ///< For kExprLexRegister.
+
+    struct {
+      bool closed;  ///< True if quote was closed.
+    } str;  ///< For kExprLexSingleQuotedString and kExprLexDoubleQuotedString.
+
+    struct {
+      const char *name;  ///< Option name start.
+      size_t len;  ///< Option name length.
+      ExprOptScope scope;  ///< Option scope: &l:, &g: or not specified.
+    } opt;  ///< Option properties.
+
+    struct {
+      ExprVarScope scope;  ///< Scope character or 0 if not present.
+      bool autoload;  ///< Has autoload characters.
+    } var;  ///< For kExprLexPlainIdentifier
+
+    struct {
+      LexExprTokenType type;  ///< Suggested type for parsing incorrect code.
+      const char *msg;  ///< Error message.
+    } err;  ///< For kExprLexInvalid
+
+    struct {
+      union {
+        float_T floating;
+        uvarnumber_T integer;
+      } val;  ///< Number value.
+      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;
+
+typedef enum {
+  /// If set, “pointer” to the current byte in pstate will not be shifted
+  kELFlagPeek = (1 << 0),
+  /// Determines whether scope is allowed to come before the identifier
+  kELFlagForbidScope = (1 << 1),
+  /// Determines whether floating-point numbers are allowed
+  ///
+  /// I.e. whether dot is a decimal point separator or is not a part of
+  /// a number at all.
+  kELFlagAllowFloat = (1 << 2),
+  /// Determines whether `is` and `isnot` are seen as comparison operators
+  ///
+  /// If set they are supposed to be just regular identifiers.
+  kELFlagIsNotCmp = (1 << 3),
+  /// Determines whether EOC tokens are allowed
+  ///
+  /// If set then it will yield Invalid token with E15 in place of EOC one if
+  /// “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),
+  // XXX Whenever you add a new flag, alter klee_assume() statement in
+  //     viml_expressions_lexer.c.
+} LexExprFlags;
+
+/// Expression AST node type
+typedef enum {
+  kExprNodeMissing = 0,
+  kExprNodeOpMissing,
+  kExprNodeTernary,  ///< Ternary operator.
+  kExprNodeTernaryValue,  ///< Ternary operator, colon.
+  kExprNodeRegister,  ///< Register.
+  kExprNodeSubscript,  ///< Subscript.
+  kExprNodeListLiteral,  ///< List literal.
+  kExprNodeUnaryPlus,
+  kExprNodeBinaryPlus,
+  kExprNodeNested,  ///< Nested parenthesised expression.
+  kExprNodeCall,  ///< Function call.
+  /// Plain identifier: simple variable/function name
+  ///
+  /// Looks like "string", "g:Foo", etc: consists from a single
+  /// kExprLexPlainIdentifier token.
+  kExprNodePlainIdentifier,
+  /// Plain dictionary key, for use with kExprNodeConcatOrSubscript
+  kExprNodePlainKey,
+  /// Complex identifier: variable/function name with curly braces
+  kExprNodeComplexIdentifier,
+  /// Figure brace expression which is not yet known
+  ///
+  /// May resolve to any of kExprNodeDictLiteral, kExprNodeLambda or
+  /// kExprNodeCurlyBracesIdentifier.
+  kExprNodeUnknownFigure,
+  kExprNodeLambda,  ///< Lambda.
+  kExprNodeDictLiteral,  ///< Dictionary literal.
+  kExprNodeCurlyBracesIdentifier,  ///< Part of the curly braces name.
+  kExprNodeComma,  ///< Comma “operator”.
+  kExprNodeColon,  ///< Colon “operator”.
+  kExprNodeArrow,  ///< Arrow “operator”.
+  kExprNodeComparison,  ///< Various comparison operators.
+  /// Concat operator
+  ///
+  /// To be only used in cases when it is known for sure it is not a subscript.
+  kExprNodeConcat,
+  /// Concat or subscript operator
+  ///
+  /// For cases when it is not obvious whether expression is a concat or
+  /// a subscript. May only have either number or plain identifier as the second
+  /// child. To make it easier to avoid curly braces in place of
+  /// kExprNodePlainIdentifier node kExprNodePlainKey is used.
+  kExprNodeConcatOrSubscript,
+  kExprNodeInteger,  ///< Integral number.
+  kExprNodeFloat,  ///< Floating-point number.
+  kExprNodeSingleQuotedString,
+  kExprNodeDoubleQuotedString,
+  kExprNodeOr,
+  kExprNodeAnd,
+  kExprNodeUnaryMinus,
+  kExprNodeBinaryMinus,
+  kExprNodeNot,
+  kExprNodeMultiplication,
+  kExprNodeDivision,
+  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;
+
+/// Structure representing one AST node
+struct expr_ast_node {
+  ExprASTNodeType type;  ///< Node type.
+  /// Node children: e.g. for 1 + 2 nodes 1 and 2 will be children of +.
+  ExprASTNode *children;
+  /// Next node: e.g. for 1 + 2 child nodes 1 and 2 are put into a single-linked
+  /// list: `(+)->children` references only node 1, node 2 is in
+  /// `(+)->children->next`.
+  ExprASTNode *next;
+  ParserPosition start;
+  size_t len;
+  union {
+    struct {
+      int name;  ///< Register name, may be -1 if name not present.
+    } reg;  ///< For kExprNodeRegister.
+    struct {
+      /// Which nodes UnknownFigure can’t possibly represent.
+      struct {
+        /// True if UnknownFigure may actually represent dictionary literal.
+        bool allow_dict;
+        /// True if UnknownFigure may actually represent lambda.
+        bool allow_lambda;
+        /// True if UnknownFigure may actually be part of curly braces name.
+        bool allow_ident;
+      } type_guesses;
+      /// Highlight chunk index, used for rehighlighting if needed
+      size_t opening_hl_idx;
+    } fig;  ///< For kExprNodeUnknownFigure.
+    struct {
+      ExprVarScope scope;  ///< Scope character or 0 if not present.
+      /// Actual identifier without scope.
+      ///
+      /// Points to inside parser reader state.
+      const char *ident;
+      size_t ident_len;  ///< Actual identifier length.
+    } var;  ///< For kExprNodePlainIdentifier and kExprNodePlainKey.
+    struct {
+      bool got_colon;  ///< True if colon was seen.
+    } ter;  ///< For kExprNodeTernaryValue.
+    struct {
+      ExprComparisonType type;  ///< Comparison type.
+      ExprCaseCompareStrategy ccs;  ///< Case comparison strategy.
+      bool inv;  ///< True if comparison is to be inverted.
+    } cmp;  ///< For kExprNodeComparison.
+    struct {
+      uvarnumber_T value;
+    } num;  ///< For kExprNodeInteger.
+    struct {
+      float_T value;
+    } flt;  ///< For kExprNodeFloat.
+    struct {
+      char *value;
+      size_t size;
+    } str;  ///< For kExprNodeSingleQuotedString and
+            ///< kExprNodeDoubleQuotedString.
+    struct {
+      const char *ident;  ///< Option name start.
+      size_t ident_len;  ///< Option name length.
+      ExprOptScope scope;  ///< Option scope: &l:, &g: or not specified.
+    } opt;  ///< For kExprNodeOption.
+    struct {
+      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;
+};
+
+enum {
+  /// Allow multiple expressions in a row: e.g. for :echo
+  ///
+  /// Parser will still parse only one of them though.
+  kExprFlagsMulti = (1 << 0),
+  /// Allow NL, NUL and bar to be EOC
+  ///
+  /// When parsing expressions input by user bar is assumed to be a binary
+  /// operator and other two are spacings.
+  kExprFlagsDisallowEOC = (1 << 1),
+  /// 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
+typedef struct {
+  /// Error message. Must contain a single printf format atom: %.*s.
+  const char *msg;
+  /// Error message argument: points to the location of the error.
+  const char *arg;
+  /// Message argument length: length till the end of string.
+  int arg_len;
+} ExprASTError;
+
+/// Structure representing complety AST for one expression
+typedef struct {
+  /// When AST is not correct this message will be printed.
+  ///
+  /// Uses `emsgf(msg, arg_len, arg);`, `msg` is assumed to contain only `%.*s`.
+  ExprASTError err;
+  /// Root node of the AST.
+  ExprASTNode *root;
+} ExprAST;
+
+/// Array mapping ExprASTNodeType to maximum amount of children node may have
+extern const uint8_t node_maxchildren[];
+
+/// Array mapping ExprASTNodeType values to their stringified versions
+extern const char *const east_node_type_tab[];
+
+/// Array mapping ExprComparisonType values to their stringified versions
+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
+
+#endif  // NVIM_VIML_PARSER_EXPRESSIONS_H