Check pyproject.toml correctly when it is passed via stdin (#16971)

## Summary

Resolves #16950 and [a 1.5-year-old TODO
comment](8d16a5c8c9/crates/ruff/src/diagnostics.rs (L380)).

After this change, a `pyproject.toml` will be linted the same as any
Python files would when passed via stdin.

## Test Plan

Integration tests.

crates/ruff/src/diagnostics.rs

@@ -367,8 +367,7 @@ pub(crate) fn lint_path(
     })
 }
 
-/// Generate `Diagnostic`s from source code content derived from
-/// stdin.
+/// Generate `Diagnostic`s from source code content derived from stdin.
 pub(crate) fn lint_stdin(
     path: Option<&Path>,
     package: Option<PackageRoot<'_>>,
@@ -377,13 +376,37 @@ pub(crate) fn lint_stdin(
     noqa: flags::Noqa,
     fix_mode: flags::FixMode,
 ) -> Result<Diagnostics> {
-    // TODO(charlie): Support `pyproject.toml`.
     let source_type = match path.and_then(|path| settings.linter.extension.get(path)) {
         None => match path.map(SourceType::from).unwrap_or_default() {
             SourceType::Python(source_type) => source_type,
-            SourceType::Toml(_) => {
-                return Ok(Diagnostics::default());
+
+            SourceType::Toml(source_type) if source_type.is_pyproject() => {
+                if !settings
+                    .linter
+                    .rules
+                    .iter_enabled()
+                    .any(|rule_code| rule_code.lint_source().is_pyproject_toml())
+                {
+                    return Ok(Diagnostics::default());
+                }
+
+                let path = path.unwrap();
+                let source_file =
+                    SourceFileBuilder::new(path.to_string_lossy(), contents.clone()).finish();
+
+                match fix_mode {
+                    flags::FixMode::Diff | flags::FixMode::Generate => {}
+                    flags::FixMode::Apply => write!(&mut io::stdout().lock(), "{contents}")?,
+                }
+
+                return Ok(Diagnostics {
+                    messages: lint_pyproject_toml(source_file, &settings.linter),
+                    fixed: FixMap::from_iter([(fs::relativize_path(path), FixTable::default())]),
+                    notebook_indexes: FxHashMap::default(),
+                });
             }
+
+            SourceType::Toml(_) => return Ok(Diagnostics::default()),
         },
         Some(language) => PySourceType::from(language),
     };

crates/ruff/tests/integration_test.rs

@@ -2233,3 +2233,195 @@ unfixable = ["RUF"]
 
     Ok(())
 }
+
+#[test]
+fn pyproject_toml_stdin_syntax_error() {
+    let mut cmd = RuffCheck::default()
+        .args(["--stdin-filename", "pyproject.toml", "--select", "RUF200"])
+        .build();
+
+    assert_cmd_snapshot!(
+        cmd.pass_stdin("[project"),
+        @r"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+    pyproject.toml:1:9: RUF200 Failed to parse pyproject.toml: invalid table header
+    expected `.`, `]`
+      |
+    1 | [project
+      |         ^ RUF200
+      |
+
+    Found 1 error.
+
+    ----- stderr -----
+    "
+    );
+}
+
+#[test]
+fn pyproject_toml_stdin_schema_error() {
+    let mut cmd = RuffCheck::default()
+        .args(["--stdin-filename", "pyproject.toml", "--select", "RUF200"])
+        .build();
+
+    assert_cmd_snapshot!(
+        cmd.pass_stdin("[project]\nname = 1"),
+        @r"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+    pyproject.toml:2:8: RUF200 Failed to parse pyproject.toml: invalid type: integer `1`, expected a string
+      |
+    1 | [project]
+    2 | name = 1
+      |        ^ RUF200
+      |
+
+    Found 1 error.
+
+    ----- stderr -----
+    "
+    );
+}
+
+#[test]
+fn pyproject_toml_stdin_no_applicable_rules_selected() {
+    let mut cmd = RuffCheck::default()
+        .args(["--stdin-filename", "pyproject.toml"])
+        .build();
+
+    assert_cmd_snapshot!(
+        cmd.pass_stdin("[project"),
+        @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    All checks passed!
+
+    ----- stderr -----
+    "
+    );
+}
+
+#[test]
+fn pyproject_toml_stdin_no_applicable_rules_selected_2() {
+    let mut cmd = RuffCheck::default()
+        .args(["--stdin-filename", "pyproject.toml", "--select", "F401"])
+        .build();
+
+    assert_cmd_snapshot!(
+        cmd.pass_stdin("[project"),
+        @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    All checks passed!
+
+    ----- stderr -----
+    "
+    );
+}
+
+#[test]
+fn pyproject_toml_stdin_no_errors() {
+    let mut cmd = RuffCheck::default()
+        .args(["--stdin-filename", "pyproject.toml"])
+        .build();
+
+    assert_cmd_snapshot!(
+        cmd.pass_stdin(r#"[project]\nname = "ruff"\nversion = "0.0.0""#),
+        @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    All checks passed!
+
+    ----- stderr -----
+    "
+    );
+}
+
+#[test]
+fn pyproject_toml_stdin_schema_error_fix() {
+    let mut cmd = RuffCheck::default()
+        .args([
+            "--stdin-filename",
+            "pyproject.toml",
+            "--select",
+            "RUF200",
+            "--fix",
+        ])
+        .build();
+
+    assert_cmd_snapshot!(
+        cmd.pass_stdin("[project]\nname = 1"),
+        @r"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+    [project]
+    name = 1
+    ----- stderr -----
+    pyproject.toml:2:8: RUF200 Failed to parse pyproject.toml: invalid type: integer `1`, expected a string
+      |
+    1 | [project]
+    2 | name = 1
+      |        ^ RUF200
+      |
+
+    Found 1 error.
+    "
+    );
+}
+
+#[test]
+fn pyproject_toml_stdin_schema_error_fix_only() {
+    let mut cmd = RuffCheck::default()
+        .args([
+            "--stdin-filename",
+            "pyproject.toml",
+            "--select",
+            "RUF200",
+            "--fix-only",
+        ])
+        .build();
+
+    assert_cmd_snapshot!(
+        cmd.pass_stdin("[project]\nname = 1"),
+        @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    [project]
+    name = 1
+    ----- stderr -----
+    "
+    );
+}
+
+#[test]
+fn pyproject_toml_stdin_schema_error_fix_diff() {
+    let mut cmd = RuffCheck::default()
+        .args([
+            "--stdin-filename",
+            "pyproject.toml",
+            "--select",
+            "RUF200",
+            "--fix",
+            "--diff",
+        ])
+        .build();
+
+    assert_cmd_snapshot!(
+        cmd.pass_stdin("[project]\nname = 1"),
+        @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    "
+    );
+}