Allow file descriptor be generated without --include_source_info

The file descriptor sets generated by rules_proto in bazel don't have
this by default, so this allows some flexibility for users to reuse
results that are already available to them.  It's fairly trivial
adjustment so it seemed reasonable to me to allow the flexibility.

prost-build/src/ast.rs

@@ -134,7 +134,7 @@ pub struct Service {
     /// The package name as it appears in the .proto file.
     pub package: String,
     /// The service comments.
-    pub comments: Comments,
+    pub comments: Option<Comments>,
     /// The service methods.
     pub methods: Vec<Method>,
     /// The service options.
@@ -149,7 +149,7 @@ pub struct Method {
     /// The name of the method as it appears in the .proto file.
     pub proto_name: String,
     /// The method comments.
-    pub comments: Comments,
+    pub comments: Option<Comments>,
     /// The input Rust type.
     pub input_type: String,
     /// The output Rust type.

prost-build/src/code_generator.rs

@@ -29,7 +29,7 @@ enum Syntax {
 pub struct CodeGenerator<'a> {
     config: &'a mut Config,
     package: String,
-    source_info: SourceCodeInfo,
+    source_info: Option<SourceCodeInfo>,
     syntax: Syntax,
     message_graph: &'a MessageGraph,
     extern_paths: &'a ExternPaths,
@@ -51,16 +51,14 @@ impl<'a> CodeGenerator<'a> {
         file: FileDescriptorProto,
         buf: &mut String,
     ) {
-        let mut source_info = file
-            .source_code_info
-            .expect("no source code info in request");
-        source_info.location.retain(|location| {
-            let len = location.path.len();
-            len > 0 && len % 2 == 0
+        let source_info = file.source_code_info.map(|mut s| {
+            s.location.retain(|loc| {
+                let len = loc.path.len();
+                len > 0 && len % 2 == 0
+            });
+            s.location.sort_by(|a, b| a.path.cmp(&b.path));
+            s
         });
-        source_info
-            .location
-            .sort_by_key(|location| location.path.clone());
 
         let syntax = match file.syntax.as_ref().map(String::as_str) {
             None | Some("proto2") => Syntax::Proto2,
@@ -569,14 +567,13 @@ impl<'a> CodeGenerator<'a> {
         self.buf.push_str("}\n");
     }
 
-    fn location(&self) -> &Location {
-        let idx = self
-            .source_info
+    fn location(&self) -> Option<&Location> {
+        let source_info = self.source_info.as_ref()?;
+        let idx = source_info
             .location
             .binary_search_by_key(&&self.path[..], |location| &location.path[..])
             .unwrap();
-
-        &self.source_info.location[idx]
+        Some(&source_info.location[idx])
     }
 
     fn append_doc(&mut self, fq_name: &str, field_name: Option<&str>) {
@@ -589,7 +586,9 @@ impl<'a> CodeGenerator<'a> {
             self.config.disable_comments.get(fq_name).next().is_none()
         };
         if append_doc {
-            Comments::from_location(self.location()).append_with_indent(self.depth, self.buf)
+            if let Some(comments) = self.location().map(Comments::from_location) {
+                comments.append_with_indent(self.depth, self.buf);
+            }
         }
     }
 
@@ -715,7 +714,7 @@ impl<'a> CodeGenerator<'a> {
 
         for variant in variant_mappings.iter() {
             self.push_indent();
-            self.buf.push_str("\"");
+            self.buf.push('\"');
             self.buf.push_str(variant.proto_name);
             self.buf.push_str("\" => Some(Self::");
             self.buf.push_str(&variant.generated_variant_name);
@@ -742,7 +741,7 @@ impl<'a> CodeGenerator<'a> {
         let name = service.name().to_owned();
         debug!("  service: {:?}", name);
 
-        let comments = Comments::from_location(self.location());
+        let comments = self.location().map(Comments::from_location);
 
         self.path.push(2);
         let methods = service
@@ -751,8 +750,9 @@ impl<'a> CodeGenerator<'a> {
             .enumerate()
             .map(|(idx, mut method)| {
                 debug!("  method: {:?}", method.name());
+
                 self.path.push(idx as i32);
-                let comments = Comments::from_location(self.location());
+                let comments = self.location().map(Comments::from_location);
                 self.path.pop();
 
                 let name = method.name.take().unwrap();

prost-build/src/lib.rs

@@ -664,8 +664,7 @@ impl Config {
 
     /// In combination with with `file_descriptor_set_path`, this can be used to provide a file
     /// descriptor set as an input file, rather than having prost-build generate the file by calling
-    /// protoc.  Prost-build does require that the descriptor set was generated with
-    /// --include_source_info.
+    /// protoc.
     ///
     /// In `build.rs`:
     ///
@@ -1333,12 +1332,16 @@ mod tests {
     impl ServiceGenerator for ServiceTraitGenerator {
         fn generate(&mut self, service: Service, buf: &mut String) {
             // Generate a trait for the service.
-            service.comments.append_with_indent(0, buf);
+            if let Some(comments) = service.comments {
+                comments.append_with_indent(0, buf);
+            }
             buf.push_str(&format!("trait {} {{\n", &service.name));
 
             // Generate the service methods.
             for method in service.methods {
-                method.comments.append_with_indent(1, buf);
+                if let Some(comments) = method.comments {
+                    comments.append_with_indent(1, buf);
+                }
                 buf.push_str(&format!(
                     "    fn {}(_: {}) -> {};\n",
                     method.name, method.input_type, method.output_type