Keep Go's indented continuation in a step's cause: the composer's reason ended at "requires" and hid the module that could not be found (issue 1966)

Claude-Session: https://claude.ai/code/session_01JG73KJNrH5EgFXMKKYSrig

modules/mesh-build-go/cmd/mesh-build-go/candidate.go

@@ -97,9 +97,10 @@ func (m *Module) candidate(ctx context.Context, workspace string) (string, error
 // compileError is the first line of the compiler's output that says an error: not the `# package` heading go build
 // prints above a package's errors, nor Go's progress.
 func compileError(out string) string {
-	for _, l := range strings.Split(out, "\n") {
+	lines := strings.Split(out, "\n")
+	for i, l := range lines {
 		if l = strings.TrimSpace(l); l != "" && !strings.HasPrefix(l, "#") && !strings.HasPrefix(l, "go: downloading ") {
-			return l
+			return withContinuation(l, lines[i+1:])
 		}
 	}
 	return firstLine(out)

modules/mesh-build-go/cmd/mesh-build-go/fetch.go

@@ -351,15 +351,33 @@ func (m *Module) now() time.Time { return m.clock() }
 // causeLine is the first line of a go command's output that says something other than Go's own progress: `go:
 // downloading ...` comes first when Go fetches a module, and `# <package>` heads a compile error of go build or go vet
 // (issue 1703: the reason said only the package), and the cause of a failure on a later line.
+//
+// A cause Go writes over several lines keeps its indented continuation lines, joined by a space: a module graph error
+// is "X requires" with the module that fails, and why, on the next line (issue 1966: the composer's reason ended at
+// "requires" and hid "golang.org/x/tools@v0.42.0: module lookup disabled").
 func causeLine(s string) string {
-	for _, l := range strings.Split(s, "\n") {
+	lines := strings.Split(s, "\n")
+	for i, l := range lines {
 		if l = strings.TrimSpace(l); l != "" && !strings.HasPrefix(l, "go: downloading ") && !strings.HasPrefix(l, "# ") {
-			return l
+			return withContinuation(l, lines[i+1:])
 		}
 	}
 	return firstLine(s)
 }
 
+// withContinuation is first with each following line Go indented under it, trimmed and joined by a space.
+func withContinuation(first string, rest []string) string {
+	for _, l := range rest {
+		if !strings.HasPrefix(l, "\t") && !strings.HasPrefix(l, "    ") {
+			break
+		}
+		if l = strings.TrimSpace(l); l != "" {
+			first += " " + l
+		}
+	}
+	return first
+}
+
 func firstLine(s string) string {
 	for _, l := range strings.Split(s, "\n") {
 		if l = strings.TrimSpace(l); l != "" {

modules/mesh-build-go/cmd/mesh-build-go/fetch_test.go

@@ -309,9 +309,14 @@ func TestCauseLineSkipsProgressAndThePackageHeading(t *testing.T) {
 		"go: downloading x v1\n# git.example/m/cmd\n./main.go:3:2: undefined: y\n": "./main.go:3:2: undefined: y",
 		"\ngo: example.com/x@v1: reading: 404\n":                                   "go: example.com/x@v1: reading: 404",
 		"go: downloading x v1\n":                                                   "go: downloading x v1",
+		// issue 1966: a module graph error goes on, indented, on the next line; a later error is not taken in.
+		"a.go:3:8: x@v1 requires\n\ty@v2: module lookup disabled by GOPROXY=off\nb.go:1:1: other\n": "a.go:3:8: x@v1 requires y@v2: module lookup disabled by GOPROXY=off",
 	} {
 		if got := causeLine(in); got != want {
 			t.Errorf("causeLine(%q) = %q, want %q", in, got, want)
 		}
+		if got := compileError(in); got != want {
+			t.Errorf("compileError(%q) = %q, want %q", in, got, want)
+		}
 	}
 }