From 38a3364f5f098906316f4ed0f3ee3cc27833b682 Mon Sep 17 00:00:00 2001
From: testing <testing@example.com>
Date: Sun, 5 Apr 2026 07:17:05 +0200
Subject: [PATCH] fix: normalize stdin path to be relative to tree root

When using --stdin with an absolute path, the RelPath was incorrectly
set to the absolute path, causing pattern matching for includes and
excludes to fail against project-relative globs.

This change ensures that the path provided with --stdin is converted
to a relative path from the tree root before processing.
---
 cmd/root_test.go | 36 ++++++++++++++++++++++++++++++++++++
 walk/walk.go     | 20 +++++++++++++++++---
 2 files changed, 53 insertions(+), 3 deletions(-)

diff --git a/cmd/root_test.go b/cmd/root_test.go
index 11cbcfed..119d27db 100644
--- a/cmd/root_test.go
+++ b/cmd/root_test.go
@@ -2117,6 +2117,42 @@ func TestStdin(t *testing.T) {
 		}),
 	)
 
+	// verify that absolute paths work
+	absPath, err := filepath.Abs("test.nix")
+	as.NoError(err)
+	os.Stdin = test.TempFile(t, "", "stdin", &contents)
+
+	treefmt(t,
+		withArgs("--stdin", absPath),
+		withNoError(t),
+		withStats(t, map[stats.Type]int{
+			stats.Traversed: 1,
+			stats.Matched:   1,
+			stats.Formatted: 1,
+			stats.Changed:   1,
+		}),
+		withStdout(func(out []byte) {
+			as.Equal(`{ ...}: "hello"
+`, string(out))
+		}),
+	)
+
+	// verify that absolute paths in subdirectories work with glob patterns
+	absPathInSubdir, err := filepath.Abs(filepath.Join("go", "main.go"))
+	as.NoError(err)
+	goContents := "package main\n"
+	os.Stdin = test.TempFile(t, "", "stdin", &goContents)
+
+	treefmt(t,
+		withArgs("--stdin", absPathInSubdir),
+		withNoError(t),
+		withStats(t, map[stats.Type]int{
+			stats.Traversed: 1,
+			stats.Matched:   1,
+			stats.Formatted: 1,
+		}),
+	)
+
 	// the nix formatters should have reduced the example to the following
 
 	// try a file that's outside of the project root
diff --git a/walk/walk.go b/walk/walk.go
index b851bfa2..b767a217 100644
--- a/walk/walk.go
+++ b/walk/walk.go
@@ -283,12 +283,26 @@ func NewCompositeReader(
 		}
 
 		path := paths[0]
+		resolvedPath, err := resolvePath(path)
+		if err != nil {
+			// If the path doesn't exist, we still want to make it relative to the root if possible.
+			// We use the absolute path without resolving symlinks.
+			resolvedPath, err = filepath.Abs(path)
+			if err != nil {
+				return nil, fmt.Errorf("error computing absolute path of %s: %w", path, err)
+			}
+		}
+
+		relativePath, err := filepath.Rel(root, resolvedPath)
+		if err != nil {
+			return nil, fmt.Errorf("error computing relative path from %s to %s: %w", root, resolvedPath, err)
+		}
 
-		if strings.HasPrefix(path, "..") {
-			return nil, fmt.Errorf("path %s not inside the tree root %s", path, root)
+		if strings.HasPrefix(relativePath, "..") {
+			return nil, fmt.Errorf("path %s not inside the tree root %s (relative path: %s)", path, root, relativePath)
 		}
 
-		return NewStdinReader(root, path, statz), nil
+		return NewStdinReader(root, relativePath, statz), nil
 	}
 
 	// create a reader for each provided path