Don't warn when `--output-file` is empty (#9417)

## Summary

Closes https://github.com/astral-sh/uv/issues/9410.

Cargo.lock

@@ -5396,7 +5396,6 @@ dependencies = [
  "uv-normalize",
  "uv-pep508",
  "uv-pypi-types",
- "uv-warnings",
 ]
 
 [[package]]

crates/uv-requirements-txt/Cargo.toml

@@ -23,7 +23,6 @@ uv-client = { workspace = true }
 uv-fs = { workspace = true }
 uv-normalize = { workspace = true }
 uv-configuration = { workspace = true }
-uv-warnings = { workspace = true }
 
 fs-err = { workspace = true }
 regex = { workspace = true }

crates/uv-requirements-txt/src/lib.rs

@@ -52,7 +52,6 @@ use uv_distribution_types::{UnresolvedRequirement, UnresolvedRequirementSpecific
 use uv_fs::Simplified;
 use uv_pep508::{expand_env_vars, Pep508Error, RequirementOrigin, VerbatimUrl};
 use uv_pypi_types::{Requirement, VerbatimParsedUrl};
-use uv_warnings::warn_user;
 
 use crate::requirement::EditableError;
 pub use crate::requirement::RequirementsTxtRequirement;
@@ -219,16 +218,6 @@ impl RequirementsTxt {
             file: requirements_txt.to_path_buf(),
             error: err,
         })?;
-        if data == Self::default() {
-            if requirements_txt == Path::new("-") {
-                warn_user!("No dependencies found in stdin");
-            } else {
-                warn_user!(
-                    "Requirements file {} does not contain any dependencies",
-                    requirements_txt.user_display()
-                );
-            }
-        }
 
         Ok(data)
     }

crates/uv-requirements/src/specification.rs

@@ -45,6 +45,7 @@ use uv_pep508::{MarkerTree, UnnamedRequirement, UnnamedRequirementUrl};
 use uv_pypi_types::Requirement;
 use uv_pypi_types::VerbatimParsedUrl;
 use uv_requirements_txt::{RequirementsTxt, RequirementsTxtRequirement};
+use uv_warnings::warn_user;
 use uv_workspace::pyproject::PyProjectToml;
 
 use crate::RequirementsSource;
@@ -113,6 +114,18 @@ impl RequirementsSpecification {
                 }
 
                 let requirements_txt = RequirementsTxt::parse(path, &*CWD, client_builder).await?;
+
+                if requirements_txt == RequirementsTxt::default() {
+                    if path == Path::new("-") {
+                        warn_user!("No dependencies found in stdin");
+                    } else {
+                        warn_user!(
+                            "Requirements file `{}` does not contain any dependencies",
+                            path.user_display()
+                        );
+                    }
+                }
+
                 Self {
                     requirements: requirements_txt
                         .requirements

crates/uv/tests/it/pip_compile.rs

@@ -132,7 +132,7 @@ fn missing_venv() -> Result<()> {
     #    uv pip compile --cache-dir [CACHE_DIR] requirements.in
 
     ----- stderr -----
-    warning: Requirements file requirements.in does not contain any dependencies
+    warning: Requirements file `requirements.in` does not contain any dependencies
     Resolved in [TIME]
     "###
     );
@@ -142,6 +142,35 @@ fn missing_venv() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn empty_output() -> Result<()> {
+    let context = TestContext::new("3.12");
+    context
+        .temp_dir
+        .child("requirements.in")
+        .write_str("iniconfig==1.1.1")?;
+    context.temp_dir.child("requirements.txt").touch()?;
+
+    uv_snapshot!(context.filters(), context.pip_compile()
+            .arg("requirements.in")
+            .arg("--output-file")
+            .arg("requirements.txt"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    # This file was autogenerated by uv via the following command:
+    #    uv pip compile --cache-dir [CACHE_DIR] requirements.in --output-file requirements.txt
+    iniconfig==1.1.1
+        # via -r requirements.in
+
+    ----- stderr -----
+    Resolved 1 package in [TIME]
+    "###
+    );
+
+    Ok(())
+}
+
 /// Resolve a specific version of `anyio` from a `pyproject.toml` file.
 #[test]
 fn compile_pyproject_toml() -> Result<()> {

crates/uv/tests/it/pip_install.rs

@@ -50,7 +50,7 @@ fn empty_requirements_txt() -> Result<()> {
     ----- stdout -----
 
     ----- stderr -----
-    warning: Requirements file requirements.txt does not contain any dependencies
+    warning: Requirements file `requirements.txt` does not contain any dependencies
     Audited in [TIME]
     "###
     );

crates/uv/tests/it/pip_sync.rs

@@ -360,7 +360,7 @@ fn pip_sync_empty() -> Result<()> {
     ----- stdout -----
 
     ----- stderr -----
-    warning: Requirements file requirements.txt does not contain any dependencies
+    warning: Requirements file `requirements.txt` does not contain any dependencies
     No requirements found (hint: use `--allow-empty-requirements` to clear the environment)
     "###
     );
@@ -373,7 +373,7 @@ fn pip_sync_empty() -> Result<()> {
     ----- stdout -----
 
     ----- stderr -----
-    warning: Requirements file requirements.txt does not contain any dependencies
+    warning: Requirements file `requirements.txt` does not contain any dependencies
     Resolved in [TIME]
     Audited in [TIME]
     "###
@@ -397,7 +397,7 @@ fn pip_sync_empty() -> Result<()> {
     ----- stdout -----
 
     ----- stderr -----
-    warning: Requirements file requirements.txt does not contain any dependencies
+    warning: Requirements file `requirements.txt` does not contain any dependencies
     Resolved in [TIME]
     Uninstalled 1 package in [TIME]
      - iniconfig==2.0.0

crates/uv/tests/it/run.rs

@@ -1751,7 +1751,7 @@ fn run_empty_requirements_txt() -> Result<()> {
      + foo==1.0.0 (from file://[TEMP_DIR]/)
      + idna==3.6
      + sniffio==1.3.1
-    warning: Requirements file requirements.txt does not contain any dependencies
+    warning: Requirements file `requirements.txt` does not contain any dependencies
     "###);
 
     // Then reused in subsequent invocations
@@ -1763,7 +1763,7 @@ fn run_empty_requirements_txt() -> Result<()> {
     ----- stderr -----
     Resolved 6 packages in [TIME]
     Audited 4 packages in [TIME]
-    warning: Requirements file requirements.txt does not contain any dependencies
+    warning: Requirements file `requirements.txt` does not contain any dependencies
     "###);
 
     Ok(())