Detect TOML 1.1 and automatically enable the toml backwards compatibility preview feature

Cargo.toml

@@ -77,6 +77,7 @@ uv-small-str = { version = "0.0.40", path = "crates/uv-small-str" }
 uv-state = { version = "0.0.40", path = "crates/uv-state" }
 uv-static = { version = "0.0.40", path = "crates/uv-static" }
 uv-test = { version = "0.0.40", path = "crates/uv-test" }
+uv-toml = { version = "0.0.40", path = "crates/uv-toml" }
 uv-tool = { version = "0.0.40", path = "crates/uv-tool" }
 uv-torch = { version = "0.0.40", path = "crates/uv-torch" }
 uv-trampoline-builder = { version = "0.0.40", path = "crates/uv-trampoline-builder" }
@@ -269,6 +270,7 @@ tokio-stream = { version = "0.1.16" }
 tokio-util = { version = "0.7.12", features = ["compat", "io"] }
 toml = { version = "1.1.0", features = ["fast_hash"] }
 toml_edit = { version = "0.25.8", features = ["serde"] }
+toml_parser = { version = "1.1.0" }
 tracing = { version = "0.1.40" }
 tracing-durations-export = { version = "0.3.0", features = ["plot"] }
 tracing-subscriber = { version = "0.3.18" } # Default feature set for uv_build, uv activates extra features

crates/uv-build-backend/Cargo.toml

@@ -24,6 +24,7 @@ uv-pep508 = { workspace = true }
 uv-platform-tags = { workspace = true }
 uv-preview = { workspace = true }
 uv-pypi-types = { workspace = true }
+uv-toml = { workspace = true }
 uv-warnings = { workspace = true }
 
 base64 = { workspace = true }

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

@@ -1955,4 +1955,95 @@ mod tests {
         build-backend = "uv_build"
         "#);
     }
+
+    /// Test that TOML 1.1 features in pyproject.toml trigger auto-detection and rewrite to TOML
+    /// 1.0, even without explicitly enabling `PreviewFeature::TomlBackwardsCompatibility`.
+    #[test]
+    fn toml_1_1_backward_compatibility_auto_detection() {
+        let _preview = uv_preview::test::with_features(&[]);
+        let src = TempDir::new().unwrap();
+
+        // A `pyproject.toml` with a TOML 1.1 feature, trailing commas in inline tables.
+        let pyproject_toml = indoc! {r#"
+            [project]
+            name = "toml11-project"
+            version = "0.1.0"
+            description = "A test package using TOML 1.1 features"
+            requires-python = ">=3.12"
+            # TOML 1.1 feature: Trailing comma in inline table
+            authors = [
+                { name = "Ferris", email = "ferris@example.com", },
+                { name = "Platypus", email = "platypus@example.com", },
+            ]
+
+            [build-system]
+            requires = ["uv_build>=0.5.15,<0.6.0"]
+            build-backend = "uv_build"
+        "#};
+
+        fs_err::write(src.path().join("pyproject.toml"), pyproject_toml).unwrap();
+        fs_err::create_dir_all(src.path().join("src").join("toml11_project")).unwrap();
+        File::create(
+            src.path()
+                .join("src")
+                .join("toml11_project")
+                .join("__init__.py"),
+        )
+        .unwrap();
+
+        let dist = TempDir::new().unwrap();
+        let build = build(src.path(), dist.path()).unwrap();
+
+        // Check that both `pyproject.toml` and `pyproject.toml.orig` are in the sdist.
+        assert_snapshot!(build.source_dist_contents.join("\n"), @"
+        toml11_project-0.1.0/
+        toml11_project-0.1.0/PKG-INFO
+        toml11_project-0.1.0/pyproject.toml
+        toml11_project-0.1.0/pyproject.toml.orig
+        toml11_project-0.1.0/src
+        toml11_project-0.1.0/src/toml11_project
+        toml11_project-0.1.0/src/toml11_project/__init__.py
+        ");
+
+        // Extract the sdist to verify the contents of both files.
+        let source_dist_path = dist.path().join(build.source_dist_filename.to_string());
+        let sdist_reader = BufReader::new(File::open(&source_dist_path).unwrap());
+        let mut source_dist = tar::Archive::new(GzDecoder::new(sdist_reader));
+
+        let mut pyproject_toml_content = String::new();
+        let mut pyproject_toml_orig_content = String::new();
+        for entry in source_dist.entries().unwrap() {
+            let mut entry = entry.unwrap();
+            let path = entry.path().unwrap().to_string_lossy().to_string();
+
+            if path.ends_with("pyproject.toml") && !path.eq_ignore_ascii_case(".orig") {
+                entry.read_to_string(&mut pyproject_toml_content).unwrap();
+            } else if path.ends_with("pyproject.toml.orig") {
+                entry
+                    .read_to_string(&mut pyproject_toml_orig_content)
+                    .unwrap();
+            }
+        }
+
+        assert_eq!(pyproject_toml_orig_content, pyproject_toml);
+        assert_snapshot!(pyproject_toml_content, @r#"
+        [project]
+        name = "toml11-project"
+        version = "0.1.0"
+        description = "A test package using TOML 1.1 features"
+        requires-python = ">=3.12"
+
+        [[project.authors]]
+        name = "Ferris"
+        email = "ferris@example.com"
+
+        [[project.authors]]
+        name = "Platypus"
+        email = "platypus@example.com"
+
+        [build-system]
+        requires = ["uv_build>=0.5.15,<0.6.0"]
+        build-backend = "uv_build"
+        "#);
+    }
 }

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

@@ -17,6 +17,7 @@ use uv_distribution_filename::{SourceDistExtension, SourceDistFilename};
 use uv_fs::{Simplified, normalize_path};
 use uv_globfilter::{GlobDirFilter, PortableGlobParser};
 use uv_preview::PreviewFeature;
+use uv_toml::has_toml11_features;
 use uv_warnings::warn_user_once;
 use walkdir::WalkDir;
 
@@ -236,11 +237,26 @@ fn write_source_dist(
     //
     // To work around this, we do a best-effort rewrite of `pyproject.toml` to TOML 1.0. We also
     // add the original `pyproject.toml` as `pyproject.toml.orig` for reference.
+    //
+    // The feature is enabled either explicitly via the preview flag, or automatically when the
+    // `pyproject.toml` is detected to contain TOML 1.1-only syntax.
+    let pyproject_path = source_tree.join("pyproject.toml");
+    let pyproject_contents = fs_err::read_to_string(&pyproject_path)?;
     let toml_backwards_compatibility =
-        uv_preview::is_enabled(PreviewFeature::TomlBackwardsCompatibility);
+        if uv_preview::is_enabled(PreviewFeature::TomlBackwardsCompatibility) {
+            true
+        } else if has_toml11_features(&pyproject_contents) {
+            warn_user_once!(
+                "`pyproject.toml` uses TOML 1.1 features; rewriting to TOML 1.0 for \
+                compatibility with older build tools. Use `--preview-feature \
+                {feature}` to suppress this warning.",
+                feature = PreviewFeature::TomlBackwardsCompatibility
+            );
+            true
+        } else {
+            false
+        };
     if toml_backwards_compatibility {
-        let pyproject_path = source_tree.join("pyproject.toml");
-        let pyproject_contents = fs_err::read_to_string(&pyproject_path)?;
         let pyproject_value: toml::Value = toml::from_str(&pyproject_contents)
             .map_err(|err| Error::Toml(pyproject_path.clone(), err))?;
         // See https://github.com/toml-rs/toml/issues/1088 for `to_string_pretty`.

crates/uv-toml/Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "uv-toml"
+version = "0.0.40"
+description = "This is an internal component crate of uv"
+edition = { workspace = true }
+rust-version = { workspace = true }
+homepage = { workspace = true }
+repository = { workspace = true }
+authors = { workspace = true }
+license = { workspace = true }
+
+[lib]
+doctest = false
+
+[lints]
+workspace = true
+
+[dependencies]
+toml_parser = { workspace = true }

crates/uv-toml/README.md

@@ -0,0 +1,13 @@
+<!-- This file is generated. DO NOT EDIT -->
+
+# uv-fs
+
+This crate is an internal component of [uv](https://crates.io/crates/uv). The Rust API exposed here
+is unstable and will have frequent breaking changes.
+
+This version (0.0.40) is a component of [uv 0.11.7](https://crates.io/crates/uv/0.11.7). The source
+can be found [here](https://github.com/astral-sh/uv/blob/0.11.7/crates/uv-fs).
+
+See uv's
+[crate versioning policy](https://docs.astral.sh/uv/reference/policies/versioning/#crate-versioning)
+for details on versioning.