[ty] Tell the user why we inferred a certain Python version when reporting version-specific syntax errors (#18295)

crates/ty_python_semantic/src/util/diagnostics.rs

@@ -0,0 +1,51 @@
+use crate::{Db, Program, PythonVersionWithSource};
+use ruff_db::{
+    diagnostic::{Annotation, Diagnostic, Severity, Span, SubDiagnostic},
+    files::system_path_to_file,
+};
+
+/// Add a subdiagnostic to `diagnostic` that explains why a certain Python version was inferred.
+///
+/// ty can infer the Python version from various sources, such as command-line arguments,
+/// configuration files, or defaults.
+pub fn add_inferred_python_version_hint_to_diagnostic(
+    db: &dyn Db,
+    diagnostic: &mut Diagnostic,
+    action: &str,
+) {
+    let program = Program::get(db);
+    let PythonVersionWithSource { version, source } = program.python_version_with_source(db);
+
+    match source {
+        crate::PythonVersionSource::Cli => {
+            diagnostic.info(format_args!(
+                "Python {version} was assumed when {action} because it was specified on the command line",
+            ));
+        }
+        crate::PythonVersionSource::File(path, range) => {
+            if let Ok(file) = system_path_to_file(db.upcast(), &**path) {
+                let mut sub_diagnostic = SubDiagnostic::new(
+                    Severity::Info,
+                    format_args!("Python {version} was assumed when {action}"),
+                );
+                sub_diagnostic.annotate(
+                    Annotation::primary(Span::from(file).with_optional_range(*range)).message(
+                        format_args!("Python {version} assumed due to this configuration setting"),
+                    ),
+                );
+                diagnostic.sub(sub_diagnostic);
+            } else {
+                diagnostic.info(format_args!(
+                    "Python {version} was assumed when {action} because of your configuration file(s)",
+                ));
+            }
+        }
+        crate::PythonVersionSource::Default => {
+            diagnostic.info(format_args!(
+                "Python {version} was assumed when {action} \
+                because it is the newest Python version supported by ty, \
+                and neither a command-line argument nor a configuration setting was provided",
+            ));
+        }
+    }
+}

crates/ty_python_semantic/src/util/mod.rs

@@ -1 +1,2 @@
+pub(crate) mod diagnostics;
 pub(crate) mod subscript;