check if expectation can be fulfilled in check_unsafe_derive_deserialize

This commit adds fulfilling expectations to `check_unsafe_derive_deserialize`.

The utility function `clippy_utils::fulfill_or_allowed` is not used because
using it would require to move the check for allowed after the check
iterating over all inherent impls of the type, doing possibly
unnecessary work.
Instead, `is_lint_allowed` is called as before, but additionally, once
certain that the lint should be emitted, expectations are checked and
fulfilled if found. In that case actually emitting the lint is skipped.

fixes: rust-lang#12802

changelog: fulfill expectations in `check_unsafe_derive_deserialize`

Author: B14CK313

clippy_lints/src/derive.rs

@@ -7,7 +7,7 @@ use rustc_hir::intravisit::{walk_expr, walk_fn, walk_item, FnKind, Visitor};
 use rustc_hir::{
     self as hir, BlockCheckMode, BodyId, Expr, ExprKind, FnDecl, Impl, Item, ItemKind, UnsafeSource, Unsafety,
 };
-use rustc_lint::{LateContext, LateLintPass};
+use rustc_lint::{LateContext, LateLintPass, LintContext};
 use rustc_middle::hir::nested_filter;
 use rustc_middle::traits::Reveal;
 use rustc_middle::ty::{
@@ -390,6 +390,12 @@ fn check_unsafe_derive_deserialize<'tcx>(
             .map(|imp_did| cx.tcx.hir().expect_item(imp_did.expect_local()))
             .any(|imp| has_unsafe(cx, imp))
     {
+        let (level, _) = cx.tcx.lint_level_at_node(UNSAFE_DERIVE_DESERIALIZE, adt_hir_id);
+        if let Some(expectation) = level.get_expectation_id() {
+            cx.fulfill_expectation(expectation);
+            return;
+        }
+
         span_lint_and_help(
             cx,
             UNSAFE_DERIVE_DESERIALIZE,