Auto merge of #69484 - Dylan-DPC:rollup-j6ripxy, r=Dylan-DPC

Rollup of 5 pull requests

Successful merges:

 - #68712 (Add methods to 'leak' RefCell borrows as references with the lifetime of the original reference)
 - #69209 (Miscellaneous cleanup to formatting)
 - #69381 (Allow getting `no_std` from the config file)
 - #69434 (rustc_metadata: Use binary search from standard library)
 - #69447 (Minor refactoring of statement parsing)

Failed merges:

r? @ghost

Author: bors

src/bootstrap/config.rs

@@ -177,6 +177,15 @@ pub struct Target {
     pub no_std: bool,
 }
 
+impl Target {
+    pub fn from_triple(triple: &str) -> Self {
+        let mut target: Self = Default::default();
+        if triple.contains("-none-") || triple.contains("nvptx") {
+            target.no_std = true;
+        }
+        target
+    }
+}
 /// Structure of the `config.toml` file that configuration is read from.
 ///
 /// This structure uses `Decodable` to automatically decode a TOML configuration
@@ -353,6 +362,7 @@ struct TomlTarget {
     musl_root: Option<String>,
     wasi_root: Option<String>,
     qemu_rootfs: Option<String>,
+    no_std: Option<bool>,
 }
 
 impl Config {
@@ -595,7 +605,7 @@ impl Config {
 
         if let Some(ref t) = toml.target {
             for (triple, cfg) in t {
-                let mut target = Target::default();
+                let mut target = Target::from_triple(triple);
 
                 if let Some(ref s) = cfg.llvm_config {
                     target.llvm_config = Some(config.src.join(s));
@@ -606,6 +616,9 @@ impl Config {
                 if let Some(ref s) = cfg.android_ndk {
                     target.ndk = Some(config.src.join(s));
                 }
+                if let Some(s) = cfg.no_std {
+                    target.no_std = s;
+                }
                 target.cc = cfg.cc.clone().map(PathBuf::from);
                 target.cxx = cfg.cxx.clone().map(PathBuf::from);
                 target.ar = cfg.ar.clone().map(PathBuf::from);

src/bootstrap/sanity.rs

@@ -17,6 +17,7 @@ use std::process::Command;
 
 use build_helper::{output, t};
 
+use crate::config::Target;
 use crate::Build;
 
 struct Finder {
@@ -192,13 +193,9 @@ pub fn check(build: &mut Build) {
             panic!("the iOS target is only supported on macOS");
         }
 
-        if target.contains("-none-") || target.contains("nvptx") {
-            if build.no_std(*target).is_none() {
-                let target = build.config.target_config.entry(target.clone()).or_default();
-
-                target.no_std = true;
-            }
+        build.config.target_config.entry(target.clone()).or_insert(Target::from_triple(target));
 
+        if target.contains("-none-") || target.contains("nvptx") {
             if build.no_std(*target) == Some(false) {
                 panic!("All the *-none-* and nvptx* targets are no-std targets")
             }

src/libcore/cell.rs

@@ -1245,6 +1245,38 @@ impl<'b, T: ?Sized> Ref<'b, T> {
         let borrow = orig.borrow.clone();
         (Ref { value: a, borrow }, Ref { value: b, borrow: orig.borrow })
     }
+
+    /// Convert into a reference to the underlying data.
+    ///
+    /// The underlying `RefCell` can never be mutably borrowed from again and will always appear
+    /// already immutably borrowed. It is not a good idea to leak more than a constant number of
+    /// references. The `RefCell` can be immutably borrowed again if only a smaller number of leaks
+    /// have occurred in total.
+    ///
+    /// This is an associated function that needs to be used as
+    /// `Ref::leak(...)`. A method would interfere with methods of the
+    /// same name on the contents of a `RefCell` used through `Deref`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(cell_leak)]
+    /// use std::cell::{RefCell, Ref};
+    /// let cell = RefCell::new(0);
+    ///
+    /// let value = Ref::leak(cell.borrow());
+    /// assert_eq!(*value, 0);
+    ///
+    /// assert!(cell.try_borrow().is_ok());
+    /// assert!(cell.try_borrow_mut().is_err());
+    /// ```
+    #[unstable(feature = "cell_leak", issue = "69099")]
+    pub fn leak(orig: Ref<'b, T>) -> &'b T {
+        // By forgetting this Ref we ensure that the borrow counter in the RefCell never goes back
+        // to UNUSED again. No further mutable references can be created from the original cell.
+        mem::forget(orig.borrow);
+        orig.value
+    }
 }
 
 #[unstable(feature = "coerce_unsized", issue = "27732")]
@@ -1330,6 +1362,37 @@ impl<'b, T: ?Sized> RefMut<'b, T> {
         let borrow = orig.borrow.clone();
         (RefMut { value: a, borrow }, RefMut { value: b, borrow: orig.borrow })
     }
+
+    /// Convert into a mutable reference to the underlying data.
+    ///
+    /// The underlying `RefCell` can not be borrowed from again and will always appear already
+    /// mutably borrowed, making the returned reference the only to the interior.
+    ///
+    /// This is an associated function that needs to be used as
+    /// `RefMut::leak(...)`. A method would interfere with methods of the
+    /// same name on the contents of a `RefCell` used through `Deref`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(cell_leak)]
+    /// use std::cell::{RefCell, RefMut};
+    /// let cell = RefCell::new(0);
+    ///
+    /// let value = RefMut::leak(cell.borrow_mut());
+    /// assert_eq!(*value, 0);
+    /// *value = 1;
+    ///
+    /// assert!(cell.try_borrow_mut().is_err());
+    /// ```
+    #[unstable(feature = "cell_leak", issue = "69099")]
+    pub fn leak(orig: RefMut<'b, T>) -> &'b mut T {
+        // By forgetting this BorrowRefMut we ensure that the borrow counter in the RefCell never
+        // goes back to UNUSED again. No further references can be created from the original cell,
+        // making the current borrow the only reference for the remaining lifetime.
+        mem::forget(orig.borrow);
+        orig.value
+    }
 }
 
 struct BorrowRefMut<'b> {

src/libcore/fmt/float.rs

@@ -29,7 +29,6 @@ where
             *num,
             sign,
             precision,
-            false,
             buf.get_mut(),
             parts.get_mut(),
         );
@@ -59,7 +58,6 @@ where
             *num,
             sign,
             precision,
-            false,
             buf.get_mut(),
             parts.get_mut(),
         );

src/libcore/fmt/mod.rs

@@ -238,16 +238,8 @@ pub struct Formatter<'a> {
 // NB. Argument is essentially an optimized partially applied formatting function,
 // equivalent to `exists T.(&T, fn(&T, &mut Formatter<'_>) -> Result`.
 
-struct Void {
-    _priv: (),
-    /// Erases all oibits, because `Void` erases the type of the object that
-    /// will be used to produce formatted output. Since we do not know what
-    /// oibits the real types have (and they can have any or none), we need to
-    /// take the most conservative approach and forbid all oibits.
-    ///
-    /// It was added after #45197 showed that one could share a `!Sync`
-    /// object across threads by passing it into `format_args!`.
-    _oibit_remover: PhantomData<*mut dyn Fn()>,
+extern "C" {
+    type Opaque;
 }
 
 /// This struct represents the generic "argument" which is taken by the Xprintf
@@ -259,16 +251,23 @@ struct Void {
 #[unstable(feature = "fmt_internals", reason = "internal to format_args!", issue = "none")]
 #[doc(hidden)]
 pub struct ArgumentV1<'a> {
-    value: &'a Void,
-    formatter: fn(&Void, &mut Formatter<'_>) -> Result,
+    value: &'a Opaque,
+    formatter: fn(&Opaque, &mut Formatter<'_>) -> Result,
 }
 
-impl<'a> ArgumentV1<'a> {
-    #[inline(never)]
-    fn show_usize(x: &usize, f: &mut Formatter<'_>) -> Result {
-        Display::fmt(x, f)
-    }
+// This gurantees a single stable value for the function pointer associated with
+// indices/counts in the formatting infrastructure.
+//
+// Note that a function defined as such would not be correct as functions are
+// always tagged unnamed_addr with the current lowering to LLVM IR, so their
+// address is not considered important to LLVM and as such the as_usize cast
+// could have been miscompiled. In practice, we never call as_usize on non-usize
+// containing data (as a matter of static generation of the formatting
+// arguments), so this is merely an additional check.
+#[unstable(feature = "fmt_internals", reason = "internal to format_args!", issue = "none")]
+static USIZE_MARKER: fn(&usize, &mut Formatter<'_>) -> Result = |_, _| loop {};
 
+impl<'a> ArgumentV1<'a> {
     #[doc(hidden)]
     #[unstable(feature = "fmt_internals", reason = "internal to format_args!", issue = "none")]
     pub fn new<'b, T>(x: &'b T, f: fn(&T, &mut Formatter<'_>) -> Result) -> ArgumentV1<'b> {
@@ -278,11 +277,13 @@ impl<'a> ArgumentV1<'a> {
     #[doc(hidden)]
     #[unstable(feature = "fmt_internals", reason = "internal to format_args!", issue = "none")]
     pub fn from_usize(x: &usize) -> ArgumentV1<'_> {
-        ArgumentV1::new(x, ArgumentV1::show_usize)
+        ArgumentV1::new(x, USIZE_MARKER)
     }
 
     fn as_usize(&self) -> Option<usize> {
-        if self.formatter as usize == ArgumentV1::show_usize as usize {
+        if self.formatter as usize == USIZE_MARKER as usize {
+            // SAFETY: The `formatter` field is only set to USIZE_MARKER if
+            // the value is a usize, so this is safe
             Some(unsafe { *(self.value as *const _ as *const usize) })
         } else {
             None
@@ -1356,11 +1357,11 @@ impl<'a> Formatter<'a> {
             let mut align = old_align;
             if self.sign_aware_zero_pad() {
                 // a sign always goes first
-                let sign = unsafe { str::from_utf8_unchecked(formatted.sign) };
+                let sign = formatted.sign;
                 self.buf.write_str(sign)?;
 
                 // remove the sign from the formatted parts
-                formatted.sign = b"";
+                formatted.sign = "";
                 width = width.saturating_sub(sign.len());
                 align = rt::v1::Alignment::Right;
                 self.fill = '0';
@@ -1392,7 +1393,7 @@ impl<'a> Formatter<'a> {
         }
 
         if !formatted.sign.is_empty() {
-            write_bytes(self.buf, formatted.sign)?;
+            self.buf.write_str(formatted.sign)?;
         }
         for part in formatted.parts {
             match *part {

src/libcore/fmt/num.rs

@@ -369,11 +369,11 @@ macro_rules! impl_Exp {
                 flt2dec::Part::Copy(exp_slice)
             ];
             let sign = if !is_nonnegative {
-                &b"-"[..]
+                "-"
             } else if f.sign_plus() {
-                &b"+"[..]
+                "+"
             } else {
-                &b""[..]
+                ""
             };
             let formatted = flt2dec::Formatted{sign, parts};
             f.pad_formatted_parts(&formatted)

src/libcore/num/flt2dec/mod.rs

@@ -237,7 +237,7 @@ impl<'a> Part<'a> {
 #[derive(Clone)]
 pub struct Formatted<'a> {
     /// A byte slice representing a sign, either `""`, `"-"` or `"+"`.
-    pub sign: &'static [u8],
+    pub sign: &'static str,
     /// Formatted parts to be rendered after a sign and optional zero padding.
     pub parts: &'a [Part<'a>],
 }
@@ -259,7 +259,7 @@ impl<'a> Formatted<'a> {
         if out.len() < self.sign.len() {
             return None;
         }
-        out[..self.sign.len()].copy_from_slice(self.sign);
+        out[..self.sign.len()].copy_from_slice(self.sign.as_bytes());
 
         let mut written = self.sign.len();
         for part in self.parts {
@@ -402,38 +402,38 @@ pub enum Sign {
 }
 
 /// Returns the static byte string corresponding to the sign to be formatted.
-/// It can be either `b""`, `b"+"` or `b"-"`.
-fn determine_sign(sign: Sign, decoded: &FullDecoded, negative: bool) -> &'static [u8] {
+/// It can be either `""`, `"+"` or `"-"`.
+fn determine_sign(sign: Sign, decoded: &FullDecoded, negative: bool) -> &'static str {
     match (*decoded, sign) {
-        (FullDecoded::Nan, _) => b"",
-        (FullDecoded::Zero, Sign::Minus) => b"",
+        (FullDecoded::Nan, _) => "",
+        (FullDecoded::Zero, Sign::Minus) => "",
         (FullDecoded::Zero, Sign::MinusRaw) => {
             if negative {
-                b"-"
+                "-"
             } else {
-                b""
+                ""
             }
         }
-        (FullDecoded::Zero, Sign::MinusPlus) => b"+",
+        (FullDecoded::Zero, Sign::MinusPlus) => "+",
         (FullDecoded::Zero, Sign::MinusPlusRaw) => {
             if negative {
-                b"-"
+                "-"
             } else {
-                b"+"
+                "+"
             }
         }
         (_, Sign::Minus) | (_, Sign::MinusRaw) => {
             if negative {
-                b"-"
+                "-"
             } else {
-                b""
+                ""
             }
         }
         (_, Sign::MinusPlus) | (_, Sign::MinusPlusRaw) => {
             if negative {
-                b"-"
+                "-"
             } else {
-                b"+"
+                "+"
             }
         }
     }
@@ -462,7 +462,6 @@ pub fn to_shortest_str<'a, T, F>(
     v: T,
     sign: Sign,
     frac_digits: usize,
-    _upper: bool,
     buf: &'a mut [u8],
     parts: &'a mut [Part<'a>],
 ) -> Formatted<'a>
@@ -679,7 +678,6 @@ pub fn to_exact_fixed_str<'a, T, F>(
     v: T,
     sign: Sign,
     frac_digits: usize,
-    _upper: bool,
     buf: &'a mut [u8],
     parts: &'a mut [Part<'a>],
 ) -> Formatted<'a>

src/libcore/tests/num/flt2dec/mod.rs

@@ -408,20 +408,12 @@ impl<'a, 'tcx> SpecializedDecoder<Span> for DecodeContext<'a, 'tcx> {
             {
                 last_source_file
             } else {
-                let mut a = 0;
-                let mut b = imported_source_files.len();
-
-                while b - a > 1 {
-                    let m = (a + b) / 2;
-                    if imported_source_files[m].original_start_pos > lo {
-                        b = m;
-                    } else {
-                        a = m;
-                    }
-                }
+                let index = imported_source_files
+                    .binary_search_by_key(&lo, |source_file| source_file.original_start_pos)
+                    .unwrap_or_else(|index| index - 1);
 
-                self.last_source_file_index = a;
-                &imported_source_files[a]
+                self.last_source_file_index = index;
+                &imported_source_files[index]
             }
         };
 

src/librustc_metadata/rmeta/decoder.rs

@@ -35,61 +35,32 @@ impl<'a> Parser<'a> {
         let attrs = self.parse_outer_attributes()?;
         let lo = self.token.span;
 
-        if self.eat_keyword(kw::Let) {
-            return self.parse_local_mk(lo, attrs.into()).map(Some);
-        }
-        if self.is_kw_followed_by_ident(kw::Mut) {
-            return self.recover_stmt_local(lo, attrs.into(), "missing keyword", "let mut");
-        }
-        if self.is_kw_followed_by_ident(kw::Auto) {
+        let stmt = if self.eat_keyword(kw::Let) {
+            self.parse_local_mk(lo, attrs.into())?
+        } else if self.is_kw_followed_by_ident(kw::Mut) {
+            self.recover_stmt_local(lo, attrs.into(), "missing keyword", "let mut")?
+        } else if self.is_kw_followed_by_ident(kw::Auto) {
             self.bump(); // `auto`
             let msg = "write `let` instead of `auto` to introduce a new variable";
-            return self.recover_stmt_local(lo, attrs.into(), msg, "let");
-        }
-        if self.is_kw_followed_by_ident(sym::var) {
+            self.recover_stmt_local(lo, attrs.into(), msg, "let")?
+        } else if self.is_kw_followed_by_ident(sym::var) {
             self.bump(); // `var`
             let msg = "write `let` instead of `var` to introduce a new variable";
-            return self.recover_stmt_local(lo, attrs.into(), msg, "let");
-        }
-
-        // Starts like a simple path, being careful to avoid contextual keywords,
-        // e.g., `union`, items with `crate` visibility, or `auto trait` items.
-        // We aim to parse an arbitrary path `a::b` but not something that starts like a path
-        // (1 token), but it fact not a path. Also, we avoid stealing syntax from `parse_item_`.
-        if self.token.is_path_start() && !self.token.is_qpath_start() && !self.is_path_start_item()
+            self.recover_stmt_local(lo, attrs.into(), msg, "let")?
+        } else if self.token.is_path_start()
+            && !self.token.is_qpath_start()
+            && !self.is_path_start_item()
         {
-            let path = self.parse_path(PathStyle::Expr)?;
-
-            if self.eat(&token::Not) {
-                return self.parse_stmt_mac(lo, attrs.into(), path);
-            }
-
-            let expr = if self.check(&token::OpenDelim(token::Brace)) {
-                self.parse_struct_expr(lo, path, AttrVec::new())?
-            } else {
-                let hi = self.prev_span;
-                self.mk_expr(lo.to(hi), ExprKind::Path(None, path), AttrVec::new())
-            };
-
-            let expr = self.with_res(Restrictions::STMT_EXPR, |this| {
-                let expr = this.parse_dot_or_call_expr_with(expr, lo, attrs.into())?;
-                this.parse_assoc_expr_with(0, LhsExpr::AlreadyParsed(expr))
-            })?;
-            return Ok(Some(self.mk_stmt(lo.to(self.prev_span), StmtKind::Expr(expr))));
-        }
-
-        // FIXME: Bad copy of attrs
-        let old_directory_ownership =
-            mem::replace(&mut self.directory.ownership, DirectoryOwnership::UnownedViaBlock);
-        let item = self.parse_item_common(attrs.clone(), false, true, |_| true)?;
-        self.directory.ownership = old_directory_ownership;
-
-        if let Some(item) = item {
-            return Ok(Some(self.mk_stmt(lo.to(item.span), StmtKind::Item(P(item)))));
-        }
-
-        // Do not attempt to parse an expression if we're done here.
-        if self.token == token::Semi {
+            // We have avoided contextual keywords like `union`, items with `crate` visibility,
+            // or `auto trait` items. We aim to parse an arbitrary path `a::b` but not something
+            // that starts like a path (1 token), but it fact not a path.
+            // Also, we avoid stealing syntax from `parse_item_`.
+            self.parse_stmt_path_start(lo, attrs)?
+        } else if let Some(item) = self.parse_stmt_item(attrs.clone())? {
+            // FIXME: Bad copy of attrs
+            self.mk_stmt(lo.to(item.span), StmtKind::Item(P(item)))
+        } else if self.token == token::Semi {
+            // Do not attempt to parse an expression if we're done here.
             self.error_outer_attrs(&attrs);
             self.bump();
             let mut last_semi = lo;
@@ -104,27 +75,49 @@ impl<'a> Parser<'a> {
                 ExprKind::Tup(Vec::new()),
                 AttrVec::new(),
             ));
-            return Ok(Some(self.mk_stmt(lo.to(last_semi), kind)));
-        }
-
-        if self.token == token::CloseDelim(token::Brace) {
+            self.mk_stmt(lo.to(last_semi), kind)
+        } else if self.token != token::CloseDelim(token::Brace) {
+            // Remainder are line-expr stmts.
+            let e = self.parse_expr_res(Restrictions::STMT_EXPR, Some(attrs.into()))?;
+            self.mk_stmt(lo.to(e.span), StmtKind::Expr(e))
+        } else {
             self.error_outer_attrs(&attrs);
             return Ok(None);
+        };
+        Ok(Some(stmt))
+    }
+
+    fn parse_stmt_item(&mut self, attrs: Vec<Attribute>) -> PResult<'a, Option<ast::Item>> {
+        let old = mem::replace(&mut self.directory.ownership, DirectoryOwnership::UnownedViaBlock);
+        let item = self.parse_item_common(attrs.clone(), false, true, |_| true)?;
+        self.directory.ownership = old;
+        Ok(item)
+    }
+
+    fn parse_stmt_path_start(&mut self, lo: Span, attrs: Vec<Attribute>) -> PResult<'a, Stmt> {
+        let path = self.parse_path(PathStyle::Expr)?;
+
+        if self.eat(&token::Not) {
+            return self.parse_stmt_mac(lo, attrs.into(), path);
         }
 
-        // Remainder are line-expr stmts.
-        let e = self.parse_expr_res(Restrictions::STMT_EXPR, Some(attrs.into()))?;
-        Ok(Some(self.mk_stmt(lo.to(e.span), StmtKind::Expr(e))))
+        let expr = if self.check(&token::OpenDelim(token::Brace)) {
+            self.parse_struct_expr(lo, path, AttrVec::new())?
+        } else {
+            let hi = self.prev_span;
+            self.mk_expr(lo.to(hi), ExprKind::Path(None, path), AttrVec::new())
+        };
+
+        let expr = self.with_res(Restrictions::STMT_EXPR, |this| {
+            let expr = this.parse_dot_or_call_expr_with(expr, lo, attrs.into())?;
+            this.parse_assoc_expr_with(0, LhsExpr::AlreadyParsed(expr))
+        })?;
+        Ok(self.mk_stmt(lo.to(self.prev_span), StmtKind::Expr(expr)))
     }
 
     /// Parses a statement macro `mac!(args)` provided a `path` representing `mac`.
     /// At this point, the `!` token after the path has already been eaten.
-    fn parse_stmt_mac(
-        &mut self,
-        lo: Span,
-        attrs: AttrVec,
-        path: ast::Path,
-    ) -> PResult<'a, Option<Stmt>> {
+    fn parse_stmt_mac(&mut self, lo: Span, attrs: AttrVec, path: ast::Path) -> PResult<'a, Stmt> {
         let args = self.parse_mac_args()?;
         let delim = args.delim();
         let hi = self.prev_span;
@@ -145,7 +138,7 @@ impl<'a> Parser<'a> {
             let e = self.parse_assoc_expr_with(0, LhsExpr::AlreadyParsed(e))?;
             StmtKind::Expr(e)
         };
-        Ok(Some(self.mk_stmt(lo.to(hi), kind)))
+        Ok(self.mk_stmt(lo.to(hi), kind))
     }
 
     /// Error on outer attributes in this context.
@@ -167,12 +160,12 @@ impl<'a> Parser<'a> {
         attrs: AttrVec,
         msg: &str,
         sugg: &str,
-    ) -> PResult<'a, Option<Stmt>> {
+    ) -> PResult<'a, Stmt> {
         let stmt = self.parse_local_mk(lo, attrs)?;
         self.struct_span_err(lo, "invalid variable declaration")
             .span_suggestion(lo, msg, sugg.to_string(), Applicability::MachineApplicable)
             .emit();
-        Ok(Some(stmt))
+        Ok(stmt)
     }
 
     fn parse_local_mk(&mut self, lo: Span, attrs: AttrVec) -> PResult<'a, Stmt> {
@@ -372,36 +365,36 @@ impl<'a> Parser<'a> {
 
         let mut eat_semi = true;
         match stmt.kind {
-            StmtKind::Expr(ref expr) if self.token != token::Eof => {
-                // expression without semicolon
-                if classify::expr_requires_semi_to_be_stmt(expr) {
-                    // Just check for errors and recover; do not eat semicolon yet.
-                    if let Err(mut e) =
-                        self.expect_one_of(&[], &[token::Semi, token::CloseDelim(token::Brace)])
-                    {
-                        if let TokenKind::DocComment(..) = self.token.kind {
-                            if let Ok(snippet) = self.span_to_snippet(self.token.span) {
-                                let sp = self.token.span;
-                                let marker = &snippet[..3];
-                                let (comment_marker, doc_comment_marker) = marker.split_at(2);
-
-                                e.span_suggestion(
-                                    sp.with_hi(sp.lo() + BytePos(marker.len() as u32)),
-                                    &format!(
-                                        "add a space before `{}` to use a regular comment",
-                                        doc_comment_marker,
-                                    ),
-                                    format!("{} {}", comment_marker, doc_comment_marker),
-                                    Applicability::MaybeIncorrect,
-                                );
-                            }
+            // Expression without semicolon.
+            StmtKind::Expr(ref expr)
+                if self.token != token::Eof && classify::expr_requires_semi_to_be_stmt(expr) =>
+            {
+                // Just check for errors and recover; do not eat semicolon yet.
+                if let Err(mut e) =
+                    self.expect_one_of(&[], &[token::Semi, token::CloseDelim(token::Brace)])
+                {
+                    if let TokenKind::DocComment(..) = self.token.kind {
+                        if let Ok(snippet) = self.span_to_snippet(self.token.span) {
+                            let sp = self.token.span;
+                            let marker = &snippet[..3];
+                            let (comment_marker, doc_comment_marker) = marker.split_at(2);
+
+                            e.span_suggestion(
+                                sp.with_hi(sp.lo() + BytePos(marker.len() as u32)),
+                                &format!(
+                                    "add a space before `{}` to use a regular comment",
+                                    doc_comment_marker,
+                                ),
+                                format!("{} {}", comment_marker, doc_comment_marker),
+                                Applicability::MaybeIncorrect,
+                            );
                         }
-                        e.emit();
-                        self.recover_stmt();
-                        // Don't complain about type errors in body tail after parse error (#57383).
-                        let sp = expr.span.to(self.prev_span);
-                        stmt.kind = StmtKind::Expr(self.mk_expr_err(sp));
                     }
+                    e.emit();
+                    self.recover_stmt();
+                    // Don't complain about type errors in body tail after parse error (#57383).
+                    let sp = expr.span.to(self.prev_span);
+                    stmt.kind = StmtKind::Expr(self.mk_expr_err(sp));
                 }
             }
             StmtKind::Local(..) => {

src/librustc_parse/parser/stmt.rs

@@ -1,34 +1,30 @@
-error[E0277]: `*mut (dyn std::ops::Fn() + 'static)` cannot be shared between threads safely
+error[E0277]: `core::fmt::Opaque` cannot be shared between threads safely
   --> $DIR/send-sync.rs:8:5
    |
 LL | fn send<T: Send>(_: T) {}
    |    ----    ---- required by this bound in `send`
 ...
 LL |     send(format_args!("{:?}", c));
-   |     ^^^^ `*mut (dyn std::ops::Fn() + 'static)` cannot be shared between threads safely
+   |     ^^^^ `core::fmt::Opaque` cannot be shared between threads safely
    |
-   = help: within `[std::fmt::ArgumentV1<'_>]`, the trait `std::marker::Sync` is not implemented for `*mut (dyn std::ops::Fn() + 'static)`
-   = note: required because it appears within the type `std::marker::PhantomData<*mut (dyn std::ops::Fn() + 'static)>`
-   = note: required because it appears within the type `core::fmt::Void`
-   = note: required because it appears within the type `&core::fmt::Void`
+   = help: within `[std::fmt::ArgumentV1<'_>]`, the trait `std::marker::Sync` is not implemented for `core::fmt::Opaque`
+   = note: required because it appears within the type `&core::fmt::Opaque`
    = note: required because it appears within the type `std::fmt::ArgumentV1<'_>`
    = note: required because it appears within the type `[std::fmt::ArgumentV1<'_>]`
    = note: required because of the requirements on the impl of `std::marker::Send` for `&[std::fmt::ArgumentV1<'_>]`
    = note: required because it appears within the type `std::fmt::Arguments<'_>`
 
-error[E0277]: `*mut (dyn std::ops::Fn() + 'static)` cannot be shared between threads safely
+error[E0277]: `core::fmt::Opaque` cannot be shared between threads safely
   --> $DIR/send-sync.rs:9:5
    |
 LL | fn sync<T: Sync>(_: T) {}
    |    ----    ---- required by this bound in `sync`
 ...
 LL |     sync(format_args!("{:?}", c));
-   |     ^^^^ `*mut (dyn std::ops::Fn() + 'static)` cannot be shared between threads safely
+   |     ^^^^ `core::fmt::Opaque` cannot be shared between threads safely
    |
-   = help: within `std::fmt::Arguments<'_>`, the trait `std::marker::Sync` is not implemented for `*mut (dyn std::ops::Fn() + 'static)`
-   = note: required because it appears within the type `std::marker::PhantomData<*mut (dyn std::ops::Fn() + 'static)>`
-   = note: required because it appears within the type `core::fmt::Void`
-   = note: required because it appears within the type `&core::fmt::Void`
+   = help: within `std::fmt::Arguments<'_>`, the trait `std::marker::Sync` is not implemented for `core::fmt::Opaque`
+   = note: required because it appears within the type `&core::fmt::Opaque`
    = note: required because it appears within the type `std::fmt::ArgumentV1<'_>`
    = note: required because it appears within the type `[std::fmt::ArgumentV1<'_>]`
    = note: required because it appears within the type `&[std::fmt::ArgumentV1<'_>]`