src/tools/clippy/clippy_lints/src/undocumented_unsafe_blocks.rs

   1 use std::ops::ControlFlow;
   2
   3 use clippy_utils::diagnostics::span_lint_and_help;
   4 use clippy_utils::source::walk_span_to_context;
   5 use clippy_utils::visitors::{for_each_expr_with_closures, Descend};
   6 use clippy_utils::{get_parent_node, is_lint_allowed};
   7 use hir::HirId;
   8 use rustc_data_structures::sync::Lrc;
   9 use rustc_hir as hir;
  10 use rustc_hir::{Block, BlockCheckMode, ItemKind, Node, UnsafeSource};
  11 use rustc_lexer::{tokenize, TokenKind};
  12 use rustc_lint::{LateContext, LateLintPass, LintContext};
  13 use rustc_middle::lint::in_external_macro;
  14 use rustc_session::{declare_lint_pass, declare_tool_lint};
  15 use rustc_span::{BytePos, Pos, Span, SyntaxContext};
  16
  17 declare_clippy_lint! {
  18     /// ### What it does
  19     /// Checks for `unsafe` blocks and impls without a `// SAFETY: ` comment
  20     /// explaining why the unsafe operations performed inside
  21     /// the block are safe.
  22     ///
  23     /// Note the comment must appear on the line(s) preceding the unsafe block
  24     /// with nothing appearing in between. The following is ok:
  25     /// ```ignore
  26     /// foo(
  27     ///     // SAFETY:
  28     ///     // This is a valid safety comment
  29     ///     unsafe { *x }
  30     /// )
  31     /// ```
  32     /// But neither of these are:
  33     /// ```ignore
  34     /// // SAFETY:
  35     /// // This is not a valid safety comment
  36     /// foo(
  37     ///     /* SAFETY: Neither is this */ unsafe { *x },
  38     /// );
  39     /// ```
  40     ///
  41     /// ### Why is this bad?
  42     /// Undocumented unsafe blocks and impls can make it difficult to
  43     /// read and maintain code, as well as uncover unsoundness
  44     /// and bugs.
  45     ///
  46     /// ### Example
  47     /// ```rust
  48     /// use std::ptr::NonNull;
  49     /// let a = &mut 42;
  50     ///
  51     /// let ptr = unsafe { NonNull::new_unchecked(a) };
  52     /// ```
  53     /// Use instead:
  54     /// ```rust
  55     /// use std::ptr::NonNull;
  56     /// let a = &mut 42;
  57     ///
  58     /// // SAFETY: references are guaranteed to be non-null.
  59     /// let ptr = unsafe { NonNull::new_unchecked(a) };
  60     /// ```
  61     #[clippy::version = "1.58.0"]
  62     pub UNDOCUMENTED_UNSAFE_BLOCKS,
  63     restriction,
  64     "creating an unsafe block without explaining why it is safe"
  65 }
  66 declare_clippy_lint! {
  67     /// ### What it does
  68     /// Checks for `// SAFETY: ` comments on safe code.
  69     ///
  70     /// ### Why is this bad?
  71     /// Safe code has no safety requirements, so there is no need to
  72     /// describe safety invariants.
  73     ///
  74     /// ### Example
  75     /// ```rust
  76     /// use std::ptr::NonNull;
  77     /// let a = &mut 42;
  78     ///
  79     /// // SAFETY: references are guaranteed to be non-null.
  80     /// let ptr = NonNull::new(a).unwrap();
  81     /// ```
  82     /// Use instead:
  83     /// ```rust
  84     /// use std::ptr::NonNull;
  85     /// let a = &mut 42;
  86     ///
  87     /// let ptr = NonNull::new(a).unwrap();
  88     /// ```
  89     #[clippy::version = "1.67.0"]
  90     pub UNNECESSARY_SAFETY_COMMENT,
  91     restriction,
  92     "annotating safe code with a safety comment"
  93 }
  94
  95 declare_lint_pass!(UndocumentedUnsafeBlocks => [UNDOCUMENTED_UNSAFE_BLOCKS, UNNECESSARY_SAFETY_COMMENT]);
  96
  97 impl<'tcx> LateLintPass<'tcx> for UndocumentedUnsafeBlocks {
  98     fn check_block(&mut self, cx: &LateContext<'tcx>, block: &'tcx Block<'tcx>) {
  99         if block.rules == BlockCheckMode::UnsafeBlock(UnsafeSource::UserProvided)
 100             && !in_external_macro(cx.tcx.sess, block.span)
 101             && !is_lint_allowed(cx, UNDOCUMENTED_UNSAFE_BLOCKS, block.hir_id)
 102             && !is_unsafe_from_proc_macro(cx, block.span)
 103             && !block_has_safety_comment(cx, block.span)
 104             && !block_parents_have_safety_comment(cx, block.hir_id)
 105         {
 106             let source_map = cx.tcx.sess.source_map();
 107             let span = if source_map.is_multiline(block.span) {
 108                 source_map.span_until_char(block.span, '\n')
 109             } else {
 110                 block.span
 111             };
 112
 113             span_lint_and_help(
 114                 cx,
 115                 UNDOCUMENTED_UNSAFE_BLOCKS,
 116                 span,
 117                 "unsafe block missing a safety comment",
 118                 None,
 119                 "consider adding a safety comment on the preceding line",
 120             );
 121         }
 122
 123         if let Some(tail) = block.expr
 124             && !is_lint_allowed(cx, UNNECESSARY_SAFETY_COMMENT, tail.hir_id)
 125             && !in_external_macro(cx.tcx.sess, tail.span)
 126             && let HasSafetyComment::Yes(pos) = stmt_has_safety_comment(cx, tail.span, tail.hir_id)
 127             && let Some(help_span) = expr_has_unnecessary_safety_comment(cx, tail, pos)
 128         {
 129             span_lint_and_help(
 130                 cx,
 131                 UNNECESSARY_SAFETY_COMMENT,
 132                 tail.span,
 133                 "expression has unnecessary safety comment",
 134                 Some(help_span),
 135                 "consider removing the safety comment",
 136             );
 137         }
 138     }
 139
 140     fn check_stmt(&mut self, cx: &LateContext<'tcx>, stmt: &hir::Stmt<'tcx>) {
 141         let (
 142             hir::StmtKind::Local(&hir::Local { init: Some(expr), .. })
 143             | hir::StmtKind::Expr(expr)
 144             | hir::StmtKind::Semi(expr)
 145         ) = stmt.kind else { return };
 146         if !is_lint_allowed(cx, UNNECESSARY_SAFETY_COMMENT, stmt.hir_id)
 147             && !in_external_macro(cx.tcx.sess, stmt.span)
 148             && let HasSafetyComment::Yes(pos) = stmt_has_safety_comment(cx, stmt.span, stmt.hir_id)
 149             && let Some(help_span) = expr_has_unnecessary_safety_comment(cx, expr, pos)
 150         {
 151             span_lint_and_help(
 152                 cx,
 153                 UNNECESSARY_SAFETY_COMMENT,
 154                 stmt.span,
 155                 "statement has unnecessary safety comment",
 156                 Some(help_span),
 157                 "consider removing the safety comment",
 158             );
 159         }
 160     }
 161
 162     fn check_item(&mut self, cx: &LateContext<'_>, item: &hir::Item<'_>) {
 163         if in_external_macro(cx.tcx.sess, item.span) {
 164             return;
 165         }
 166
 167         let mk_spans = |pos: BytePos| {
 168             let source_map = cx.tcx.sess.source_map();
 169             let span = Span::new(pos, pos, SyntaxContext::root(), None);
 170             let help_span = source_map.span_extend_to_next_char(span, '\n', true);
 171             let span = if source_map.is_multiline(item.span) {
 172                 source_map.span_until_char(item.span, '\n')
 173             } else {
 174                 item.span
 175             };
 176             (span, help_span)
 177         };
 178
 179         let item_has_safety_comment = item_has_safety_comment(cx, item);
 180         match (&item.kind, item_has_safety_comment) {
 181             // lint unsafe impl without safety comment
 182             (hir::ItemKind::Impl(impl_), HasSafetyComment::No) if impl_.unsafety == hir::Unsafety::Unsafe => {
 183                 if !is_lint_allowed(cx, UNDOCUMENTED_UNSAFE_BLOCKS, item.hir_id())
 184                     && !is_unsafe_from_proc_macro(cx, item.span)
 185                 {
 186                     let source_map = cx.tcx.sess.source_map();
 187                     let span = if source_map.is_multiline(item.span) {
 188                         source_map.span_until_char(item.span, '\n')
 189                     } else {
 190                         item.span
 191                     };
 192
 193                     span_lint_and_help(
 194                         cx,
 195                         UNDOCUMENTED_UNSAFE_BLOCKS,
 196                         span,
 197                         "unsafe impl missing a safety comment",
 198                         None,
 199                         "consider adding a safety comment on the preceding line",
 200                     );
 201                 }
 202             },
 203             // lint safe impl with unnecessary safety comment
 204             (hir::ItemKind::Impl(impl_), HasSafetyComment::Yes(pos)) if impl_.unsafety == hir::Unsafety::Normal => {
 205                 if !is_lint_allowed(cx, UNNECESSARY_SAFETY_COMMENT, item.hir_id()) {
 206                     let (span, help_span) = mk_spans(pos);
 207
 208                     span_lint_and_help(
 209                         cx,
 210                         UNNECESSARY_SAFETY_COMMENT,
 211                         span,
 212                         "impl has unnecessary safety comment",
 213                         Some(help_span),
 214                         "consider removing the safety comment",
 215                     );
 216                 }
 217             },
 218             (hir::ItemKind::Impl(_), _) => {},
 219             // const and static items only need a safety comment if their body is an unsafe block, lint otherwise
 220             (&hir::ItemKind::Const(.., body) | &hir::ItemKind::Static(.., body), HasSafetyComment::Yes(pos)) => {
 221                 if !is_lint_allowed(cx, UNNECESSARY_SAFETY_COMMENT, body.hir_id) {
 222                     let body = cx.tcx.hir().body(body);
 223                     if !matches!(
 224                         body.value.kind, hir::ExprKind::Block(block, _)
 225                         if block.rules == BlockCheckMode::UnsafeBlock(UnsafeSource::UserProvided)
 226                     ) {
 227                         let (span, help_span) = mk_spans(pos);
 228
 229                         span_lint_and_help(
 230                             cx,
 231                             UNNECESSARY_SAFETY_COMMENT,
 232                             span,
 233                             &format!("{} has unnecessary safety comment", item.kind.descr()),
 234                             Some(help_span),
 235                             "consider removing the safety comment",
 236                         );
 237                     }
 238                 }
 239             },
 240             // Aside from unsafe impls and consts/statics with an unsafe block, items in general
 241             // do not have safety invariants that need to be documented, so lint those.
 242             (_, HasSafetyComment::Yes(pos)) => {
 243                 if !is_lint_allowed(cx, UNNECESSARY_SAFETY_COMMENT, item.hir_id()) {
 244                     let (span, help_span) = mk_spans(pos);
 245
 246                     span_lint_and_help(
 247                         cx,
 248                         UNNECESSARY_SAFETY_COMMENT,
 249                         span,
 250                         &format!("{} has unnecessary safety comment", item.kind.descr()),
 251                         Some(help_span),
 252                         "consider removing the safety comment",
 253                     );
 254                 }
 255             },
 256             _ => (),
 257         }
 258     }
 259 }
 260
 261 fn expr_has_unnecessary_safety_comment<'tcx>(
 262     cx: &LateContext<'tcx>,
 263     expr: &'tcx hir::Expr<'tcx>,
 264     comment_pos: BytePos,
 265 ) -> Option<Span> {
 266     // this should roughly be the reverse of `block_parents_have_safety_comment`
 267     if for_each_expr_with_closures(cx, expr, |expr| match expr.kind {
 268         hir::ExprKind::Block(
 269             Block {
 270                 rules: BlockCheckMode::UnsafeBlock(UnsafeSource::UserProvided),
 271                 ..
 272             },
 273             _,
 274         ) => ControlFlow::Break(()),
 275         // statements will be handled by check_stmt itself again
 276         hir::ExprKind::Block(..) => ControlFlow::Continue(Descend::No),
 277         _ => ControlFlow::Continue(Descend::Yes),
 278     })
 279     .is_some()
 280     {
 281         return None;
 282     }
 283
 284     let source_map = cx.tcx.sess.source_map();
 285     let span = Span::new(comment_pos, comment_pos, SyntaxContext::root(), None);
 286     let help_span = source_map.span_extend_to_next_char(span, '\n', true);
 287
 288     Some(help_span)
 289 }
 290
 291 fn is_unsafe_from_proc_macro(cx: &LateContext<'_>, span: Span) -> bool {
 292     let source_map = cx.sess().source_map();
 293     let file_pos = source_map.lookup_byte_offset(span.lo());
 294     file_pos
 295         .sf
 296         .src
 297         .as_deref()
 298         .and_then(|src| src.get(file_pos.pos.to_usize()..))
 299         .map_or(true, |src| !src.starts_with("unsafe"))
 300 }
 301
 302 // Checks if any parent {expression, statement, block, local, const, static}
 303 // has a safety comment
 304 fn block_parents_have_safety_comment(cx: &LateContext<'_>, id: hir::HirId) -> bool {
 305     if let Some(node) = get_parent_node(cx.tcx, id) {
 306         return match node {
 307             Node::Expr(expr) => !is_branchy(expr) && span_in_body_has_safety_comment(cx, expr.span),
 308             Node::Stmt(hir::Stmt {
 309                 kind:
 310                     hir::StmtKind::Local(hir::Local { span, .. })
 311                     | hir::StmtKind::Expr(hir::Expr { span, .. })
 312                     | hir::StmtKind::Semi(hir::Expr { span, .. }),
 313                 ..
 314             })
 315             | Node::Local(hir::Local { span, .. })
 316             | Node::Item(hir::Item {
 317                 kind: hir::ItemKind::Const(..) | ItemKind::Static(..),
 318                 span,
 319                 ..
 320             }) => span_in_body_has_safety_comment(cx, *span),
 321             _ => false,
 322         };
 323     }
 324     false
 325 }
 326
 327 /// Checks if an expression is "branchy", e.g. loop, match/if/etc.
 328 fn is_branchy(expr: &hir::Expr<'_>) -> bool {
 329     matches!(
 330         expr.kind,
 331         hir::ExprKind::If(..) | hir::ExprKind::Loop(..) | hir::ExprKind::Match(..)
 332     )
 333 }
 334
 335 /// Checks if the lines immediately preceding the block contain a safety comment.
 336 fn block_has_safety_comment(cx: &LateContext<'_>, span: Span) -> bool {
 337     // This intentionally ignores text before the start of a function so something like:
 338     // ```
 339     //     // SAFETY: reason
 340     //     fn foo() { unsafe { .. } }
 341     // ```
 342     // won't work. This is to avoid dealing with where such a comment should be place relative to
 343     // attributes and doc comments.
 344
 345     matches!(
 346         span_from_macro_expansion_has_safety_comment(cx, span),
 347         HasSafetyComment::Yes(_)
 348     ) || span_in_body_has_safety_comment(cx, span)
 349 }
 350
 351 enum HasSafetyComment {
 352     Yes(BytePos),
 353     No,
 354     Maybe,
 355 }
 356
 357 /// Checks if the lines immediately preceding the item contain a safety comment.
 358 #[allow(clippy::collapsible_match)]
 359 fn item_has_safety_comment(cx: &LateContext<'_>, item: &hir::Item<'_>) -> HasSafetyComment {
 360     match span_from_macro_expansion_has_safety_comment(cx, item.span) {
 361         HasSafetyComment::Maybe => (),
 362         has_safety_comment => return has_safety_comment,
 363     }
 364
 365     if item.span.ctxt() != SyntaxContext::root() {
 366         return HasSafetyComment::No;
 367     }
 368     if let Some(parent_node) = get_parent_node(cx.tcx, item.hir_id()) {
 369         let comment_start = match parent_node {
 370             Node::Crate(parent_mod) => {
 371                 comment_start_before_item_in_mod(cx, parent_mod, parent_mod.spans.inner_span, item)
 372             },
 373             Node::Item(parent_item) => {
 374                 if let ItemKind::Mod(parent_mod) = &parent_item.kind {
 375                     comment_start_before_item_in_mod(cx, parent_mod, parent_item.span, item)
 376                 } else {
 377                     // Doesn't support impls in this position. Pretend a comment was found.
 378                     return HasSafetyComment::Maybe;
 379                 }
 380             },
 381             Node::Stmt(stmt) => {
 382                 if let Some(Node::Block(block)) = get_parent_node(cx.tcx, stmt.hir_id) {
 383                     walk_span_to_context(block.span, SyntaxContext::root()).map(Span::lo)
 384                 } else {
 385                     // Problem getting the parent node. Pretend a comment was found.
 386                     return HasSafetyComment::Maybe;
 387                 }
 388             },
 389             _ => {
 390                 // Doesn't support impls in this position. Pretend a comment was found.
 391                 return HasSafetyComment::Maybe;
 392             },
 393         };
 394
 395         let source_map = cx.sess().source_map();
 396         if let Some(comment_start) = comment_start
 397             && let Ok(unsafe_line) = source_map.lookup_line(item.span.lo())
 398             && let Ok(comment_start_line) = source_map.lookup_line(comment_start)
 399             && Lrc::ptr_eq(&unsafe_line.sf, &comment_start_line.sf)
 400             && let Some(src) = unsafe_line.sf.src.as_deref()
 401         {
 402             return unsafe_line.sf.lines(|lines| {
 403                 if comment_start_line.line >= unsafe_line.line {
 404                     HasSafetyComment::No
 405                 } else {
 406                     match text_has_safety_comment(
 407                         src,
 408                         &lines[comment_start_line.line + 1..=unsafe_line.line],
 409                         unsafe_line.sf.start_pos.to_usize(),
 410                     ) {
 411                         Some(b) => HasSafetyComment::Yes(b),
 412                         None => HasSafetyComment::No,
 413                     }
 414                 }
 415             });
 416         }
 417     }
 418     HasSafetyComment::Maybe
 419 }
 420
 421 /// Checks if the lines immediately preceding the item contain a safety comment.
 422 #[allow(clippy::collapsible_match)]
 423 fn stmt_has_safety_comment(cx: &LateContext<'_>, span: Span, hir_id: HirId) -> HasSafetyComment {
 424     match span_from_macro_expansion_has_safety_comment(cx, span) {
 425         HasSafetyComment::Maybe => (),
 426         has_safety_comment => return has_safety_comment,
 427     }
 428
 429     if span.ctxt() != SyntaxContext::root() {
 430         return HasSafetyComment::No;
 431     }
 432
 433     if let Some(parent_node) = get_parent_node(cx.tcx, hir_id) {
 434         let comment_start = match parent_node {
 435             Node::Block(block) => walk_span_to_context(block.span, SyntaxContext::root()).map(Span::lo),
 436             _ => return HasSafetyComment::Maybe,
 437         };
 438
 439         let source_map = cx.sess().source_map();
 440         if let Some(comment_start) = comment_start
 441             && let Ok(unsafe_line) = source_map.lookup_line(span.lo())
 442             && let Ok(comment_start_line) = source_map.lookup_line(comment_start)
 443             && Lrc::ptr_eq(&unsafe_line.sf, &comment_start_line.sf)
 444             && let Some(src) = unsafe_line.sf.src.as_deref()
 445         {
 446             return unsafe_line.sf.lines(|lines| {
 447                 if comment_start_line.line >= unsafe_line.line {
 448                     HasSafetyComment::No
 449                 } else {
 450                     match text_has_safety_comment(
 451                         src,
 452                         &lines[comment_start_line.line + 1..=unsafe_line.line],
 453                         unsafe_line.sf.start_pos.to_usize(),
 454                     ) {
 455                         Some(b) => HasSafetyComment::Yes(b),
 456                         None => HasSafetyComment::No,
 457                     }
 458                 }
 459             });
 460         }
 461     }
 462     HasSafetyComment::Maybe
 463 }
 464
 465 fn comment_start_before_item_in_mod(
 466     cx: &LateContext<'_>,
 467     parent_mod: &hir::Mod<'_>,
 468     parent_mod_span: Span,
 469     item: &hir::Item<'_>,
 470 ) -> Option<BytePos> {
 471     parent_mod.item_ids.iter().enumerate().find_map(|(idx, item_id)| {
 472         if *item_id == item.item_id() {
 473             if idx == 0 {
 474                 // mod A { /* comment */ unsafe impl T {} ... }
 475                 // ^------------------------------------------^ returns the start of this span
 476                 // ^---------------------^ finally checks comments in this range
 477                 if let Some(sp) = walk_span_to_context(parent_mod_span, SyntaxContext::root()) {
 478                     return Some(sp.lo());
 479                 }
 480             } else {
 481                 // some_item /* comment */ unsafe impl T {}
 482                 // ^-------^ returns the end of this span
 483                 //         ^---------------^ finally checks comments in this range
 484                 let prev_item = cx.tcx.hir().item(parent_mod.item_ids[idx - 1]);
 485                 if let Some(sp) = walk_span_to_context(prev_item.span, SyntaxContext::root()) {
 486                     return Some(sp.hi());
 487                 }
 488             }
 489         }
 490         None
 491     })
 492 }
 493
 494 fn span_from_macro_expansion_has_safety_comment(cx: &LateContext<'_>, span: Span) -> HasSafetyComment {
 495     let source_map = cx.sess().source_map();
 496     let ctxt = span.ctxt();
 497     if ctxt == SyntaxContext::root() {
 498         HasSafetyComment::Maybe
 499     } else {
 500         // From a macro expansion. Get the text from the start of the macro declaration to start of the
 501         // unsafe block.
 502         //     macro_rules! foo { () => { stuff }; (x) => { unsafe { stuff } }; }
 503         //     ^--------------------------------------------^
 504         if let Ok(unsafe_line) = source_map.lookup_line(span.lo())
 505             && let Ok(macro_line) = source_map.lookup_line(ctxt.outer_expn_data().def_site.lo())
 506             && Lrc::ptr_eq(&unsafe_line.sf, &macro_line.sf)
 507             && let Some(src) = unsafe_line.sf.src.as_deref()
 508         {
 509             unsafe_line.sf.lines(|lines| {
 510                 if macro_line.line < unsafe_line.line {
 511                     match text_has_safety_comment(
 512                         src,
 513                         &lines[macro_line.line + 1..=unsafe_line.line],
 514                         unsafe_line.sf.start_pos.to_usize(),
 515                     ) {
 516                         Some(b) => HasSafetyComment::Yes(b),
 517                         None => HasSafetyComment::No,
 518                     }
 519                 } else {
 520                     HasSafetyComment::No
 521                 }
 522             })
 523         } else {
 524             // Problem getting source text. Pretend a comment was found.
 525             HasSafetyComment::Maybe
 526         }
 527     }
 528 }
 529
 530 fn get_body_search_span(cx: &LateContext<'_>) -> Option<Span> {
 531     let body = cx.enclosing_body?;
 532     let map = cx.tcx.hir();
 533     let mut span = map.body(body).value.span;
 534     for (_, node) in map.parent_iter(body.hir_id) {
 535         match node {
 536             Node::Expr(e) => span = e.span,
 537             Node::Block(_) | Node::Arm(_) | Node::Stmt(_) | Node::Local(_) => (),
 538             _ => break,
 539         }
 540     }
 541     Some(span)
 542 }
 543
 544 fn span_in_body_has_safety_comment(cx: &LateContext<'_>, span: Span) -> bool {
 545     let source_map = cx.sess().source_map();
 546     let ctxt = span.ctxt();
 547     if ctxt == SyntaxContext::root()
 548         && let Some(search_span) = get_body_search_span(cx)
 549     {
 550         if let Ok(unsafe_line) = source_map.lookup_line(span.lo())
 551             && let Some(body_span) = walk_span_to_context(search_span, SyntaxContext::root())
 552             && let Ok(body_line) = source_map.lookup_line(body_span.lo())
 553             && Lrc::ptr_eq(&unsafe_line.sf, &body_line.sf)
 554             && let Some(src) = unsafe_line.sf.src.as_deref()
 555         {
 556             // Get the text from the start of function body to the unsafe block.
 557             //     fn foo() { some_stuff; unsafe { stuff }; other_stuff; }
 558             //              ^-------------^
 559             unsafe_line.sf.lines(|lines| {
 560                 body_line.line < unsafe_line.line && text_has_safety_comment(
 561                     src,
 562                     &lines[body_line.line + 1..=unsafe_line.line],
 563                     unsafe_line.sf.start_pos.to_usize(),
 564                 ).is_some()
 565             })
 566         } else {
 567             // Problem getting source text. Pretend a comment was found.
 568             true
 569         }
 570     } else {
 571         false
 572     }
 573 }
 574
 575 /// Checks if the given text has a safety comment for the immediately proceeding line.
 576 fn text_has_safety_comment(src: &str, line_starts: &[BytePos], offset: usize) -> Option<BytePos> {
 577     let mut lines = line_starts
 578         .array_windows::<2>()
 579         .rev()
 580         .map_while(|[start, end]| {
 581             let start = start.to_usize() - offset;
 582             let end = end.to_usize() - offset;
 583             let text = src.get(start..end)?;
 584             let trimmed = text.trim_start();
 585             Some((start + (text.len() - trimmed.len()), trimmed))
 586         })
 587         .filter(|(_, text)| !text.is_empty());
 588
 589     let Some((line_start, line)) = lines.next() else {
 590         return None;
 591     };
 592     // Check for a sequence of line comments.
 593     if line.starts_with("//") {
 594         let (mut line, mut line_start) = (line, line_start);
 595         loop {
 596             if line.to_ascii_uppercase().contains("SAFETY:") {
 597                 return Some(BytePos(
 598                     u32::try_from(line_start).unwrap() + u32::try_from(offset).unwrap(),
 599                 ));
 600             }
 601             match lines.next() {
 602                 Some((s, x)) if x.starts_with("//") => (line, line_start) = (x, s),
 603                 _ => return None,
 604             }
 605         }
 606     }
 607     // No line comments; look for the start of a block comment.
 608     // This will only find them if they are at the start of a line.
 609     let (mut line_start, mut line) = (line_start, line);
 610     loop {
 611         if line.starts_with("/*") {
 612             let src = &src[line_start..line_starts.last().unwrap().to_usize() - offset];
 613             let mut tokens = tokenize(src);
 614             return (src[..tokens.next().unwrap().len as usize]
 615                 .to_ascii_uppercase()
 616                 .contains("SAFETY:")
 617                 && tokens.all(|t| t.kind == TokenKind::Whitespace))
 618             .then_some(BytePos(
 619                 u32::try_from(line_start).unwrap() + u32::try_from(offset).unwrap(),
 620             ));
 621         }
 622         match lines.next() {
 623             Some(x) => (line_start, line) = x,
 624             None => return None,
 625         }
 626     }
 627 }