cmd/deadcode: respect unused symbols inside all loaded modules

Current deadcode uses the first package module path as a regular expression to
filter when check deadcode code for packages. However, when users load multiple
modules, the filter will respect one of them only.

This CL constructs the filter regex based on all loaded modules rather than use
the first privileging the first package.

Fixes golang/go#73652

Change-Id: Id7b9eb8274141cd2d6362da01366cbc45c87eebc
Reviewed-on: https://go-review.googlesource.com/c/tools/+/671916
Reviewed-by: Alan Donovan <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>

Author: xieyuschen

cmd/deadcode/deadcode.go

@@ -132,8 +132,17 @@ func main() {
 
 	// If -filter is unset, use first module (if available).
 	if *filterFlag == "<module>" {
-		if mod := initial[0].Module; mod != nil && mod.Path != "" {
-			*filterFlag = "^" + regexp.QuoteMeta(mod.Path) + "\\b"
+		seen := make(map[string]bool)
+		var patterns []string
+		for _, pkg := range initial {
+			if pkg.Module != nil && pkg.Module.Path != "" && !seen[pkg.Module.Path] {
+				seen[pkg.Module.Path] = true
+				patterns = append(patterns, regexp.QuoteMeta(pkg.Module.Path))
+			}
+		}
+
+		if patterns != nil {
+			*filterFlag = "^(" + strings.Join(patterns, "|") + ")\\b"
 		} else {
 			*filterFlag = "" // match any
 		}

cmd/deadcode/doc.go

@@ -5,7 +5,7 @@
 /*
 The deadcode command reports unreachable functions in Go programs.
 
-Usage: deadcode [flags] package...
+	Usage: deadcode [flags] package...
 
 The deadcode command loads a Go program from source then uses Rapid
 Type Analysis (RTA) to build a call graph of all the functions
@@ -25,8 +25,8 @@ function without an "Output:" comment is merely documentation:
 it is dead code, and does not contribute coverage.
 
 The -filter flag restricts results to packages that match the provided
-regular expression; its default value is the module name of the first
-package. Use -filter= to display all results.
+regular expression; its default value matches the listed packages and any other
+packages belonging to the same modules. Use -filter= to display all results.
 
 Example: show all dead code within the gopls module:
 

cmd/deadcode/testdata/issue73652.txtar

@@ -0,0 +1,39 @@
+# Test deadcode usage under go.work.
+
+ deadcode ./svc/... ./lib/...
+ want "unreachable func: A"
+
+# different order of path under the same go.work should behave the same.
+
+ deadcode ./svc/... ./lib/...
+ want "unreachable func: A"
+
+
+-- go.work --
+go 1.18
+
+use (
+	./lib
+	./svc
+)
+
+-- lib/go.mod --
+module lib.com
+
+go 1.18
+
+-- lib/a/a.go --
+package a
+
+func A() {}
+
+-- svc/go.mod --
+module svc.com
+
+go 1.18
+
+-- svc/s/main.go --
+package main
+
+func main() { println("main") }
+