Build backend: Add fast path (#9556)

Going through PEP 517 to build a package is slow, so when building a
package with the uv build backend, we can call into the uv build backend
directly. This is the basis for the `uv build --list`.

This does not enable the fast path for general source dependencies.

There is a possible difference in execution if the latest uv version is
newer than the one currently running: The PEP 517 path would use the
latest version, while the fast path uses the current version.

Please review commit-by-commit

### Benchmark

`built_with_uv`, using the fast path:
```
$ hyperfine "~/projects/uv/target/profiling/uv build"
Time (mean ± σ):       9.2 ms ±   1.1 ms    [User: 4.6 ms, System: 4.6 ms]
Range (min … max):     6.4 ms …  12.7 ms    290 runs
```

`hatcling_editable`, with hatchling being optimized for fast startup
times:
```
$ hyperfine "~/projects/uv/target/profiling/uv build"
Time (mean ± σ):     270.5 ms ±  18.4 ms    [User: 230.8 ms, System: 44.5 ms]
Range (min … max):   250.7 ms … 298.4 ms    10 runs
```

Author: konstin

crates/uv-build-backend/src/lib.rs

@@ -1,6 +1,8 @@
 mod metadata;
 
-use crate::metadata::{BuildBackendSettings, PyProjectToml, ValidationError, DEFAULT_EXCLUDES};
+pub use metadata::PyProjectToml;
+
+use crate::metadata::{BuildBackendSettings, ValidationError, DEFAULT_EXCLUDES};
 use flate2::write::GzEncoder;
 use flate2::Compression;
 use fs_err::File;
@@ -304,7 +306,9 @@ pub fn build_wheel(
 ) -> Result<WheelFilename, Error> {
     let contents = fs_err::read_to_string(source_tree.join("pyproject.toml"))?;
     let pyproject_toml = PyProjectToml::parse(&contents)?;
-    pyproject_toml.check_build_system(uv_version);
+    for warning in pyproject_toml.check_build_system(uv_version) {
+        warn_user_once!("{warning}");
+    }
     let settings = pyproject_toml
         .settings()
         .cloned()
@@ -465,7 +469,9 @@ pub fn build_editable(
 ) -> Result<WheelFilename, Error> {
     let contents = fs_err::read_to_string(source_tree.join("pyproject.toml"))?;
     let pyproject_toml = PyProjectToml::parse(&contents)?;
-    pyproject_toml.check_build_system(uv_version);
+    for warning in pyproject_toml.check_build_system(uv_version) {
+        warn_user_once!("{warning}");
+    }
     let settings = pyproject_toml
         .settings()
         .cloned()
@@ -601,7 +607,9 @@ pub fn build_source_dist(
 ) -> Result<SourceDistFilename, Error> {
     let contents = fs_err::read_to_string(source_tree.join("pyproject.toml"))?;
     let pyproject_toml = PyProjectToml::parse(&contents)?;
-    pyproject_toml.check_build_system(uv_version);
+    for warning in pyproject_toml.check_build_system(uv_version) {
+        warn_user_once!("{warning}");
+    }
     let settings = pyproject_toml
         .settings()
         .cloned()
@@ -851,7 +859,9 @@ pub fn metadata(
 ) -> Result<String, Error> {
     let contents = fs_err::read_to_string(source_tree.join("pyproject.toml"))?;
     let pyproject_toml = PyProjectToml::parse(&contents)?;
-    pyproject_toml.check_build_system(uv_version);
+    for warning in pyproject_toml.check_build_system(uv_version) {
+        warn_user_once!("{warning}");
+    }
 
     let filename = WheelFilename {
         name: pyproject_toml.name().clone(),

crates/uv-build-backend/src/metadata.rs

@@ -12,7 +12,6 @@ use uv_normalize::{ExtraName, PackageName};
 use uv_pep440::{Version, VersionSpecifiers};
 use uv_pep508::{Requirement, VersionOrUrl};
 use uv_pypi_types::{Metadata23, VerbatimParsedUrl};
-use uv_warnings::warn_user_once;
 use version_ranges::Ranges;
 use walkdir::WalkDir;
 
@@ -58,7 +57,7 @@ pub enum ValidationError {
     expecting = "The project table needs to follow \
     https://packaging.python.org/en/latest/guides/writing-pyproject-toml"
 )]
-pub(crate) struct PyProjectToml {
+pub struct PyProjectToml {
     /// Project metadata
     project: Project,
     /// uv-specific configuration
@@ -92,7 +91,7 @@ impl PyProjectToml {
         self.tool.as_ref()?.uv.as_ref()?.build_backend.as_ref()
     }
 
-    /// Warn if the `[build-system]` table looks suspicious.
+    /// Returns user-facing warnings if the `[build-system]` table looks suspicious.
     ///
     /// Example of a valid table:
     ///
@@ -101,16 +100,13 @@ impl PyProjectToml {
     /// requires = ["uv>=0.4.15,<5"]
     /// build-backend = "uv"
     /// ```
-    ///
-    /// Returns whether all checks passed.
-    pub(crate) fn check_build_system(&self, uv_version: &str) -> bool {
-        let mut passed = true;
+    pub fn check_build_system(&self, uv_version: &str) -> Vec<String> {
+        let mut warnings = Vec::new();
         if self.build_system.build_backend.as_deref() != Some("uv") {
-            warn_user_once!(
+            warnings.push(format!(
                 r#"The value for `build_system.build-backend` should be `"uv"`, not `"{}"`"#,
                 self.build_system.build_backend.clone().unwrap_or_default()
-            );
-            passed = false;
+            ));
         }
 
         let uv_version =
@@ -126,12 +122,12 @@ impl PyProjectToml {
         };
 
         let [uv_requirement] = &self.build_system.requires.as_slice() else {
-            warn_user_once!("{}", expected());
-            return false;
+            warnings.push(expected());
+            return warnings;
         };
         if uv_requirement.name.as_str() != "uv" {
-            warn_user_once!("{}", expected());
-            return false;
+            warnings.push(expected());
+            return warnings;
         }
         let bounded = match &uv_requirement.version_or_url {
             None => false,
@@ -147,11 +143,10 @@ impl PyProjectToml {
                 // the existing immutable source distributions on pypi.
                 if !specifier.contains(&uv_version) {
                     // This is allowed to happen when testing prereleases, but we should still warn.
-                    warn_user_once!(
+                    warnings.push(format!(
                         r#"`build_system.requires = ["{uv_requirement}"]` does not contain the
                         current uv version {uv_version}"#,
-                    );
-                    passed = false;
+                    ));
                 }
                 Ranges::from(specifier.clone())
                     .bounding_range()
@@ -161,16 +156,15 @@ impl PyProjectToml {
         };
 
         if !bounded {
-            warn_user_once!(
-                r#"`build_system.requires = ["{uv_requirement}"]` is missing an
-                upper bound on the uv version such as `<{next_breaking}`.
-                Without bounding the uv version, the source distribution will break
-                when a future, breaking version of uv is released."#,
-            );
-            passed = false;
+            warnings.push(format!(
+                "`build_system.requires = [\"{uv_requirement}\"]` is missing an \
+                upper bound on the uv version such as `<{next_breaking}`. \
+                Without bounding the uv version, the source distribution will break \
+                when a future, breaking version of uv is released.",
+            ));
         }
 
-        passed
+        warnings
     }
 
     /// Validate and convert a `pyproject.toml` to core metadata.
@@ -995,7 +989,10 @@ mod tests {
     fn build_system_valid() {
         let contents = extend_project("");
         let pyproject_toml = PyProjectToml::parse(&contents).unwrap();
-        assert!(pyproject_toml.check_build_system("1.0.0+test"));
+        assert_snapshot!(
+            pyproject_toml.check_build_system("1.0.0+test").join("\n"),
+            @""
+        );
     }
 
     #[test]
@@ -1010,7 +1007,10 @@ mod tests {
             build-backend = "uv"
         "#};
         let pyproject_toml = PyProjectToml::parse(contents).unwrap();
-        assert!(!pyproject_toml.check_build_system("1.0.0+test"));
+        assert_snapshot!(
+            pyproject_toml.check_build_system("0.4.15+test").join("\n"),
+            @r###"`build_system.requires = ["uv"]` is missing an upper bound on the uv version such as `<0.5`. Without bounding the uv version, the source distribution will break when a future, breaking version of uv is released."###
+        );
     }
 
     #[test]
@@ -1025,7 +1025,10 @@ mod tests {
             build-backend = "uv"
         "#};
         let pyproject_toml = PyProjectToml::parse(contents).unwrap();
-        assert!(!pyproject_toml.check_build_system("1.0.0+test"));
+        assert_snapshot!(
+            pyproject_toml.check_build_system("0.4.15+test").join("\n"),
+            @"Expected a single uv requirement in `build-system.requires`, found ``"
+        );
     }
 
     #[test]
@@ -1040,7 +1043,10 @@ mod tests {
             build-backend = "uv"
         "#};
         let pyproject_toml = PyProjectToml::parse(contents).unwrap();
-        assert!(!pyproject_toml.check_build_system("1.0.0+test"));
+        assert_snapshot!(
+            pyproject_toml.check_build_system("0.4.15+test").join("\n"),
+            @"Expected a single uv requirement in `build-system.requires`, found ``"
+        );
     }
 
     #[test]
@@ -1055,7 +1061,10 @@ mod tests {
             build-backend = "setuptools"
         "#};
         let pyproject_toml = PyProjectToml::parse(contents).unwrap();
-        assert!(!pyproject_toml.check_build_system("1.0.0+test"));
+        assert_snapshot!(
+            pyproject_toml.check_build_system("0.4.15+test").join("\n"),
+            @r###"The value for `build_system.build-backend` should be `"uv"`, not `"setuptools"`"###
+        );
     }
 
     #[test]

crates/uv-cli/src/lib.rs

@@ -2178,6 +2178,14 @@ pub struct BuildArgs {
     #[arg(long, overrides_with("build_logs"), hide = true)]
     pub no_build_logs: bool,
 
+    /// Always build through PEP 517, don't use the fast path for the uv build backend.
+    ///
+    /// By default, uv won't create a PEP 517 build environment for packages using the uv build
+    /// backend, but use a fast path that calls into the build backend directly. This option forces
+    /// always using PEP 517.
+    #[arg(long)]
+    pub no_fast_path: bool,
+
     /// Constrain build dependencies using the given requirements files when building
     /// distributions.
     ///