fix: unnecessary_safety_comment FP on desugared assign (rust-lang#14371)

Closes rust-lang#13039

changelog: [`unnecessary_safety_comment`]: fix FP on desugared assign

Author: Alexendoo

clippy_lints/src/undocumented_unsafe_blocks.rs

@@ -312,6 +312,25 @@ fn expr_has_unnecessary_safety_comment<'tcx>(
             },
             _,
         ) => ControlFlow::Break(()),
+        // `_ = foo()` is desugared to `{ let _ = foo(); }`
+        hir::ExprKind::Block(
+            Block {
+                rules: BlockCheckMode::DefaultBlock,
+                stmts:
+                    [
+                        hir::Stmt {
+                            kind:
+                                hir::StmtKind::Let(hir::LetStmt {
+                                    source: hir::LocalSource::AssignDesugar(_),
+                                    ..
+                                }),
+                            ..
+                        },
+                    ],
+                ..
+            },
+            _,
+        ) => ControlFlow::Continue(Descend::Yes),
         // statements will be handled by check_stmt itself again
         hir::ExprKind::Block(..) => ControlFlow::Continue(Descend::No),
         _ => ControlFlow::Continue(Descend::Yes),

tests/ui-toml/undocumented_unsafe_blocks/undocumented_unsafe_blocks.default.stderr

@@ -342,5 +342,17 @@ LL |         const NO_SAFETY_IN_IMPL: i32 = unsafe { 1 };
    |
    = help: consider adding a safety comment on the preceding line
 
-error: aborting due to 39 previous errors
+error: statement has unnecessary safety comment
+  --> tests/ui-toml/undocumented_unsafe_blocks/undocumented_unsafe_blocks.rs:719:5
+   |
+LL |     _ = bar();
+   |     ^^^^^^^^^^
+   |
+help: consider removing the safety comment
+  --> tests/ui-toml/undocumented_unsafe_blocks/undocumented_unsafe_blocks.rs:718:5
+   |
+LL |     // SAFETY: unnecessary_safety_comment triggers here
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 40 previous errors
 

tests/ui-toml/undocumented_unsafe_blocks/undocumented_unsafe_blocks.disabled.stderr

@@ -438,5 +438,17 @@ LL |         unsafe { Date::__from_ordinal_date_unchecked(1970, 1) }.into_julian
    |
    = help: consider adding a safety comment on the preceding line
 
-error: aborting due to 51 previous errors
+error: statement has unnecessary safety comment
+  --> tests/ui-toml/undocumented_unsafe_blocks/undocumented_unsafe_blocks.rs:719:5
+   |
+LL |     _ = bar();
+   |     ^^^^^^^^^^
+   |
+help: consider removing the safety comment
+  --> tests/ui-toml/undocumented_unsafe_blocks/undocumented_unsafe_blocks.rs:718:5
+   |
+LL |     // SAFETY: unnecessary_safety_comment triggers here
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 52 previous errors
 

tests/ui-toml/undocumented_unsafe_blocks/undocumented_unsafe_blocks.rs

@@ -703,4 +703,24 @@ mod issue_11709_regression {
     //~[disabled]^ undocumented_unsafe_blocks
 }
 
+fn issue_13039() {
+    unsafe fn foo() -> usize {
+        1234
+    }
+
+    fn bar() -> usize {
+        1234
+    }
+
+    // SAFETY: unnecessary_safety_comment should not trigger here
+    _ = unsafe { foo() };
+
+    // SAFETY: unnecessary_safety_comment triggers here
+    _ = bar();
+    //~^ unnecessary_safety_comment
+
+    // SAFETY: unnecessary_safety_comment should not trigger here
+    _ = unsafe { foo() }
+}
+
 fn main() {}