Write relative paths with unnamed requirement syntax (#3682)

## Summary

This PR falls back to writing an unnamed requirement if it appears to be
a relative URL. pip is way more flexible when providing an unnamed
requirement than when providing a PEP 508 requirement. For example,
_only_ this works:

```
black @ file:///Users/crmarsh/workspace/uv/scripts/packages/black_editable
```

Any other form will fail.

Meanwhile, _all_ of these work:

```
file:///Users/crmarsh/workspace/uv/scripts/packages/black_editable
scripts/packages/black_editable
./scripts/packages/black_editable
file:./scripts/packages/black_editable
file:scripts/packages/black_editable
```

Closes https://github.com/astral-sh/uv/issues/3180.

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

@@ -1,14 +1,11 @@
 use std::borrow::Cow;
 use std::collections::BTreeSet;
 
-use itertools::Itertools;
 use owo_colors::OwoColorize;
 use petgraph::visit::EdgeRef;
 use petgraph::Direction;
 
-use distribution_types::{
+use distribution_types::{IndexUrl, LocalEditable, Name, SourceAnnotations, Verbatim};
-    DistributionMetadata, IndexUrl, LocalEditable, Name, SourceAnnotations, Verbatim,
-};
 use pypi_types::HashDigest;
 use uv_normalize::PackageName;
 
@@ -155,19 +152,7 @@ impl std::fmt::Display for DisplayResolutionGraph<'_> {
             let mut line = match node {
                 Node::Editable(editable) => format!("-e {}", editable.verbatim()),
                 Node::Distribution(dist) => {
-                    if self.include_extras && !dist.extras.is_empty() {
+                    dist.to_requirements_txt(self.include_extras).to_string()
-                        let mut extras = dist.extras.clone();
-                        extras.sort_unstable();
-                        extras.dedup();
-                        format!(
-                            "{}[{}]{}",
-                            dist.name(),
-                            extras.into_iter().join(", "),
-                            dist.version_or_url().verbatim()
-                        )
-                    } else {
-                        dist.verbatim().to_string()
-                    }
                 }
             };
 

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

@@ -1,6 +1,11 @@
+use std::borrow::Cow;
 use std::fmt::Display;
+use std::path::Path;
 
-use distribution_types::{DistributionMetadata, Name, ResolvedDist, VersionOrUrlRef};
+use itertools::Itertools;
+
+use distribution_types::{DistributionMetadata, Name, ResolvedDist, Verbatim, VersionOrUrlRef};
+use pep508_rs::{split_scheme, Scheme};
 use pypi_types::{HashDigest, Metadata23};
 use uv_normalize::{ExtraName, PackageName};
 
@@ -13,7 +18,7 @@ mod graph;
 /// A pinned package with its resolved distribution and metadata. The [`ResolvedDist`] refers to a
 /// specific distribution (e.g., a specific wheel), while the [`Metadata23`] refers to the metadata
 /// for the package-version pair.
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub(crate) struct AnnotatedDist {
     pub(crate) dist: ResolvedDist,
     pub(crate) extras: Vec<ExtraName>,
@@ -21,6 +26,74 @@ pub(crate) struct AnnotatedDist {
     pub(crate) metadata: Metadata23,
 }
 
+impl AnnotatedDist {
+    /// Convert the [`AnnotatedDist`] to a requirement that adheres to the `requirements.txt`
+    /// format.
+    ///
+    /// This typically results in a PEP 508 representation of the requirement, but will write an
+    /// unnamed requirement for relative paths, which can't be represented with PEP 508 (but are
+    /// supported in `requirements.txt`).
+    pub(crate) fn to_requirements_txt(&self, include_extras: bool) -> Cow<str> {
+        // If the URL is not _definitively_ an absolute `file://` URL, write it as a relative path.
+        if let VersionOrUrlRef::Url(url) = self.dist.version_or_url() {
+            let given = url.verbatim();
+            match split_scheme(&given) {
+                Some((scheme, path)) => {
+                    match Scheme::parse(scheme) {
+                        Some(Scheme::File) => {
+                            if path
+                                .strip_prefix("//localhost")
+                                .filter(|path| path.starts_with('/'))
+                                .is_some()
+                            {
+                                // Always absolute; nothing to do.
+                            } else if let Some(path) = path.strip_prefix("//") {
+                                // Strip the prefix, to convert, e.g., `file://flask-3.0.3-py3-none-any.whl` to `flask-3.0.3-py3-none-any.whl`.
+                                //
+                                // However, we should allow any of the following:
+                                // - `file://flask-3.0.3-py3-none-any.whl`
+                                // - `file://C:\Users\user\flask-3.0.3-py3-none-any.whl`
+                                // - `file:///C:\Users\user\flask-3.0.3-py3-none-any.whl`
+                                if !path.starts_with("${PROJECT_ROOT}")
+                                    && !Path::new(path).has_root()
+                                {
+                                    return Cow::Owned(path.to_string());
+                                }
+                            } else {
+                                // Ex) `file:./flask-3.0.3-py3-none-any.whl`
+                                return given;
+                            }
+                        }
+                        Some(_) => {}
+                        None => {
+                            // Ex) `flask @ C:\Users\user\flask-3.0.3-py3-none-any.whl`
+                            return given;
+                        }
+                    }
+                }
+                None => {
+                    // Ex) `flask @ flask-3.0.3-py3-none-any.whl`
+                    return given;
+                }
+            }
+        }
+
+        if self.extras.is_empty() || !include_extras {
+            self.dist.verbatim()
+        } else {
+            let mut extras = self.extras.clone();
+            extras.sort_unstable();
+            extras.dedup();
+            Cow::Owned(format!(
+                "{}[{}]{}",
+                self.name(),
+                extras.into_iter().join(", "),
+                self.version_or_url().verbatim()
+            ))
+        }
+    }
+}
+
 impl Name for AnnotatedDist {
     fn name(&self) -> &PackageName {
         self.dist.name()

crates/uv/tests/pip_compile.rs

@@ -2065,7 +2065,7 @@ fn allowed_transitive_url_path_dependency() -> 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 requirements.in
-    hatchling-editable @ ${HATCH_PATH}
+    ${HATCH_PATH}
         # via -r requirements.in
     iniconfig @ git+https://github.com/pytest-dev/iniconfig@9cae43103df70bac6fde7b9f35ad11a9f1be0cb4
         # via hatchling-editable
@@ -2510,7 +2510,7 @@ fn compile_wheel_path_dependency() -> Result<()> {
         # via flask
     click==8.1.7
         # via flask
-    flask @ file:flask-3.0.0-py3-none-any.whl
+    file:flask-3.0.0-py3-none-any.whl
         # via -r requirements.in
     itsdangerous==2.1.2
         # via flask
@@ -2543,7 +2543,7 @@ fn compile_wheel_path_dependency() -> Result<()> {
         # via flask
     click==8.1.7
         # via flask
-    flask @ file://flask-3.0.0-py3-none-any.whl
+    flask-3.0.0-py3-none-any.whl
         # via -r requirements.in
     itsdangerous==2.1.2
         # via flask
@@ -2576,7 +2576,7 @@ fn compile_wheel_path_dependency() -> Result<()> {
         # via flask
     click==8.1.7
         # via flask
-    flask @ ./flask-3.0.0-py3-none-any.whl
+    ./flask-3.0.0-py3-none-any.whl
         # via -r requirements.in
     itsdangerous==2.1.2
         # via flask
@@ -2609,7 +2609,7 @@ fn compile_wheel_path_dependency() -> Result<()> {
         # via flask
     click==8.1.7
         # via flask
-    flask @ [TEMP_DIR]/flask-3.0.0-py3-none-any.whl
+    [TEMP_DIR]/flask-3.0.0-py3-none-any.whl
         # via -r requirements.in
     itsdangerous==2.1.2
         # via flask
@@ -3192,7 +3192,7 @@ fn respect_http_env_var() -> Result<()> {
         # via flask
     click==8.1.7
         # via flask
-    flask @ ${URL}
+    ${URL}
         # via -r requirements.in
     itsdangerous==2.1.2
         # via flask
@@ -3233,7 +3233,7 @@ fn respect_unnamed_env_var() -> Result<()> {
         # via flask
     click==8.1.7
         # via flask
-    flask @ ${URL}
+    ${URL}
         # via -r requirements.in
     itsdangerous==2.1.2
         # via flask
@@ -3305,7 +3305,7 @@ fn respect_file_env_var() -> Result<()> {
         # via flask
     click==8.1.7
         # via flask
-    flask @ ${FILE_PATH}
+    ${FILE_PATH}
         # via -r requirements.in
     itsdangerous==2.1.2
         # via flask
@@ -3468,7 +3468,7 @@ fn recursive_extras_direct_url() -> Result<()> {
         # via aiohttp
     attrs==23.2.0
         # via aiohttp
-    black @ ../../scripts/packages/black_editable
+    ../../scripts/packages/black_editable
         # via -r [TEMP_DIR]/requirements.in
     frozenlist==1.4.1
         # via
@@ -3964,7 +3964,7 @@ fn generate_hashes_local_directory() -> Result<()> {
         --hash=sha256:9ecdbbd083b06798ae1e86adcbfe8ab1479cf864e4ee30fe4e46a003d12491ca \
         --hash=sha256:c05567e9c24a6b9faaa835c4821bad0590fbb9d5779e7caa6e1cc4978e7eb24f
         # via anyio
-    poetry-editable @ ../../scripts/packages/poetry_editable
+    ../../scripts/packages/poetry_editable
         # via -r [TEMP_DIR]/requirements.in
     sniffio==1.3.1 \
         --hash=sha256:2f6da418d1f1e0fddd844478f41680e794e6051915791a034ff65e5f100525a2 \
@@ -6799,11 +6799,11 @@ fn compile_root_uri_non_editable() -> 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 requirements.in
-    black @ ${BLACK_PATH}
+    ${BLACK_PATH}
         # via
         #   -r requirements.in
         #   root-editable
-    root-editable @ ${ROOT_PATH}
+    ${ROOT_PATH}
         # via -r requirements.in
 
     ----- stderr -----
@@ -7186,7 +7186,7 @@ fn unnamed_path_requirement() -> Result<()> {
         # via
         #   httpx
         #   poetry-editable
-    black @ ../../scripts/packages/black_editable
+    ../../scripts/packages/black_editable
         # via -r [TEMP_DIR]/requirements.in
     certifi==2024.2.2
         # via
@@ -7206,13 +7206,13 @@ fn unnamed_path_requirement() -> Result<()> {
         #   anyio
         #   httpx
         #   requests
-    poetry-editable @ ../../scripts/packages/poetry_editable
+    ../../scripts/packages/poetry_editable
         # via -r [TEMP_DIR]/requirements.in
     requests==2.31.0
         # via setup-cfg-editable
-    setup-cfg-editable @ ../../scripts/packages/setup_cfg_editable
+    ../../scripts/packages/setup_cfg_editable
         # via -r [TEMP_DIR]/requirements.in
-    setup-py-editable @ ../../scripts/packages/setup_py_editable
+    ../../scripts/packages/setup_py_editable
         # via -r [TEMP_DIR]/requirements.in
     sniffio==1.3.1
         # via
@@ -7322,7 +7322,7 @@ fn dynamic_dependencies() -> Result<()> {
     #    uv pip compile --cache-dir [CACHE_DIR] --exclude-newer 2024-03-25T00:00:00Z [TEMP_DIR]/requirements.in
     anyio==4.3.0
         # via hatchling-dynamic
-    hatchling-dynamic @ ../../scripts/packages/hatchling_dynamic
+    ../../scripts/packages/hatchling_dynamic
         # via -r [TEMP_DIR]/requirements.in
     idna==3.6
         # via anyio
@@ -7698,7 +7698,7 @@ requires-python = ">3.8"
     #    uv pip compile --cache-dir [CACHE_DIR] --exclude-newer 2024-03-25T00:00:00Z requirements.in
     anyio @ file://[TEMP_DIR]/anyio/
         # via lib
-    example @ ./app
+    ./app
         # via -r requirements.in
     idna==3.6
         # via anyio
@@ -7786,7 +7786,7 @@ requires-python = ">3.8"
         # via
         #   --override overrides.txt
         #   lib
-    example @ ./app
+    ./app
         # via -r requirements.in
     idna==3.6
         # via anyio
@@ -7896,12 +7896,12 @@ requires-python = ">3.8"
     ----- 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 requirements.in --override overrides.txt --constraint constraints.txt
-    anyio @ ./anyio
+    ./anyio
         # via
         #   -c constraints.txt
         #   --override overrides.txt
         #   lib
-    example @ ./app
+    ./app
         # via -r requirements.in
     idna==3.6
         # via anyio
@@ -7947,7 +7947,7 @@ requires-python = ">3.8"
     #    uv pip compile --cache-dir [CACHE_DIR] --exclude-newer 2024-03-25T00:00:00Z requirements.in
     click==8.1.7
         # via flask
-    example @ .
+    .
         # via -r requirements.in
     flask==2.0.0rc1
         # via example