[red-knot] Add --python-platform CLI option (#17284)

## Summary

Add a new `--python-platform` command-line option, in analogy to
`--python-version`.

## Test Plan

Added new integration test.

crates/red_knot/src/args.rs

@@ -71,6 +71,14 @@ pub(crate) struct CheckCommand {
     #[arg(long, value_name = "VERSION", alias = "target-version")]
     pub(crate) python_version: Option<PythonVersion>,
 
+    /// Target platform to assume when resolving types.
+    ///
+    /// This is used to specialize the type of `sys.platform` and will affect the visibility
+    /// of platform-specific functions and attributes. If the value is set to `all`, no
+    /// assumptions are made about the target platform.
+    #[arg(long, value_name = "PLATFORM", alias = "platform")]
+    pub(crate) python_platform: Option<String>,
+
     #[clap(flatten)]
     pub(crate) verbosity: Verbosity,
 
@@ -116,6 +124,9 @@ impl CheckCommand {
                 python_version: self
                     .python_version
                     .map(|version| RangedValue::cli(version.into())),
+                python_platform: self
+                    .python_platform
+                    .map(|platform| RangedValue::cli(platform.into())),
                 python: self.python.map(RelativePathBuf::cli),
                 typeshed: self.typeshed.map(RelativePathBuf::cli),
                 extra_paths: self.extra_search_path.map(|extra_search_paths| {
@@ -124,7 +135,6 @@ impl CheckCommand {
                         .map(RelativePathBuf::cli)
                         .collect()
                 }),
-                ..EnvironmentOptions::default()
             }),
             terminal: Some(TerminalOptions {
                 output_format: self

crates/red_knot/tests/cli.rs

@@ -6,9 +6,9 @@ use std::process::Command;
 use tempfile::TempDir;
 
 /// Specifying an option on the CLI should take precedence over the same setting in the
-/// project's configuration.
+/// project's configuration. Here, this is tested for the Python version.
 #[test]
-fn config_override() -> anyhow::Result<()> {
+fn config_override_python_version() -> anyhow::Result<()> {
     let case = TestCase::with_files([
         (
             "pyproject.toml",
@@ -57,6 +57,67 @@ fn config_override() -> anyhow::Result<()> {
     Ok(())
 }
 
+/// Same as above, but for the Python platform.
+#[test]
+fn config_override_python_platform() -> anyhow::Result<()> {
+    let case = TestCase::with_files([
+        (
+            "pyproject.toml",
+            r#"
+            [tool.knot.environment]
+            python-platform = "linux"
+            "#,
+        ),
+        (
+            "test.py",
+            r#"
+            import sys
+            from typing_extensions import reveal_type
+
+            reveal_type(sys.platform)
+            "#,
+        ),
+    ])?;
+
+    assert_cmd_snapshot!(case.command(), @r#"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    info: revealed-type
+     --> <temp_dir>/test.py:5:1
+      |
+    3 | from typing_extensions import reveal_type
+    4 |
+    5 | reveal_type(sys.platform)
+      | ^^^^^^^^^^^^^^^^^^^^^^^^^ Revealed type is `Literal["linux"]`
+      |
+
+    Found 1 diagnostic
+
+    ----- stderr -----
+    "#);
+
+    assert_cmd_snapshot!(case.command().arg("--python-platform").arg("all"), @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    info: revealed-type
+     --> <temp_dir>/test.py:5:1
+      |
+    3 | from typing_extensions import reveal_type
+    4 |
+    5 | reveal_type(sys.platform)
+      | ^^^^^^^^^^^^^^^^^^^^^^^^^ Revealed type is `LiteralString`
+      |
+
+    Found 1 diagnostic
+
+    ----- stderr -----
+    ");
+
+    Ok(())
+}
+
 /// Paths specified on the CLI are relative to the current working directory and not the project root.
 ///
 /// We test this by adding an extra search path from the CLI to the libs directory when

crates/red_knot_project/src/metadata/options.rs

@@ -224,7 +224,7 @@ impl Options {
 #[serde(rename_all = "kebab-case", deny_unknown_fields)]
 #[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
 pub struct EnvironmentOptions {
-    /// Specifies the version of Python that will be used to execute the source code.
+    /// Specifies the version of Python that will be used to analyze the source code.
     /// The version should be specified as a string in the format `M.m` where `M` is the major version
     /// and `m` is the minor (e.g. "3.0" or "3.6").
     /// If a version is provided, knot will generate errors if the source code makes use of language features
@@ -233,7 +233,7 @@ pub struct EnvironmentOptions {
     #[serde(skip_serializing_if = "Option::is_none")]
     pub python_version: Option<RangedValue<PythonVersion>>,
 
-    /// Specifies the target platform that will be used to execute the source code.
+    /// Specifies the target platform that will be used to analyze the source code.
     /// If specified, Red Knot will tailor its use of type stub files,
     /// which conditionalize type definitions based on the platform.
     ///

crates/red_knot_python_semantic/src/python_platform.rs

@@ -21,6 +21,15 @@ pub enum PythonPlatform {
     Identifier(String),
 }
 
+impl From<String> for PythonPlatform {
+    fn from(platform: String) -> Self {
+        match platform.as_str() {
+            "all" => PythonPlatform::All,
+            _ => PythonPlatform::Identifier(platform.to_string()),
+        }
+    }
+}
+
 impl Display for PythonPlatform {
     fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
         match self {