Fix inconsistencies in handling of inert attributes on statements

When the 'early' and 'late' visitors visit an attribute target, they
activate any lint attributes (e.g. `#[allow]`) that apply to it.
This can affect warnings emitted on sibiling attributes. For example,
the following code does not produce an `unused_attributes` for
`#[inline]`, since the sibiling `#[allow(unused_attributes)]` suppressed
the warning.

```rust
trait Foo {
    #[allow(unused_attributes)] #[inline] fn first();
    #[inline] #[allow(unused_attributes)] fn second();
}
```

However, we do not do this for statements - instead, the lint attributes
only become active when we visit the struct nested inside `StmtKind`
(e.g. `Item`).

Currently, this is difficult to observe due to another issue - the
`HasAttrs` impl for `StmtKind` ignores attributes for `StmtKind::Item`.
As a result, the `unused_doc_comments` lint will never see attributes on
item statements.

This commit makes two interrelated fixes to the handling of inert
(non-proc-macro) attributes on statements:

* The `HasAttr` impl for `StmtKind` now returns attributes for
  `StmtKind::Item`, treating it just like every other `StmtKind`
  variant. The only place relying on the old behavior was macro
  which has been updated to explicitly ignore attributes on item
  statements. This allows the `unused_doc_comments` lint to fire for
  item statements.
* The `early` and `late` lint visitors now activate lint attributes when
  invoking the callback for `Stmt`. This ensures that a lint
  attribute (e.g. `#[allow(unused_doc_comments)]`) can be applied to
  sibiling attributes on an item statement.

The `unused_doc_comments` lint now fires on a number of items in the
stnadard library. I've added `#[allow(unused_doc_comments)]` to these
items.

Author: Aaron1011

Diff for: compiler/rustc_ast/src/attr/mod.rs

@@ -623,7 +623,8 @@ impl HasAttrs for StmtKind {
         match *self {
             StmtKind::Local(ref local) => local.attrs(),
             StmtKind::Expr(ref expr) | StmtKind::Semi(ref expr) => expr.attrs(),
-            StmtKind::Empty | StmtKind::Item(..) => &[],
+            StmtKind::Item(ref item) => item.attrs(),
+            StmtKind::Empty => &[],
             StmtKind::MacCall(ref mac) => mac.attrs.attrs(),
         }
     }
@@ -632,7 +633,8 @@ impl HasAttrs for StmtKind {
         match self {
             StmtKind::Local(local) => local.visit_attrs(f),
             StmtKind::Expr(expr) | StmtKind::Semi(expr) => expr.visit_attrs(f),
-            StmtKind::Empty | StmtKind::Item(..) => {}
+            StmtKind::Item(item) => item.visit_attrs(f),
+            StmtKind::Empty => {}
             StmtKind::MacCall(mac) => {
                 mac.attrs.visit_attrs(f);
             }

Diff for: compiler/rustc_expand/src/expand.rs

@@ -1357,7 +1357,8 @@ impl<'a, 'b> MutVisitor for InvocationCollector<'a, 'b> {
         // we'll expand attributes on expressions separately
         if !stmt.is_expr() {
             let (attr, derives, after_derive) = if stmt.is_item() {
-                self.classify_item(&mut stmt)
+                // FIXME: Handle custom attributes on statements (#15701)
+                (None, vec![], false)
             } else {
                 // ignore derives on non-item statements so it falls through
                 // to the unused-attributes lint

Diff for: compiler/rustc_hir/src/hir.rs

@@ -1099,11 +1099,11 @@ pub enum StmtKind<'hir> {
     Semi(&'hir Expr<'hir>),
 }
 
-impl StmtKind<'hir> {
-    pub fn attrs(&self) -> &'hir [Attribute] {
+impl<'hir> StmtKind<'hir> {
+    pub fn attrs(&self, get_item: impl FnOnce(ItemId) -> &'hir Item<'hir>) -> &'hir [Attribute] {
         match *self {
             StmtKind::Local(ref l) => &l.attrs,
-            StmtKind::Item(_) => &[],
+            StmtKind::Item(ref item_id) => &get_item(*item_id).attrs,
             StmtKind::Expr(ref e) | StmtKind::Semi(ref e) => &e.attrs,
         }
     }

Diff for: compiler/rustc_lint/src/early.rs

@@ -18,6 +18,7 @@ use crate::context::{EarlyContext, LintContext, LintStore};
 use crate::passes::{EarlyLintPass, EarlyLintPassObject};
 use rustc_ast as ast;
 use rustc_ast::visit as ast_visit;
+use rustc_attr::HasAttrs;
 use rustc_session::lint::{BufferedEarlyLint, LintBuffer, LintPass};
 use rustc_session::Session;
 use rustc_span::symbol::Ident;
@@ -119,8 +120,22 @@ impl<'a, T: EarlyLintPass> ast_visit::Visitor<'a> for EarlyContextAndPass<'a, T>
     }
 
     fn visit_stmt(&mut self, s: &'a ast::Stmt) {
-        run_early_pass!(self, check_stmt, s);
-        self.check_id(s.id);
+        // Add the statement's lint attributes to our
+        // current state when checking the statement itself.
+        // This allows us to handle attributes like
+        // `#[allow(unused_doc_comments)]`, which apply to
+        // sibling attributes on the same target
+        //
+        // Note that statements get their attributes from
+        // the AST struct that they wrap (e.g. an item)
+        self.with_lint_attrs(s.id, s.attrs(), |cx| {
+            run_early_pass!(cx, check_stmt, s);
+            cx.check_id(s.id);
+        });
+        // The visitor for the AST struct wrapped
+        // by the statement (e.g. `Item`) will call
+        // `with_lint_attrs`, so do this walk
+        // outside of the above `with_lint_attrs` call
         ast_visit::walk_stmt(self, s);
     }
 

Diff for: compiler/rustc_lint/src/late.rs

@@ -174,12 +174,13 @@ impl<'tcx, T: LateLintPass<'tcx>> hir_visit::Visitor<'tcx> for LateContextAndPas
     }
 
     fn visit_stmt(&mut self, s: &'tcx hir::Stmt<'tcx>) {
-        // statement attributes are actually just attributes on one of
-        // - item
-        // - local
-        // - expression
-        // so we keep track of lint levels there
-        lint_callback!(self, check_stmt, s);
+        let get_item = |id: hir::ItemId| self.context.tcx.hir().item(id.id);
+        let attrs = &s.kind.attrs(get_item);
+        // See `EarlyContextAndPass::visit_stmt` for an explanation
+        // of why we call `walk_stmt` outside of `with_lint_attrs`
+        self.with_lint_attrs(s.hir_id, attrs, |cx| {
+            lint_callback!(cx, check_stmt, s);
+        });
         hir_visit::walk_stmt(self, s);
     }
 

Diff for: compiler/rustc_lint/src/levels.rs

@@ -562,6 +562,13 @@ impl<'tcx> intravisit::Visitor<'tcx> for LintLevelMapBuilder<'_, 'tcx> {
         })
     }
 
+    fn visit_stmt(&mut self, e: &'tcx hir::Stmt<'tcx>) {
+        // We will call `with_lint_attrs` when we walk
+        // the `StmtKind`. The outer statement itself doesn't
+        // define the lint levels.
+        intravisit::walk_stmt(self, e);
+    }
+
     fn visit_expr(&mut self, e: &'tcx hir::Expr<'tcx>) {
         self.with_lint_attrs(e.hir_id, &e.attrs, |builder| {
             intravisit::walk_expr(builder, e);

Diff for: compiler/rustc_middle/src/hir/map/mod.rs

@@ -816,7 +816,7 @@ impl<'hir> Map<'hir> {
             Some(Node::Variant(ref v)) => Some(&v.attrs[..]),
             Some(Node::Field(ref f)) => Some(&f.attrs[..]),
             Some(Node::Expr(ref e)) => Some(&*e.attrs),
-            Some(Node::Stmt(ref s)) => Some(s.kind.attrs()),
+            Some(Node::Stmt(ref s)) => Some(s.kind.attrs(|id| self.item(id.id))),
             Some(Node::Arm(ref a)) => Some(&*a.attrs),
             Some(Node::GenericParam(param)) => Some(&param.attrs[..]),
             // Unit/tuple structs/variants take the attributes straight from

Diff for: src/test/ui/allow-doc-comments.rs

@@ -0,0 +1,12 @@
+// check-pass
+
+fn main() {
+
+    #[allow(unused_doc_comments)]
+    /// A doc comment
+    fn bar() {}
+
+    /// Another doc comment
+    #[allow(unused_doc_comments)]
+    struct Foo {}
+}

Diff for: src/test/ui/lint/reasons-forbidden.rs

@@ -5,6 +5,9 @@
     //~^ NOTE `forbid` level set here
     //~| NOTE `forbid` level set here
     //~| NOTE `forbid` level set here
+    //~| NOTE `forbid` level set here
+    //~| NOTE `forbid` level set here
+    //~| NOTE `forbid` level set here
     reason = "our errors & omissions insurance policy doesn't cover unsafe Rust"
 )]
 
@@ -17,9 +20,18 @@ fn main() {
     //~^ ERROR allow(unsafe_code) overruled by outer forbid(unsafe_code)
     //~| ERROR allow(unsafe_code) overruled by outer forbid(unsafe_code)
     //~| ERROR allow(unsafe_code) overruled by outer forbid(unsafe_code)
+    //~| ERROR allow(unsafe_code) overruled by outer forbid(unsafe_code)
+    //~| ERROR allow(unsafe_code) overruled by outer forbid(unsafe_code)
+    //~| ERROR allow(unsafe_code) overruled by outer forbid(unsafe_code)
     //~| NOTE overruled by previous forbid
     //~| NOTE overruled by previous forbid
     //~| NOTE overruled by previous forbid
+    //~| NOTE overruled by previous forbid
+    //~| NOTE overruled by previous forbid
+    //~| NOTE overruled by previous forbid
+    //~| NOTE our errors & omissions insurance policy doesn't cover unsafe Rust
+    //~| NOTE our errors & omissions insurance policy doesn't cover unsafe Rust
+    //~| NOTE our errors & omissions insurance policy doesn't cover unsafe Rust
     //~| NOTE our errors & omissions insurance policy doesn't cover unsafe Rust
     //~| NOTE our errors & omissions insurance policy doesn't cover unsafe Rust
     //~| NOTE our errors & omissions insurance policy doesn't cover unsafe Rust

Diff for: src/test/ui/lint/reasons-forbidden.stderr

@@ -1,5 +1,5 @@
 error[E0453]: allow(unsafe_code) overruled by outer forbid(unsafe_code)
-  --> $DIR/reasons-forbidden.rs:16:13
+  --> $DIR/reasons-forbidden.rs:19:13
    |
 LL |     unsafe_code,
    |     ----------- `forbid` level set here
@@ -10,7 +10,7 @@ LL |     #[allow(unsafe_code)]
    = note: our errors & omissions insurance policy doesn't cover unsafe Rust
 
 error[E0453]: allow(unsafe_code) overruled by outer forbid(unsafe_code)
-  --> $DIR/reasons-forbidden.rs:16:13
+  --> $DIR/reasons-forbidden.rs:19:13
    |
 LL |     unsafe_code,
    |     ----------- `forbid` level set here
@@ -21,7 +21,7 @@ LL |     #[allow(unsafe_code)]
    = note: our errors & omissions insurance policy doesn't cover unsafe Rust
 
 error[E0453]: allow(unsafe_code) overruled by outer forbid(unsafe_code)
-  --> $DIR/reasons-forbidden.rs:16:13
+  --> $DIR/reasons-forbidden.rs:19:13
    |
 LL |     unsafe_code,
    |     ----------- `forbid` level set here
@@ -31,6 +31,39 @@ LL |     #[allow(unsafe_code)]
    |
    = note: our errors & omissions insurance policy doesn't cover unsafe Rust
 
-error: aborting due to 3 previous errors
+error[E0453]: allow(unsafe_code) overruled by outer forbid(unsafe_code)
+  --> $DIR/reasons-forbidden.rs:19:13
+   |
+LL |     unsafe_code,
+   |     ----------- `forbid` level set here
+...
+LL |     #[allow(unsafe_code)]
+   |             ^^^^^^^^^^^ overruled by previous forbid
+   |
+   = note: our errors & omissions insurance policy doesn't cover unsafe Rust
+
+error[E0453]: allow(unsafe_code) overruled by outer forbid(unsafe_code)
+  --> $DIR/reasons-forbidden.rs:19:13
+   |
+LL |     unsafe_code,
+   |     ----------- `forbid` level set here
+...
+LL |     #[allow(unsafe_code)]
+   |             ^^^^^^^^^^^ overruled by previous forbid
+   |
+   = note: our errors & omissions insurance policy doesn't cover unsafe Rust
+
+error[E0453]: allow(unsafe_code) overruled by outer forbid(unsafe_code)
+  --> $DIR/reasons-forbidden.rs:19:13
+   |
+LL |     unsafe_code,
+   |     ----------- `forbid` level set here
+...
+LL |     #[allow(unsafe_code)]
+   |             ^^^^^^^^^^^ overruled by previous forbid
+   |
+   = note: our errors & omissions insurance policy doesn't cover unsafe Rust
+
+error: aborting due to 6 previous errors
 
 For more information about this error, try `rustc --explain E0453`.

Diff for: src/test/ui/macros/macro-literal.rs

@@ -67,6 +67,7 @@ macro_rules! match_produced_attr {
     ($lit: literal) => {
         // Struct with doc comment passed via $literal
         #[doc = $lit]
+        #[allow(unused_doc_comments)]
         struct LiteralProduced;
     };
     ($expr: expr) => {

Diff for: src/test/ui/macros/macro-nested_expr.rs

@@ -6,6 +6,7 @@
 
 pub macro m($inner_str:expr) {
     #[doc = $inner_str]
+    #[allow(unused_doc_comments)]
     struct S;
 }
 

Diff for: src/test/ui/useless-comment.rs

@@ -38,6 +38,15 @@ fn foo() {
     {
 
     }
+
+    /// foo //~ ERROR unused doc comment
+    struct Foo {}
+
+    /// bar //~ ERROR unused doc comment
+    struct Bar {};
+
+    /// my_fn //~ ERROR unused_doc_comment
+    fn my_fn() {}
 }
 
 fn main() {

Diff for: src/test/ui/useless-comment.stderr

@@ -90,5 +90,29 @@ LL | |
 LL | |     }
    | |_____- rustdoc does not generate documentation for expressions
 
-error: aborting due to 10 previous errors
+error: unused doc comment
+  --> $DIR/useless-comment.rs:42:5
+   |
+LL |     /// foo
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |     struct Foo {}
+   |     ------------- rustdoc does not generate documentation for inner items
+
+error: unused doc comment
+  --> $DIR/useless-comment.rs:45:5
+   |
+LL |     /// bar
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |     struct Bar {};
+   |     -------------- rustdoc does not generate documentation for inner items
+
+error: unused doc comment
+  --> $DIR/useless-comment.rs:48:5
+   |
+LL |     /// my_fn
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |     fn my_fn() {}
+   |     ------------- rustdoc does not generate documentation for inner items
+
+error: aborting due to 13 previous errors
 

Diff for: src/tools/clippy/clippy_lints/src/utils/author.rs

@@ -130,7 +130,7 @@ impl<'tcx> LateLintPass<'tcx> for Author {
     }
 
     fn check_stmt(&mut self, cx: &LateContext<'tcx>, stmt: &'tcx hir::Stmt<'_>) {
-        if !has_attr(cx.sess(), stmt.kind.attrs()) {
+        if !has_attr(cx.sess(), stmt.kind.attrs(|id| cx.tcx.hir().item(id.id))) {
             return;
         }
         prelude();

Diff for: src/tools/clippy/clippy_lints/src/utils/inspector.rs

@@ -109,7 +109,7 @@ impl<'tcx> LateLintPass<'tcx> for DeepCodeInspector {
     }
 
     fn check_stmt(&mut self, cx: &LateContext<'tcx>, stmt: &'tcx hir::Stmt<'_>) {
-        if !has_attr(cx.sess(), stmt.kind.attrs()) {
+        if !has_attr(cx.sess(), stmt.kind.attrs(|id| cx.tcx.hir().item(id.id))) {
             return;
         }
         match stmt.kind {