Hide empty snippets for full-file diagnostics (#19653)

Summary
--

This is the other commit I wanted to spin off from #19415, currently
stacked on #19644.

This PR suppresses blank snippets for empty ranges at the very beginning
of a file, and for empty ranges in non-existent files. Ruff includes
empty ranges for IO errors, for example.


f4e93b6335/crates/ruff_linter/src/message/text.rs (L100-L110)

The diagnostics now look like this (new snapshot test):

```
error[test-diagnostic]: main diagnostic message
--> example.py:1:1                             
```

Instead of [^*]

```
error[test-diagnostic]: main diagnostic message
--> example.py:1:1
 |
 |
```

Test Plan
--

A new `ruff_db` test showing the expected output format

[^*]: This doesn't correspond precisely to the example in the PR because
of some details of the diagnostic builder helper methods in `ruff_db`,
but you can see another example in the current version of the summary in
#19415.

crates/ruff_db/src/diagnostic/mod.rs

@@ -712,6 +712,11 @@ pub struct Annotation {
     is_primary: bool,
     /// The diagnostic tags associated with this annotation.
     tags: Vec<DiagnosticTag>,
+    /// Whether this annotation is a file-level or full-file annotation.
+    ///
+    /// When set, rendering will only include the file's name and (optional) range. Everything else
+    /// is omitted, including any file snippet or message.
+    is_file_level: bool,
 }
 
 impl Annotation {
@@ -730,6 +735,7 @@ impl Annotation {
             message: None,
             is_primary: true,
             tags: Vec::new(),
+            is_file_level: false,
         }
     }
 
@@ -746,6 +752,7 @@ impl Annotation {
             message: None,
             is_primary: false,
             tags: Vec::new(),
+            is_file_level: false,
         }
     }
 
@@ -811,6 +818,21 @@ impl Annotation {
     pub fn push_tag(&mut self, tag: DiagnosticTag) {
         self.tags.push(tag);
     }
+
+    /// Set whether or not this annotation is file-level.
+    ///
+    /// File-level annotations are only rendered with their file name and range, if available. This
+    /// is intended for backwards compatibility with Ruff diagnostics, which historically used
+    /// `TextRange::default` to indicate a file-level diagnostic. In the new diagnostic model, a
+    /// [`Span`] with a range of `None` should be used instead, as mentioned in the `Span`
+    /// documentation.
+    ///
+    /// TODO(brent) update this usage in Ruff and remove `is_file_level` entirely. See
+    /// <https://github.com/astral-sh/ruff/issues/19688>, especially my first comment, for more
+    /// details.
+    pub fn set_file_level(&mut self, yes: bool) {
+        self.is_file_level = yes;
+    }
 }
 
 /// Tags that can be associated with an annotation.

crates/ruff_db/src/diagnostic/render.rs

@@ -387,6 +387,7 @@ struct ResolvedAnnotation<'a> {
     line_end: OneIndexed,
     message: Option<&'a str>,
     is_primary: bool,
+    is_file_level: bool,
 }
 
 impl<'a> ResolvedAnnotation<'a> {
@@ -432,6 +433,7 @@ impl<'a> ResolvedAnnotation<'a> {
             line_end,
             message: ann.get_message(),
             is_primary: ann.is_primary,
+            is_file_level: ann.is_file_level,
         })
     }
 }
@@ -653,6 +655,8 @@ struct RenderableAnnotation<'r> {
     message: Option<&'r str>,
     /// Whether this annotation is considered "primary" or not.
     is_primary: bool,
+    /// Whether this annotation applies to an entire file, rather than a snippet within it.
+    is_file_level: bool,
 }
 
 impl<'r> RenderableAnnotation<'r> {
@@ -670,6 +674,7 @@ impl<'r> RenderableAnnotation<'r> {
             range,
             message: ann.message,
             is_primary: ann.is_primary,
+            is_file_level: ann.is_file_level,
         }
     }
 
@@ -695,7 +700,7 @@ impl<'r> RenderableAnnotation<'r> {
         if let Some(message) = self.message {
             ann = ann.label(message);
         }
-        ann
+        ann.is_file_level(self.is_file_level)
     }
 }
 
@@ -2551,7 +2556,12 @@ watermelon
         /// of the corresponding line minus one. (The "minus one" is because
         /// otherwise, the span will end where the next line begins, and this
         /// confuses `ruff_annotate_snippets` as of 2025-03-13.)
-        fn span(&self, path: &str, line_offset_start: &str, line_offset_end: &str) -> Span {
+        pub(super) fn span(
+            &self,
+            path: &str,
+            line_offset_start: &str,
+            line_offset_end: &str,
+        ) -> Span {
             let span = self.path(path);
 
             let file = span.expect_ty_file();
@@ -2574,7 +2584,7 @@ watermelon
         }
 
         /// Like `span`, but only attaches a file path.
-        fn path(&self, path: &str) -> Span {
+        pub(super) fn path(&self, path: &str) -> Span {
             let file = system_path_to_file(&self.db, path).unwrap();
             Span::from(file)
         }

crates/ruff_db/src/diagnostic/render/full.rs

@@ -1,9 +1,10 @@
 #[cfg(test)]
 mod tests {
     use ruff_diagnostics::Applicability;
+    use ruff_text_size::TextRange;
 
     use crate::diagnostic::{
-        DiagnosticFormat, Severity,
+        Annotation, DiagnosticFormat, Severity,
         render::tests::{TestEnvironment, create_diagnostics, create_syntax_error_diagnostics},
     };
 
@@ -264,4 +265,24 @@ print()
           |
         ");
     }
+
+    /// For file-level diagnostics, we expect to see the header line with the diagnostic information
+    /// and the `-->` line with the file information but no lines of source code.
+    #[test]
+    fn file_level() {
+        let mut env = TestEnvironment::new();
+        env.add("example.py", "");
+        env.format(DiagnosticFormat::Full);
+
+        let mut diagnostic = env.err().build();
+        let span = env.path("example.py").with_range(TextRange::default());
+        let mut annotation = Annotation::primary(span);
+        annotation.set_file_level(true);
+        diagnostic.annotate(annotation);
+
+        insta::assert_snapshot!(env.render(&diagnostic), @r"
+        error[test-diagnostic]: main diagnostic message
+        --> example.py:1:1
+        ");
+    }
 }