Add diagnostics for unsafe_op_in_unsafe_fn

Turns out it's pretty easy, but I did have to add support for allowed-by-default lints.

crates/ide-diagnostics/src/handlers/missing_unsafe.rs

@@ -11,9 +11,14 @@ use crate::{fix, Diagnostic, DiagnosticCode, DiagnosticsContext};
 //
 // This diagnostic is triggered if an operation marked as `unsafe` is used outside of an `unsafe` function or block.
 pub(crate) fn missing_unsafe(ctx: &DiagnosticsContext<'_>, d: &hir::MissingUnsafe) -> Diagnostic {
+    let code = if d.only_lint {
+        DiagnosticCode::RustcLint("unsafe_op_in_unsafe_fn")
+    } else {
+        DiagnosticCode::RustcHardError("E0133")
+    };
     Diagnostic::new_with_syntax_node_ptr(
         ctx,
-        DiagnosticCode::RustcHardError("E0133"),
+        code,
         "this operation is unsafe and requires an unsafe function or block",
         d.expr.map(|it| it.into()),
     )
@@ -562,6 +567,30 @@ fn main() {
     ed2021::safe();
     ed2024::not_safe();
   //^^^^^^^^^^^^^^^^^^💡 error: this operation is unsafe and requires an unsafe function or block
+}
+            "#,
+        )
+    }
+
+    #[test]
+    fn unsafe_op_in_unsafe_fn_allowed_by_default() {
+        check_diagnostics(
+            r#"
+unsafe fn foo(p: *mut i32) {
+    *p = 123;
+}
+            "#,
+        )
+    }
+
+    #[test]
+    fn unsafe_op_in_unsafe_fn() {
+        check_diagnostics(
+            r#"
+#![warn(unsafe_op_in_unsafe_fn)]
+unsafe fn foo(p: *mut i32) {
+    *p = 123;
+  //^^💡 warn: this operation is unsafe and requires an unsafe function or block
 }
             "#,
         )

crates/ide-diagnostics/src/lib.rs

@@ -491,7 +491,7 @@ pub fn semantic_diagnostics(
 
     // The edition isn't accurate (each diagnostics may have its own edition due to macros),
     // but it's okay as it's only being used for error recovery.
-    handle_lint_attributes(
+    handle_lints(
         &ctx.sema,
         &mut FxHashMap::default(),
         &mut lints,
@@ -551,7 +551,12 @@ fn build_group_dict(
     map_with_prefixes.into_iter().map(|(k, v)| (k.strip_prefix(prefix).unwrap(), v)).collect()
 }
 
-fn handle_lint_attributes(
+/// Thd default severity for lints that are not warn by default.
+// FIXME: Autogenerate this instead of write manually.
+static LINTS_DEFAULT_SEVERITY: LazyLock<FxHashMap<&str, Severity>> =
+    LazyLock::new(|| FxHashMap::from_iter([("unsafe_op_in_unsafe_fn", Severity::Allow)]));
+
+fn handle_lints(
     sema: &Semantics<'_, RootDatabase>,
     cache: &mut FxHashMap<HirFileId, FxHashMap<SmolStr, SeverityAttr>>,
     diagnostics: &mut [(InFile<SyntaxNode>, &mut Diagnostic)],
@@ -559,6 +564,14 @@ fn handle_lint_attributes(
     edition: Edition,
 ) {
     for (node, diag) in diagnostics {
+        let lint = match diag.code {
+            DiagnosticCode::RustcLint(lint) | DiagnosticCode::Clippy(lint) => lint,
+            _ => panic!("non-lint passed to `handle_lints()`"),
+        };
+        if let Some(&default_severity) = LINTS_DEFAULT_SEVERITY.get(lint) {
+            diag.severity = default_severity;
+        }
+
         let mut diag_severity = fill_lint_attrs(sema, node, cache, cache_stack, diag, edition);
 
         if let outline_diag_severity @ Some(_) =