Refactor do keyword parsing

Mirror much more closely the parse.y COND and CMDARG stacks. This
fixes a whole host of really really weird edge cases.

Author: kddnewton

config.yml

@@ -496,6 +496,8 @@ tokens:
     comment: "defined?"
   - name: KEYWORD_DO_BLOCK
     comment: "do keyword for a block attached to a command"
+  - name: KEYWORD_DO_LAMBDA
+    comment: "do keyword that opens the body of a lambda literal"
   - name: KEYWORD_DO_LOOP
     comment: "do keyword for a predicate in a while, until, or for loop"
   - name: KEYWORD_END_UPCASE

lib/prism/lex_compat.rb

@@ -141,6 +141,7 @@ def deconstruct_keys(keys) # :nodoc:
       KEYWORD_DEFINED: :on_kw,
       KEYWORD_DO: :on_kw,
       KEYWORD_DO_BLOCK: :on_kw,
+      KEYWORD_DO_LAMBDA: :on_kw,
       KEYWORD_DO_LOOP: :on_kw,
       KEYWORD_ELSE: :on_kw,
       KEYWORD_ELSIF: :on_kw,

lib/prism/translation/parser/lexer.rb

@@ -88,6 +88,7 @@ class Lexer # :nodoc:
           KEYWORD_DEFINED: :kDEFINED,
           KEYWORD_DO: :kDO,
           KEYWORD_DO_BLOCK: :kDO_BLOCK,
+          KEYWORD_DO_LAMBDA: :kDO_LAMBDA,
           KEYWORD_DO_LOOP: :kDO_COND,
           KEYWORD_END: :kEND,
           KEYWORD_END_UPCASE: :klEND,
@@ -192,14 +193,6 @@ class Lexer # :nodoc:
         EXPR_BEG = 0x1
         EXPR_LABEL = 0x400
 
-        # It is used to determine whether `do` is of the token type `kDO` or
-        # `kDO_LAMBDA`.
-        #
-        # NOTE: In edge cases like `-> (foo = -> (bar) {}) do end`, please note
-        # that `kDO` is still returned instead of `kDO_LAMBDA`, which is
-        # expected: https://github.com/ruby/prism/pull/3046
-        LAMBDA_TOKEN_TYPES = Set.new([:kDO_LAMBDA, :tLAMBDA, :tLAMBEG])
-
         # The `PARENTHESIS_LEFT` token in Prism is classified as either
         # `tLPAREN` or `tLPAREN2` in the Parser gem. The following token types
         # are listed as those classified as `tLPAREN`.
@@ -221,7 +214,7 @@ class Lexer # :nodoc:
         # Heredocs are complex and require us to keep track of a bit of info to refer to later
         HeredocData = Struct.new(:identifier, :common_whitespace, keyword_init: true)
 
-        private_constant :TYPES, :EXPR_BEG, :EXPR_LABEL, :LAMBDA_TOKEN_TYPES, :LPAREN_CONVERSION_TOKEN_TYPES, :HeredocData
+        private_constant :TYPES, :EXPR_BEG, :EXPR_LABEL, :LPAREN_CONVERSION_TOKEN_TYPES, :HeredocData
 
         # The Parser::Source::Buffer that the tokens were lexed from.
         attr_reader :source_buffer
@@ -269,14 +262,6 @@ def to_a
             location = range(token.location.start_offset, token.location.end_offset)
 
             case type
-            when :kDO
-              nearest_lambda_token = tokens.reverse_each.find do |token|
-                LAMBDA_TOKEN_TYPES.include?(token.first)
-              end
-
-              if nearest_lambda_token&.first == :tLAMBDA
-                type = :kDO_LAMBDA
-              end
             when :tCHARACTER
               value.delete_prefix!("?")
               # Character literals behave similar to double-quoted strings. We can use the same escaping mechanism.