ruff_db: add a new diagnostic renderer

We don't actually hook this up to anything in this PR, but we do
go to some trouble to granularly unit test it. The unit tests caught
plenty of bugs after I initially wrote down the implementation, so they
were very much worth it.

Closes #16506

crates/ruff_db/src/diagnostic/mod.rs

@@ -9,10 +9,14 @@ pub use crate::diagnostic::old::{
     OldDiagnosticTrait, OldDisplayDiagnostic, OldParseDiagnostic, OldSecondaryDiagnosticMessage,
 };
 use crate::files::File;
+use crate::Db;
+
+use self::render::{DisplayDiagnostic, FileResolver};
 
 // This module should not be exported. We are planning to migrate off
 // the APIs in this module.
 mod old;
+mod render;
 
 /// A collection of information that can be rendered into a diagnostic.
 ///
@@ -99,6 +103,45 @@ impl Diagnostic {
     pub fn sub(&mut self, sub: SubDiagnostic) {
         self.inner.subs.push(sub);
     }
+
+    /// Print this diagnostic to the given writer.
+    ///
+    /// This also marks the diagnostic as having been printed. If a
+    /// diagnostic is not rendered this way, then it will panic when
+    /// it's dropped when `debug_assertions` is enabled.
+    pub fn print(
+        &mut self,
+        db: &dyn Db,
+        config: &DisplayDiagnosticConfig,
+        mut wtr: impl std::io::Write,
+    ) -> std::io::Result<()> {
+        let resolver = FileResolver::new(db);
+        let display = DisplayDiagnostic::new(&resolver, config, self);
+        let result = writeln!(wtr, "{display}");
+        // NOTE: This is called specifically even if
+        // `writeln!` fails. The reasoning here is that
+        // the `Drop` impl panicking is meant as a guard
+        // against *programmers* forgetting to print a
+        // diagnostic. We should not punish them if they
+        // remember to do that when the operation itself
+        // failed for some reason. ---AG
+        self.printed();
+        result
+    }
+
+    /// Mark this diagnostic as having been printed.
+    ///
+    /// If a diagnostic has not been marked as printed before being dropped,
+    /// then its `Drop` implementation will panic in debug mode.
+    pub(crate) fn printed(&mut self) {
+        #[cfg(debug_assertions)]
+        {
+            self.inner.printed = true;
+            for sub in &mut self.inner.subs {
+                sub.printed();
+            }
+        }
+    }
 }
 
 #[derive(Debug, Clone, Eq, PartialEq)]
@@ -187,6 +230,17 @@ impl SubDiagnostic {
     pub fn annotate(&mut self, ann: Annotation) {
         self.inner.annotations.push(ann);
     }
+
+    /// Mark this sub-diagnostic as having been printed.
+    ///
+    /// If a sub-diagnostic has not been marked as printed before being
+    /// dropped, then its `Drop` implementation will panic in debug mode.
+    pub(crate) fn printed(&mut self) {
+        #[cfg(debug_assertions)]
+        {
+            self.inner.printed = true;
+        }
+    }
 }
 
 #[derive(Debug, Clone, Eq, PartialEq)]