Adding configuration to allow safety comment above stmt containing unsafe block

CHANGELOG.md

@@ -5383,4 +5383,5 @@ Released 2018-09-13
 [`allow-private-module-inception`]: https://doc.rust-lang.org/clippy/lint_configuration.html#allow-private-module-inception
 [`allowed-idents-below-min-chars`]: https://doc.rust-lang.org/clippy/lint_configuration.html#allowed-idents-below-min-chars
 [`min-ident-chars-threshold`]: https://doc.rust-lang.org/clippy/lint_configuration.html#min-ident-chars-threshold
+[`accept-comment-above-statement`]: https://doc.rust-lang.org/clippy/lint_configuration.html#accept-comment-above-statement
 <!-- end autogenerated links to configuration documentation -->

book/src/lint_configuration.md

@@ -695,3 +695,13 @@ Minimum chars an ident can have, anything below or equal to this will be linted.
 * [`min_ident_chars`](https://rust-lang.github.io/rust-clippy/master/index.html#min_ident_chars)
 
 
+## `accept-comment-above-statement`
+Whether to accept a safety comment to be placed above the statement containing the `unsafe` block
+
+**Default Value:** `false` (`bool`)
+
+---
+**Affected lints:**
+* [`undocumented_unsafe_blocks`](https://rust-lang.github.io/rust-clippy/master/index.html#undocumented_unsafe_blocks)
+
+

clippy_lints/src/lib.rs

@@ -929,7 +929,12 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
             enable_raw_pointer_heuristic_for_send,
         ))
     });
-    store.register_late_pass(move |_| Box::new(undocumented_unsafe_blocks::UndocumentedUnsafeBlocks));
+    let accept_comment_above_statement = conf.accept_comment_above_statement;
+    store.register_late_pass(move |_| {
+        Box::new(undocumented_unsafe_blocks::UndocumentedUnsafeBlocks::new(
+            accept_comment_above_statement,
+        ))
+    });
     let allow_mixed_uninlined = conf.allow_mixed_uninlined_format_args;
     store.register_late_pass(move |_| Box::new(format_args::FormatArgs::new(msrv(), allow_mixed_uninlined)));
     store.register_late_pass(|_| Box::new(trailing_empty_array::TrailingEmptyArray));

clippy_lints/src/undocumented_unsafe_blocks.rs

@@ -11,7 +11,7 @@ use rustc_hir::{Block, BlockCheckMode, ItemKind, Node, UnsafeSource};
 use rustc_lexer::{tokenize, TokenKind};
 use rustc_lint::{LateContext, LateLintPass, LintContext};
 use rustc_middle::lint::in_external_macro;
-use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_session::{declare_tool_lint, impl_lint_pass};
 use rustc_span::{BytePos, Pos, Span, SyntaxContext};
 
 declare_clippy_lint! {
@@ -92,7 +92,20 @@ declare_clippy_lint! {
     "annotating safe code with a safety comment"
 }
 
-declare_lint_pass!(UndocumentedUnsafeBlocks => [UNDOCUMENTED_UNSAFE_BLOCKS, UNNECESSARY_SAFETY_COMMENT]);
+#[derive(Copy, Clone)]
+pub struct UndocumentedUnsafeBlocks {
+    accept_comment_above_statement: bool,
+}
+
+impl UndocumentedUnsafeBlocks {
+    pub fn new(accept_comment_above_statement: bool) -> Self {
+        Self {
+            accept_comment_above_statement,
+        }
+    }
+}
+
+impl_lint_pass!(UndocumentedUnsafeBlocks => [UNDOCUMENTED_UNSAFE_BLOCKS, UNNECESSARY_SAFETY_COMMENT]);
 
 impl<'tcx> LateLintPass<'tcx> for UndocumentedUnsafeBlocks {
     fn check_block(&mut self, cx: &LateContext<'tcx>, block: &'tcx Block<'tcx>) {
@@ -101,7 +114,7 @@ impl<'tcx> LateLintPass<'tcx> for UndocumentedUnsafeBlocks {
             && !is_lint_allowed(cx, UNDOCUMENTED_UNSAFE_BLOCKS, block.hir_id)
             && !is_unsafe_from_proc_macro(cx, block.span)
             && !block_has_safety_comment(cx, block.span)
-            && !block_parents_have_safety_comment(cx, block.hir_id)
+            && !block_parents_have_safety_comment(self.accept_comment_above_statement, cx, block.hir_id)
         {
             let source_map = cx.tcx.sess.source_map();
             let span = if source_map.is_multiline(block.span) {
@@ -313,10 +326,31 @@ fn is_unsafe_from_proc_macro(cx: &LateContext<'_>, span: Span) -> bool {
 
 // Checks if any parent {expression, statement, block, local, const, static}
 // has a safety comment
-fn block_parents_have_safety_comment(cx: &LateContext<'_>, id: hir::HirId) -> bool {
+fn block_parents_have_safety_comment(
+    accept_comment_above_statement: bool,
+    cx: &LateContext<'_>,
+    id: hir::HirId,
+) -> bool {
     if let Some(node) = get_parent_node(cx.tcx, id) {
         return match node {
-            Node::Expr(expr) => !is_branchy(expr) && span_in_body_has_safety_comment(cx, expr.span),
+            Node::Expr(expr) => {
+                if let Some(
+                    Node::Local(hir::Local { span, .. })
+                    | Node::Item(hir::Item {
+                        kind: hir::ItemKind::Const(..) | ItemKind::Static(..),
+                        span,
+                        ..
+                    }),
+                ) = get_parent_node(cx.tcx, expr.hir_id)
+                {
+                    // if unsafe block is part of a let/const/static statement,
+                    // and accept_comment_above_statement is set to true
+                    // we accept the safety comment in the line the precedes this statement.
+                    accept_comment_above_statement && span_in_body_has_safety_comment(cx, *span)
+                } else {
+                    !is_branchy(expr) && span_in_body_has_safety_comment(cx, expr.span)
+                }
+            },
             Node::Stmt(hir::Stmt {
                 kind:
                     hir::StmtKind::Local(hir::Local { span, .. })
@@ -546,7 +580,14 @@ fn get_body_search_span(cx: &LateContext<'_>) -> Option<Span> {
     for (_, node) in map.parent_iter(body.hir_id) {
         match node {
             Node::Expr(e) => span = e.span,
-            Node::Block(_) | Node::Arm(_) | Node::Stmt(_) | Node::Local(_) => (),
+            Node::Block(_)
+            | Node::Arm(_)
+            | Node::Stmt(_)
+            | Node::Local(_)
+            | Node::Item(hir::Item {
+                kind: hir::ItemKind::Const(..) | ItemKind::Static(..),
+                ..
+            }) => (),
             _ => break,
         }
     }

clippy_lints/src/utils/conf.rs

@@ -538,6 +538,10 @@ define_Conf! {
     ///
     /// Minimum chars an ident can have, anything below or equal to this will be linted.
     (min_ident_chars_threshold: u64 = 1),
+    /// Lint: UNDOCUMENTED_UNSAFE_BLOCKS.
+    ///
+    /// Whether to accept a safety comment to be placed above the statement containing the `unsafe` block
+    (accept_comment_above_statement: bool = false),
 }
 
 /// Search for the configuration file.

tests/ui-toml/toml_unknown_key/conf_unknown_key.stderr

@@ -1,4 +1,5 @@
 error: error reading Clippy's configuration file: unknown field `foobar`, expected one of
+           accept-comment-above-statement
            allow-dbg-in-tests
            allow-expect-in-tests
            allow-mixed-uninlined-format-args
@@ -65,6 +66,7 @@ LL | foobar = 42
    | ^^^^^^
 
 error: error reading Clippy's configuration file: unknown field `barfoo`, expected one of
+           accept-comment-above-statement
            allow-dbg-in-tests
            allow-expect-in-tests
            allow-mixed-uninlined-format-args

tests/ui-toml/undocumented_unsafe_blocks/auxiliary/proc_macro_unsafe.rs

@@ -0,0 +1,18 @@
+//@compile-flags: --emit=link
+//@no-prefer-dynamic
+
+#![crate_type = "proc-macro"]
+
+extern crate proc_macro;
+
+use proc_macro::{Delimiter, Group, Ident, TokenStream, TokenTree};
+
+#[proc_macro]
+pub fn unsafe_block(input: TokenStream) -> TokenStream {
+    let span = input.into_iter().next().unwrap().span();
+    TokenStream::from_iter([TokenTree::Ident(Ident::new("unsafe", span)), {
+        let mut group = Group::new(Delimiter::Brace, TokenStream::new());
+        group.set_span(span);
+        TokenTree::Group(group)
+    }])
+}

tests/ui-toml/undocumented_unsafe_blocks/clippy.toml

@@ -0,0 +1 @@
+accept-comment-above-statement = true