Warn about pyproject.toml with maturin version constraint

Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 authors = ["konstin <[email protected]>"]
 name = "maturin"
-version = "0.10.5-beta.6"
+version = "0.10.5"
 description = "Build and publish crates with pyo3, rust-cpython and cffi bindings as well as rust binaries as python packages"
 exclude = ["test-crates/**/*", "sysconfig/*", "test-data/*", "ci/*", "tests/*"]
 readme = "Readme.md"

src/pyproject_toml.rs

@@ -7,8 +7,13 @@ use std::path::Path;
 #[derive(Serialize, Deserialize, Debug, Clone)]
 #[serde(rename_all = "kebab-case")]
 pub struct BuildSystem {
-    requires: Vec<String>,
-    build_backend: String,
+    /// PEP 518: This key must have a value of a list of strings representing PEP 508 dependencies
+    /// required to execute the build system (currently that means what dependencies are required
+    /// to execute a setup.py file).
+    pub requires: Vec<String>,
+    /// PEP 517: `build-backend` is a string naming a Python object that will be used to perform
+    /// the build
+    pub build_backend: String,
 }
 
 /// The `[tool]` section of a pyproject.toml
@@ -29,8 +34,20 @@ pub struct ToolMaturin {
 #[derive(Serialize, Deserialize, Debug, Clone)]
 #[serde(rename_all = "kebab-case")]
 pub struct PyProjectToml {
-    build_system: BuildSystem,
-    tool: Option<Tool>,
+    /// PEP 518: The [build-system] table is used to store build-related data. Initially only one
+    /// key of the table will be valid and is mandatory for the table: requires. This key must have
+    /// a value of a list of strings representing PEP 508 dependencies required to execute the
+    /// build system (currently that means what dependencies are required to execute a setup.py
+    /// file).
+    ///
+    /// We also mandate `build_backend`
+    pub build_system: BuildSystem,
+    /// PEP 518: The `[tool]` table is where any tool related to your Python project, not just build
+    /// tools, can have users specify configuration data as long as they use a sub-table within
+    /// `[tool]`, e.g. the flit tool would store its configuration in `[tool.flit]`.
+    ///
+    /// We use it for `[tool.maturin]`
+    pub tool: Option<Tool>,
 }
 
 impl PyProjectToml {
@@ -44,13 +61,69 @@ impl PyProjectToml {
             "Couldn't find pyproject.toml at {}",
             path.display()
         ))?;
-        let cargo_toml = toml::from_str(&contents)
+        let cargo_toml: PyProjectToml = toml::from_str(&contents)
             .map_err(|err| format_err!("pyproject.toml is not PEP 517 compliant: {}", err))?;
+        cargo_toml.warn_missing_maturin_version();
         Ok(cargo_toml)
     }
 
     /// Returns the value of `[maturin.sdist-include]` in pyproject.toml
     pub fn sdist_include(&self) -> Option<&Vec<String>> {
         self.tool.as_ref()?.maturin.as_ref()?.sdist_include.as_ref()
     }
+
+    /// Having a pyproject.toml without a version constraint is a bad idea
+    /// because at some point we'll have to do breaking changes and then source
+    /// distributions would break
+    ///
+    /// Returns true if the pyproject.toml has the constraint
+    pub fn warn_missing_maturin_version(&self) -> bool {
+        let maturin = env!("CARGO_PKG_NAME");
+        if let Some(requires_maturin) = self
+            .build_system
+            .requires
+            .iter()
+            .find(|x| x.starts_with(maturin))
+        {
+            // Note: Update this once 1.0 is out
+            assert_eq!(env!("CARGO_PKG_VERSION_MAJOR"), "0");
+            let current_minor: usize = env!("CARGO_PKG_VERSION_MINOR").parse().unwrap();
+            if requires_maturin == maturin {
+                eprintln!(
+                    "⚠  Warning: Please use {maturin} in pyproject.toml with a version constraint, \
+                    e.g. `requires = [\"{maturin}>=0.{current},<0.{next}\"]`. \
+                    This will become an error.",
+                    maturin = maturin,
+                    current = current_minor,
+                    next = current_minor + 1,
+                );
+                return false;
+            }
+        }
+        true
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::PyProjectToml;
+    use fs_err as fs;
+    use tempfile::TempDir;
+
+    #[test]
+    fn test_warn_missing_maturin_version() {
+        let with_constraint = PyProjectToml::new("test-crates/pyo3-pure").unwrap();
+        assert!(with_constraint.warn_missing_maturin_version());
+        let without_constraint_dir = TempDir::new().unwrap();
+
+        fs::write(
+            without_constraint_dir.path().join("pyproject.toml"),
+            r#"[build-system]
+            requires = ["maturin"]
+            build-backend = "maturin""#,
+        )
+        .unwrap();
+        let without_constraint = PyProjectToml::new(without_constraint_dir).unwrap();
+        assert!(!without_constraint.warn_missing_maturin_version());
+    }
 }