Add comments around user code blocks to generated Rust module

This takes the place of #line directives

doc/user_guide.md

@@ -695,10 +695,21 @@ generated output without any surrounding `#line` directives.
 This can be useful when debugging the generated parser itself, or when the
 `#line` directives interfere with other tooling.
 
-The `noline` statement only affects the C, C++, and D targets.
+Rust has no `#line` directive equivalent.
-Rust has no `#line` directive equivalent, so `#line` directives are never
+For a Rust target, Propane instead emits a comment before and after each
-emitted into Rust output and the `noline` statement has no effect for the Rust
+section of user code naming the grammar file and the line number the code was
-target.
+taken from:
+
+```
+/* Begin user code from myparser.propane line 42. */
+  let mut v: i64 = 0;
+/* End user code from myparser.propane line 42. */
+```
+
+A compiler diagnostic that points into the generated Rust module can be traced
+back to the grammar by reading up to the nearest such comment.
+The `noline` statement suppresses these comments for a Rust target, in the same
+way that it suppresses `#line` directives for the other targets.
 
 ##> `on_tree_node` statement -  custom initialization of a token tree node
 

lib/propane/generator.rb

@@ -41,12 +41,23 @@ class Propane
           output_file = @output_file
         end
         erb = ERB.new(template, trim_mode: "<>")
+        # Rust has no #line directive support. For a Rust target the directives
+        # that the grammar embeds around user code blocks are replaced with
+        # comments naming the grammar file and line number the code came from,
+        # so that the origin of a section of user code can still be found by
+        # reading up from a compiler diagnostic pointing into the generated
+        # module.
+        user_code_origin = nil
         result = erb.result(binding.clone).lines.each_with_index.map do |line, i|
           if @language == "rust"
-            # Rust has no #line directive support, so strip the directives that
+            if md = line.match(/^#line (\d+) "([^"]*)"/)
-            # the grammar embeds in user code blocks.
+              user_code_origin = "#{md[2]} line #{md[1]}"
-            line = line.sub(/^#line \d+ "[^"]*"/, "")
+              line.sub(/^#line \d+ "[^"]*"/, %[/* Begin user code from #{user_code_origin}. */])
-            line == "#linereset\n" ? "" : line
+            elsif line == "#linereset\n"
+              %[/* End user code from #{user_code_origin}. */\n]
+            else
+              line
+            end
           elsif line == "#linereset\n"
             %[#line #{i + 2} "#{output_file}"\n]
           else

spec/propane_spec.rb

@@ -2113,6 +2113,44 @@ EOF
         end
       end
 
+      if language == "rust"
+        it "marks user code sections with their grammar file and line number" do
+          write_grammar <<EOF
+ptype i64;
+drop /\\s+/;
+token num /\\d+/ <<
+  $$ = 42;
+>>
+Start -> num << $$ = $1; >>
+EOF
+          run_propane(language: language)
+          parser = File.binread("spec/run/testparser.rs")
+          # The lexer code block body begins on grammar line 4 and the parser
+          # rule code block is on grammar line 6.
+          expect(parser).to include %[/* Begin user code from spec/run/testparser.propane line 4. */]
+          expect(parser).to include %[/* End user code from spec/run/testparser.propane line 4. */]
+          expect(parser).to include %[/* Begin user code from spec/run/testparser.propane line 6. */]
+          expect(parser).to include %[/* End user code from spec/run/testparser.propane line 6. */]
+          expect(parser).to_not include "#line"
+        end
+
+        it "omits user code section markers when noline is specified" do
+          write_grammar <<EOF
+noline;
+ptype i64;
+drop /\\s+/;
+token num /\\d+/ <<
+  $$ = 42;
+>>
+Start -> num << $$ = $1; >>
+EOF
+          run_propane(language: language)
+          parser = File.binread("spec/run/testparser.rs")
+          expect(parser).to_not include "user code from"
+          expect(parser).to_not include "#line"
+        end
+      end
+
       it "executes code blocks associated with drop statements" do
         if language == "rust"
           write_grammar <<EOF