[red-knot] Make `Diagnostic::file` optional (#15640)

crates/red_knot_project/src/lib.rs

@@ -402,8 +402,8 @@ impl Diagnostic for IOErrorDiagnostic {
         self.error.to_string().into()
     }
 
-    fn file(&self) -> File {
-        self.file
+    fn file(&self) -> Option<File> {
+        Some(self.file)
     }
 
     fn range(&self) -> Option<TextRange> {

crates/red_knot_project/src/metadata/options.rs

@@ -5,6 +5,7 @@ use red_knot_python_semantic::{
 use ruff_db::system::{System, SystemPath};
 use ruff_macros::Combine;
 use serde::{Deserialize, Serialize};
+use std::fmt::Debug;
 use thiserror::Error;
 
 /// The options for the project.

crates/red_knot_python_semantic/src/types/diagnostic.rs

@@ -802,8 +802,8 @@ impl Diagnostic for TypeCheckDiagnostic {
         TypeCheckDiagnostic::message(self).into()
     }
 
-    fn file(&self) -> File {
-        TypeCheckDiagnostic::file(self)
+    fn file(&self) -> Option<File> {
+        Some(TypeCheckDiagnostic::file(self))
     }
 
     fn range(&self) -> Option<TextRange> {

crates/red_knot_server/src/server/api/requests/diagnostic.rs

@@ -75,9 +75,9 @@ fn to_lsp_diagnostic(
     diagnostic: &dyn ruff_db::diagnostic::Diagnostic,
     encoding: crate::PositionEncoding,
 ) -> Diagnostic {
-    let range = if let Some(range) = diagnostic.range() {
-        let index = line_index(db.upcast(), diagnostic.file());
-        let source = source_text(db.upcast(), diagnostic.file());
+    let range = if let (Some(file), Some(range)) = (diagnostic.file(), diagnostic.range()) {
+        let index = line_index(db.upcast(), file);
+        let source = source_text(db.upcast(), file);
 
         range.to_range(&source, &index, encoding)
     } else {

crates/red_knot_test/src/diagnostic.rs

@@ -198,8 +198,8 @@ mod tests {
             "dummy".into()
         }
 
-        fn file(&self) -> File {
-            self.file
+        fn file(&self) -> Option<File> {
+            Some(self.file)
         }
 
         fn range(&self) -> Option<TextRange> {

crates/red_knot_test/src/matcher.rs

@@ -385,8 +385,8 @@ mod tests {
             self.message.into()
         }
 
-        fn file(&self) -> File {
-            self.file
+        fn file(&self) -> Option<File> {
+            Some(self.file)
         }
 
         fn range(&self) -> Option<TextRange> {

crates/ruff_db/src/diagnostic.rs

@@ -152,8 +152,18 @@ pub trait Diagnostic: Send + Sync + std::fmt::Debug {
 
     fn message(&self) -> Cow<str>;
 
-    fn file(&self) -> File;
+    /// The file this diagnostic is associated with.
+    ///
+    /// File can be `None` for diagnostics that don't originate from a file.
+    /// For example:
+    /// * A diagnostic indicating that a directory couldn't be read.
+    /// * A diagnostic related to a CLI argument
+    fn file(&self) -> Option<File>;
 
+    /// The primary range of the diagnostic in `file`.
+    ///
+    /// The range can be `None` if the diagnostic doesn't have a file
+    /// or it applies to the entire file (e.g. the file should be executable but isn't).
     fn range(&self) -> Option<TextRange>;
 
     fn severity(&self) -> Severity;
@@ -197,16 +207,15 @@ impl std::fmt::Display for DisplayDiagnostic<'_> {
             Severity::Fatal => f.write_str("fatal")?,
         }
 
-        write!(
-            f,
-            "[{rule}] {path}",
-            rule = self.diagnostic.id(),
-            path = self.diagnostic.file().path(self.db)
-        )?;
+        write!(f, "[{rule}]", rule = self.diagnostic.id())?;
 
-        if let Some(range) = self.diagnostic.range() {
-            let index = line_index(self.db, self.diagnostic.file());
-            let source = source_text(self.db, self.diagnostic.file());
+        if let Some(file) = self.diagnostic.file() {
+            write!(f, " {path}", path = file.path(self.db))?;
+        }
+
+        if let (Some(file), Some(range)) = (self.diagnostic.file(), self.diagnostic.range()) {
+            let index = line_index(self.db, file);
+            let source = source_text(self.db, file);
 
             let start = index.source_location(range.start(), &source);
 
@@ -229,7 +238,7 @@ where
         (**self).message()
     }
 
-    fn file(&self) -> File {
+    fn file(&self) -> Option<File> {
         (**self).file()
     }
 
@@ -254,7 +263,7 @@ where
         (**self).message()
     }
 
-    fn file(&self) -> File {
+    fn file(&self) -> Option<File> {
         (**self).file()
     }
 
@@ -276,7 +285,7 @@ impl Diagnostic for Box<dyn Diagnostic> {
         (**self).message()
     }
 
-    fn file(&self) -> File {
+    fn file(&self) -> Option<File> {
         (**self).file()
     }
 
@@ -310,8 +319,8 @@ impl Diagnostic for ParseDiagnostic {
         self.error.error.to_string().into()
     }
 
-    fn file(&self) -> File {
-        self.file
+    fn file(&self) -> Option<File> {
+        Some(self.file)
     }
 
     fn range(&self) -> Option<TextRange> {