Expose currently-silent failure in language server (#4879)

Not sure if I mixed up a merge somewhere, but this is what
`parse_error.carbon` was supposed to test. Note
`unexpected_reply.carbon` was basically doing the same test, with a
minor issue of `change` vs `result` to get better output.

Author: jonmeow

toolchain/diagnostics/diagnostic_kind.def

@@ -29,6 +29,7 @@ CARBON_DIAGNOSTIC_KIND(CompileInputNotRegularFile)
 CARBON_DIAGNOSTIC_KIND(CompileOutputFileOpenError)
 CARBON_DIAGNOSTIC_KIND(FormatMultipleFilesToOneOutput)
 CARBON_DIAGNOSTIC_KIND(LanguageServerMissingInputStream)
+CARBON_DIAGNOSTIC_KIND(LanguageServerNotificationParseError)
 CARBON_DIAGNOSTIC_KIND(LanguageServerTransportError)
 CARBON_DIAGNOSTIC_KIND(LanguageServerUnexpectedReply)
 CARBON_DIAGNOSTIC_KIND(LanguageServerUnsupportedNotification)

toolchain/language_server/incoming_messages.cpp

@@ -57,8 +57,11 @@ auto IncomingMessages::AddNotificationHandler(llvm::StringRef name,
       [name, handler](Context& context, llvm::json::Value raw_params) -> void {
     auto params = Parse<ParamsT>(name, raw_params);
     if (!params) {
-      // TODO: Maybe we should do something more with this error?
-      llvm::consumeError(params.takeError());
+      CARBON_DIAGNOSTIC(LanguageServerNotificationParseError, Warning, "{0}",
+                        std::string);
+      context.no_loc_emitter().Emit(LanguageServerNotificationParseError,
+                                    llvm::toString(params.takeError()));
+      return;
     }
     handler(context, *params);
   };

toolchain/language_server/testdata/parse_error.carbon renamed to toolchain/language_server/testdata/notify_parse_error.carbon

@@ -4,20 +4,17 @@
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
-// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/language_server/testdata/parse_error.carbon
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/language_server/testdata/notify_parse_error.carbon
 // TIP: To dump output, run:
-// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/language_server/testdata/parse_error.carbon
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/language_server/testdata/notify_parse_error.carbon
 
 
 // --- STDIN
-[[@LSP-REPLY:1]]
-[[@LSP-REPLY:2:"result": 3]]
+[[@LSP-NOTIFY:textDocument/didOpen:"nope": "bad"]]
 [[@LSP-NOTIFY:exit]]
 
 // --- AUTOUPDATE-SPLIT
 
-// CHECK:STDERR: warning: unexpected reply to request ID "1": null [LanguageServerUnexpectedReply]
-// CHECK:STDERR:
-// CHECK:STDERR: warning: unexpected reply to request ID "2": 3 [LanguageServerUnexpectedReply]
+// CHECK:STDERR: warning: -32602: in call to `textDocument/didOpen`, JSON parse failed: expected object [LanguageServerNotificationParseError]
 // CHECK:STDERR:
 // CHECK:STDOUT:

toolchain/language_server/testdata/unexpected_reply.carbon

@@ -11,13 +11,13 @@
 
 // --- STDIN
 [[@LSP-REPLY:1]]
-[[@LSP-REPLY:2:"value": 3]]
+[[@LSP-REPLY:2:"result": 3]]
 [[@LSP-NOTIFY:exit]]
 
 // --- AUTOUPDATE-SPLIT
 
 // CHECK:STDERR: warning: unexpected reply to request ID "1": null [LanguageServerUnexpectedReply]
 // CHECK:STDERR:
-// CHECK:STDERR: warning: unexpected reply to request ID "2": null [LanguageServerUnexpectedReply]
+// CHECK:STDERR: warning: unexpected reply to request ID "2": 3 [LanguageServerUnexpectedReply]
 // CHECK:STDERR:
 // CHECK:STDOUT: