Fix source annotation in pip compile annotation-style=line output (#…

…3637)

## Summary

Fixes a small discrepancy between the pip compile outputs for
`annotation-style=split` and `annotation-style=line` commands.

### Problem

Consider the following `pyproject.toml` file.
```sh
$ cat pyproject.toml
[project]
name = "uv_test"
dynamic = ["version"]
dependencies = ["click"]
```

Running uv pip compile with annotation-style=split on uv 0.1.44 version
yields the following:
```sh
❯ uv pip compile pyproject.toml --annotation-style=split
Resolved 1 package in 2ms
# This file was autogenerated by uv via the following command:
#    uv pip compile pyproject.toml --annotation-style=split
click==8.1.7
    # via uv-test (pyproject.toml)
```

However, running uv pip compile with annotation-style=line doesn't
include source info for root level dependencies.
```sh
❯ uv pip compile pyproject.toml --annotation-style=line
Resolved 1 package in 1ms
# This file was autogenerated by uv via the following command:
#    uv pip compile pyproject.toml --annotation-style=line
click==8.1.7
```

With this PR:
```sh
❯ ../target/debug/uv pip compile --annotation-style=line pyproject.toml
Resolved 1 package in 6ms
# This file was autogenerated by uv via the following command:
#    uv pip compile --annotation-style=line pyproject.toml
click==8.1.7              # via uv-test (pyproject.toml)
```

This also now matches `pip-tools` output:
```sh
❯ pip-compile --annotation-style=line pyproject.toml
#
# This file is autogenerated by pip-compile with Python 3.12
# by the following command:
#
#    pip-compile --annotation-style=line pyproject.toml
#
click==8.1.7              # via uv_test (pyproject.toml)

```

## Test Plan

`cargo test`

crates/uv-resolver/src/resolution/display.rs

@@ -207,19 +207,27 @@ impl std::fmt::Display for DisplayResolutionGraph<'_> {
                 };
 
                 match self.annotation_style {
-                    AnnotationStyle::Line => {
-                        if !edges.is_empty() {
+                    AnnotationStyle::Line => match edges.as_slice() {
+                        [] if source.is_empty() => {}
+                        [] if source.len() == 1 => {
+                            let separator = if has_hashes { "\n    " } else { "  " };
+                            let comment = format!("# via {}", source.iter().next().unwrap())
+                                .green()
+                                .to_string();
+                            annotation = Some((separator, comment));
+                        }
+                        edges => {
                             let separator = if has_hashes { "\n    " } else { "  " };
                             let deps = edges
-                                .into_iter()
+                                .iter()
                                 .map(|dependency| format!("{}", dependency.name()))
                                 .chain(source.iter().map(std::string::ToString::to_string))
                                 .collect::<Vec<_>>()
                                 .join(", ");
                             let comment = format!("# via {deps}").green().to_string();
                             annotation = Some((separator, comment));
                         }
-                    }
+                    },
                     AnnotationStyle::Split => match edges.as_slice() {
                         [] if source.is_empty() => {}
                         [] if source.len() == 1 => {

crates/uv/tests/pip_compile.rs

@@ -64,7 +64,7 @@ fn compile_requirements_in_annotation_line() -> Result<()> {
     ----- stdout -----
     # This file was autogenerated by uv via the following command:
     #    uv pip compile --cache-dir [CACHE_DIR] --exclude-newer 2024-03-25T00:00:00Z --annotation-style=line requirements.in
-    anyio==3.7.0
+    anyio==3.7.0              # via -r requirements.in
     idna==3.6                 # via anyio
     sniffio==1.3.1            # via anyio
 
@@ -193,6 +193,43 @@ dependencies = [
     Ok(())
 }
 
+/// Resolve a specific version of `anyio` from a `pyproject.toml` file with `--annotation-style=line`.
+#[test]
+fn compile_pyproject_toml_with_line_annotation() -> Result<()> {
+    let context = TestContext::new("3.12");
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(
+        r#"[build-system]
+requires = ["setuptools", "wheel"]
+
+[project]
+name = "project"
+dependencies = [
+    "anyio==3.7.0",
+]
+"#,
+    )?;
+
+    uv_snapshot!(context.compile()
+            .arg("--annotation-style=line")
+            .arg("pyproject.toml"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    # This file was autogenerated by uv via the following command:
+    #    uv pip compile --cache-dir [CACHE_DIR] --exclude-newer 2024-03-25T00:00:00Z --annotation-style=line pyproject.toml
+    anyio==3.7.0              # via project (pyproject.toml)
+    idna==3.6                 # via anyio
+    sniffio==1.3.1            # via anyio
+
+    ----- stderr -----
+    Resolved 3 packages in [TIME]
+    "###
+    );
+
+    Ok(())
+}
+
 /// Resolve a package from a `requirements.in` file, with a `constraints.txt` file.
 #[test]
 fn compile_constraints_txt() -> Result<()> {
@@ -880,7 +917,7 @@ fn compile_python_312_annotation_line() -> Result<()> {
     ----- stdout -----
     # This file was autogenerated by uv via the following command:
     #    uv pip compile --cache-dir [CACHE_DIR] --exclude-newer 2024-03-25T00:00:00Z --annotation-style=line requirements.in --python-version 3.12
-    black==23.10.1
+    black==23.10.1            # via -r requirements.in
     click==8.1.7              # via black
     mypy-extensions==1.0.0    # via black
     packaging==24.0           # via black
@@ -2208,9 +2245,9 @@ optional-dependencies.bar = [
     anyio==3.7.0              # via httpcore, project (pyproject.toml)
     certifi==2024.2.2         # via httpcore
     h11==0.14.0               # via httpcore
-    httpcore==0.18.0
+    httpcore==0.18.0          # via project (pyproject.toml)
     idna==3.6                 # via anyio
-    iniconfig==1.1.1
+    iniconfig==1.1.1          # via project (pyproject.toml)
     sniffio==1.3.1            # via anyio, httpcore
 
     ----- stderr -----
@@ -8324,7 +8361,7 @@ fn emit_index_annotation_line() -> Result<()> {
         # from https://pypi.org/simple
     idna==3.6                 # via requests
         # from https://pypi.org/simple
-    requests==2.31.0
+    requests==2.31.0          # via -r requirements.in
         # from https://pypi.org/simple
     urllib3==2.2.1            # via requests
         # from https://pypi.org/simple