internal/cmd/deadcode: use compiler-style diagnostics by default

The previous format is now relegated to an example -f template
in the documentation.

Updates golang/go#63501

Change-Id: I29548121431282edbda07e3a75c131ac33104a6f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/542375
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>

internal/cmd/deadcode/deadcode.go

@@ -18,6 +18,7 @@ import (
 	"io"
 	"log"
 	"os"
+	"path/filepath"
 	"regexp"
 	"runtime"
 	"runtime/pprof"
@@ -336,14 +337,15 @@ func main() {
 		}
 		if len(functions) > 0 {
 			packages = append(packages, jsonPackage{
+				Name:  fns[0].Pkg.Pkg.Name(),
 				Path:  pkgpath,
 				Funcs: functions,
 			})
 		}
 	}
 
-	// Default format: functions grouped by package.
-	format := `{{println .Path}}{{range .Funcs}}{{printf "\t%s\n" .Name}}{{end}}{{println}}`
+	// Default line-oriented format: "a/b/c.go:1:2: unreachable func: T.f"
+	format := `{{range .Funcs}}{{printf "%s: unreachable func: %s\n" .Position .Name}}{{end}}`
 	if *formatFlag != "" {
 		format = *formatFlag
 	}
@@ -553,8 +555,16 @@ func isStaticCall(edge *callgraph.Edge) bool {
 	return edge.Site != nil && edge.Site.Common().StaticCallee() != nil
 }
 
+var cwd, _ = os.Getwd()
+
 func toJSONPosition(posn token.Position) jsonPosition {
-	return jsonPosition{posn.Filename, posn.Line, posn.Column}
+	// Use cwd-relative filename if possible.
+	filename := posn.Filename
+	if rel, err := filepath.Rel(cwd, filename); err == nil && !strings.HasPrefix(rel, "..") {
+		filename = rel
+	}
+
+	return jsonPosition{filename, posn.Line, posn.Column}
 }
 
 func cond[T any](cond bool, t, f T) T {
@@ -578,8 +588,9 @@ type jsonFunction struct {
 func (f jsonFunction) String() string { return f.Name }
 
 type jsonPackage struct {
-	Path  string
-	Funcs []jsonFunction
+	Name  string         // declared name
+	Path  string         // full import path
+	Funcs []jsonFunction // non-empty list of package's dead functions
 }
 
 func (p jsonPackage) String() string { return p.Path }

internal/cmd/deadcode/doc.go

@@ -59,20 +59,28 @@ easier to compute the intersection of results across all runs.
 
 The command supports three output formats.
 
-With no flags, the command prints dead functions grouped by package.
+With no flags, the command prints the name and location of each dead
+function in the form of a typical compiler diagnostic, for example:
+
+	$ deadcode -f='{{range .Funcs}}{{println .Position}}{{end}}' -test ./gopls/...
+	gopls/internal/lsp/command.go:1206:6: unreachable func: openClientEditor
+	gopls/internal/lsp/template/parse.go:414:18: unreachable func: Parsed.WriteNode
+	gopls/internal/lsp/template/parse.go:419:18: unreachable func: wrNode.writeNode
 
 With the -json flag, the command prints an array of Package
 objects, as defined by the JSON schema (see below).
 
 With the -f=template flag, the command executes the specified template
-on each Package record. So, this template produces a result similar to the
-default format:
-
-	-f='{{println .Path}}{{range .Funcs}}{{printf "\t%s\n" .Name}}{{end}}{{println}}'
+on each Package record. So, this template shows dead functions grouped
+by package:
 
-And this template shows only the list of source positions of dead functions:
+	$ deadcode -f='{{println .Path}}{{range .Funcs}}{{printf "\t%s\n" .Name}}{{end}}{{println}}' -test ./gopls/...
+	golang.org/x/tools/gopls/internal/lsp
+		openClientEditor
 
-	-f='{{range .Funcs}}{{println .Position}}{{end}}'
+	golang.org/x/tools/gopls/internal/lsp/template
+		Parsed.WriteNode
+		wrNode.writeNode
 
 # Why is a function not dead?
 
@@ -104,7 +112,8 @@ is static or dynamic, and its source line number. For example:
 # JSON schema
 
 	type Package struct {
-		Path  string       // import path of package
+		Name  string       // declared name
+		Path  string       // full import path
 		Funcs []Function   // list of dead functions within it
 	}
 

internal/cmd/deadcode/testdata/generated.txtar

@@ -1,14 +1,14 @@
 # Test of -generated flag output.
 
- deadcode example.com
-!want "main"
- want "Dead1"
-!want "Dead2"
-
- deadcode -generated example.com
-!want "main"
- want "Dead1"
- want "Dead2"
+ deadcode "-f={{range .Funcs}}{{$.Name}}.{{.Name}}{{end}}" example.com
+!want "main.main"
+ want "main.Dead1"
+!want "main.Dead2"
+
+ deadcode "-f={{range .Funcs}}{{$.Name}}.{{.Name}}{{end}}" -generated example.com
+!want "main.main"
+ want "main.Dead1"
+ want "main.Dead2"
 
 -- go.mod --
 module example.com

internal/cmd/deadcode/testdata/lineflag.txtar

@@ -1,10 +1,10 @@
 # Test of line-oriented output.
 
- deadcode `-f={{range .Funcs}}{{printf "%s.%s\n" $.Path .Name}}{{end}}` -filter= example.com
+ deadcode `-f={{range .Funcs}}{{printf "%s: %s.%s\n" .Position $.Path .Name}}{{end}}` -filter= example.com
 
- want "example.com.T.Goodbye"
+ want "main.go:13:10: example.com.T.Goodbye"
 !want "example.com.T.Hello"
- want "example.com.unreferenced"
+ want "main.go:15:6: example.com.unreferenced"
 
  want "fmt.Scanf"
  want "fmt.Printf"