Add tags to diagnostics

Also removes the diagnostic.Data from the new diagnostics, which AFAICS
is unused; we can save ourselves the json marshaling.

As a part of this, added a first pass at adding tests for diagnostics
- we don't currently have any. These are a bit different because we
haven't implemented [PullDiagnostics][1] (we can add this, but we'd need
a custom implementation, or wait until [go.lsp.dev upgrades to
3.17][2]). Instead, we intercept the notifications from the
server. Lastly, since we don't actually want the tests to wait around,
adds synctest and only runs the tests on Go 1.25 for now (1.26 should be
released next month and we'll be able to upgrade to 1.25 as our
minimum).

This is in preparation to eventually landing relatedInformation (other
spans) in diagnostics, from bufbuild/protocompile#659.

[1]: https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_pullDiagnostics
[2]: go-language-server/protocol#52

Author: stefanvanburen

private/buf/buflsp/buflsp_test.go

@@ -121,26 +121,20 @@ func setupLSPServer(
 
 	stream := jsonrpc2.NewStream(serverConn)
 
-	go func() {
-		conn, err := buflsp.Serve(
-			ctx,
-			"test",
-			wktBucket,
-			appextContainer,
-			controller,
-			wasmRuntime,
-			stream,
-			queryExecutor,
-		)
-		if err != nil {
-			t.Errorf("Failed to start server: %v", err)
-			return
-		}
-		t.Cleanup(func() {
-			require.NoError(t, conn.Close())
-		})
-		<-ctx.Done()
-	}()
+	conn, err := buflsp.Serve(
+		ctx,
+		"test",
+		wktBucket,
+		appextContainer,
+		controller,
+		wasmRuntime,
+		stream,
+		queryExecutor,
+	)
+	require.NoError(t, err)
+	t.Cleanup(func() {
+		require.NoError(t, conn.Close())
+	})
 
 	clientStream := jsonrpc2.NewStream(clientConn)
 	clientJSONConn := jsonrpc2.NewConn(clientStream)

private/buf/buflsp/diagnostic.go

@@ -17,10 +17,8 @@
 package buflsp
 
 import (
-	"encoding/json"
-	"strings"
-
 	"github.com/bufbuild/protocompile/experimental/report"
+	"github.com/bufbuild/protocompile/experimental/report/tags"
 	"github.com/bufbuild/protocompile/experimental/source/length"
 	"go.lsp.dev/protocol"
 )
@@ -30,14 +28,6 @@ import (
 // https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#position
 const positionalEncoding = length.UTF16
 
-// diagnosticData is a structure to hold the [report.Diagnostic] notes, help, and debug
-// messages, to marshal into JSON for the [protocol.Diagnostic].Data field.
-type diagnosticData struct {
-	Notes string `json:"notes,omitempty"`
-	Help  string `json:"help,omitempty"`
-	Debug string `json:"debug,omitempty"`
-}
-
 // reportLevelToDiagnosticSeverity is a mapping of [report.Level] to [protocol.DiagnosticSeverity].
 var reportLevelToDiagnosticSeverity = map[report.Level]protocol.DiagnosticSeverity{
 	report.ICE:     protocol.DiagnosticSeverityError,
@@ -70,20 +60,10 @@ func reportDiagnosticToProtocolDiagnostic(
 			},
 		}
 	}
-	data := diagnosticData{
-		Notes: strings.Join(reportDiagnostic.Notes(), "\n"),
-		Help:  strings.Join(reportDiagnostic.Help(), "\n"),
-		Debug: strings.Join(reportDiagnostic.Debug(), "\n"),
-	}
-	bytes, err := json.Marshal(data)
-	if err != nil {
-		return protocol.Diagnostic{}, err
-	}
-	if bytes != nil {
-		// We serialize the bytes into a string before providing the structure to diagnostic.Data
-		// because diagnostic.Data is an interface{}, which is treated as a JSON "any", which
-		// will not cleanly deserialize.
-		diagnostic.Data = string(bytes)
+	if reportDiagnostic.Tag() == tags.UnusedImport {
+		diagnostic.Tags = []protocol.DiagnosticTag{
+			protocol.DiagnosticTagUnnecessary,
+		}
 	}
 	return diagnostic, nil
 }