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-rw-r--r--src/librustc_typeck/check/mod.rs28
-rw-r--r--src/libsyntax/parse/lexer/mod.rs3
-rw-r--r--src/libsyntax/parse/mod.rs4
-rw-r--r--src/libsyntax/parse/parser.rs64
-rw-r--r--src/libsyntax/util/parser.rs22
-rw-r--r--src/test/ui/parser/expr-as-stmt.fixed34
-rw-r--r--src/test/ui/parser/expr-as-stmt.rs34
-rw-r--r--src/test/ui/parser/expr-as-stmt.stderr80
-rw-r--r--src/test/ui/parser/match-arrows-block-then-binop.rs6
-rw-r--r--src/test/ui/parser/match-arrows-block-then-binop.stderr6
10 files changed, 270 insertions, 11 deletions
diff --git a/src/librustc_typeck/check/mod.rs b/src/librustc_typeck/check/mod.rs
index f1163847892..61270716dfc 100644
--- a/src/librustc_typeck/check/mod.rs
+++ b/src/librustc_typeck/check/mod.rs
@@ -4165,9 +4165,31 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                                 oprnd_t = self.make_overloaded_place_return_type(method).ty;
                                 self.write_method_call(expr.hir_id, method);
                             } else {
-                                type_error_struct!(tcx.sess, expr.span, oprnd_t, E0614,
-                                                   "type `{}` cannot be dereferenced",
-                                                   oprnd_t).emit();
+                                let mut err = type_error_struct!(
+                                    tcx.sess,
+                                    expr.span,
+                                    oprnd_t,
+                                    E0614,
+                                    "type `{}` cannot be dereferenced",
+                                    oprnd_t,
+                                );
+                                let sp = tcx.sess.source_map().start_point(expr.span);
+                                if let Some(sp) = tcx.sess.parse_sess.abiguous_block_expr_parse
+                                    .borrow().get(&sp)
+                                {
+                                    if let Ok(snippet) = tcx.sess.source_map()
+                                        .span_to_snippet(*sp)
+                                    {
+                                        err.span_suggestion(
+                                            *sp,
+                                            "parenthesis are required to parse this \
+                                             as an expression",
+                                            format!("({})", snippet),
+                                            Applicability::MachineApplicable,
+                                        );
+                                    }
+                                }
+                                err.emit();
                                 oprnd_t = tcx.types.err;
                             }
                         }
diff --git a/src/libsyntax/parse/lexer/mod.rs b/src/libsyntax/parse/lexer/mod.rs
index cf8f8abe2ab..e7d79a647d3 100644
--- a/src/libsyntax/parse/lexer/mod.rs
+++ b/src/libsyntax/parse/lexer/mod.rs
@@ -1899,7 +1899,7 @@ mod tests {
     use std::io;
     use std::path::PathBuf;
     use syntax_pos::{BytePos, Span, NO_EXPANSION};
-    use rustc_data_structures::fx::FxHashSet;
+    use rustc_data_structures::fx::{FxHashSet, FxHashMap};
     use rustc_data_structures::sync::Lock;
 
     fn mk_sess(sm: Lrc<SourceMap>) -> ParseSess {
@@ -1918,6 +1918,7 @@ mod tests {
             raw_identifier_spans: Lock::new(Vec::new()),
             registered_diagnostics: Lock::new(ErrorMap::new()),
             buffered_lints: Lock::new(vec![]),
+            abiguous_block_expr_parse: Lock::new(FxHashMap::default()),
         }
     }
 
diff --git a/src/libsyntax/parse/mod.rs b/src/libsyntax/parse/mod.rs
index 1abc7832ffa..94bbd5ba2f7 100644
--- a/src/libsyntax/parse/mod.rs
+++ b/src/libsyntax/parse/mod.rs
@@ -16,7 +16,7 @@ use rustc_data_structures::sync::{Lrc, Lock};
 use syntax_pos::{Span, SourceFile, FileName, MultiSpan};
 use log::debug;
 
-use rustc_data_structures::fx::FxHashSet;
+use rustc_data_structures::fx::{FxHashSet, FxHashMap};
 use std::borrow::Cow;
 use std::iter;
 use std::path::{Path, PathBuf};
@@ -47,6 +47,7 @@ pub struct ParseSess {
     included_mod_stack: Lock<Vec<PathBuf>>,
     source_map: Lrc<SourceMap>,
     pub buffered_lints: Lock<Vec<BufferedEarlyLint>>,
+    pub abiguous_block_expr_parse: Lock<FxHashMap<Span, Span>>,
 }
 
 impl ParseSess {
@@ -70,6 +71,7 @@ impl ParseSess {
             included_mod_stack: Lock::new(vec![]),
             source_map,
             buffered_lints: Lock::new(vec![]),
+            abiguous_block_expr_parse: Lock::new(FxHashMap::default()),
         }
     }
 
diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 8efe84cdf01..3c7f477cc8f 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -186,6 +186,7 @@ enum PrevTokenKind {
     Interpolated,
     Eof,
     Ident,
+    BitOr,
     Other,
 }
 
@@ -1410,6 +1411,7 @@ impl<'a> Parser<'a> {
             token::DocComment(..) => PrevTokenKind::DocComment,
             token::Comma => PrevTokenKind::Comma,
             token::BinOp(token::Plus) => PrevTokenKind::Plus,
+            token::BinOp(token::Or) => PrevTokenKind::BitOr,
             token::Interpolated(..) => PrevTokenKind::Interpolated,
             token::Eof => PrevTokenKind::Eof,
             token::Ident(..) => PrevTokenKind::Ident,
@@ -2925,6 +2927,19 @@ impl<'a> Parser<'a> {
                             let msg = format!("expected expression, found {}",
                                               self.this_token_descr());
                             let mut err = self.fatal(&msg);
+                            let sp = self.sess.source_map().start_point(self.span);
+                            if let Some(sp) = self.sess.abiguous_block_expr_parse.borrow()
+                                .get(&sp)
+                            {
+                                if let Ok(snippet) = self.sess.source_map().span_to_snippet(*sp) {
+                                    err.span_suggestion(
+                                        *sp,
+                                        "parenthesis are required to parse this as an expression",
+                                        format!("({})", snippet),
+                                        Applicability::MachineApplicable,
+                                    );
+                                }
+                            }
                             err.span_label(self.span, "expected expression");
                             return Err(err);
                         }
@@ -3616,9 +3631,41 @@ impl<'a> Parser<'a> {
             }
         };
 
-        if self.expr_is_complete(&lhs) {
-            // Semi-statement forms are odd. See https://github.com/rust-lang/rust/issues/29071
-            return Ok(lhs);
+        match (self.expr_is_complete(&lhs), AssocOp::from_token(&self.token)) {
+            (true, None) => {
+                // Semi-statement forms are odd. See https://github.com/rust-lang/rust/issues/29071
+                return Ok(lhs);
+            }
+            (false, _) => {} // continue parsing the expression
+            (true, Some(AssocOp::Multiply)) | // `{ 42 } *foo = bar;`
+            (true, Some(AssocOp::Subtract)) | // `{ 42 } -5`
+            (true, Some(AssocOp::Add)) => { // `{ 42 } + 42
+                // These cases are ambiguous and can't be identified in the parser alone
+                let sp = self.sess.source_map().start_point(self.span);
+                self.sess.abiguous_block_expr_parse.borrow_mut().insert(sp, lhs.span);
+                return Ok(lhs);
+            }
+            (true, Some(ref op)) if !op.can_continue_expr_unambiguously() => {
+                return Ok(lhs);
+            }
+            (true, Some(_)) => {
+                // #54186, #54482, #59975
+                // We've found an expression that would be parsed as a statement, but the next
+                // token implies this should be parsed as an expression.
+                let mut err = self.sess.span_diagnostic.struct_span_err(
+                    self.span,
+                    "ambiguous parse",
+                );
+                let snippet = self.sess.source_map().span_to_snippet(lhs.span)
+                    .unwrap_or_else(|_| pprust::expr_to_string(&lhs));
+                err.span_suggestion(
+                    lhs.span,
+                    "parenthesis are required to parse this as an expression",
+                    format!("({})", snippet),
+                    Applicability::MachineApplicable,
+                );
+                err.emit();
+            }
         }
         self.expected_tokens.push(TokenType::Operator);
         while let Some(op) = AssocOp::from_token(&self.token) {
@@ -4929,6 +4976,17 @@ impl<'a> Parser<'a> {
                         );
                         let mut err = self.fatal(&msg);
                         err.span_label(self.span, format!("expected {}", expected));
+                        let sp = self.sess.source_map().start_point(self.span);
+                        if let Some(sp) = self.sess.abiguous_block_expr_parse.borrow().get(&sp) {
+                            if let Ok(snippet) = self.sess.source_map().span_to_snippet(*sp) {
+                                err.span_suggestion(
+                                    *sp,
+                                    "parenthesis are required to parse this as an expression",
+                                    format!("({})", snippet),
+                                    Applicability::MachineApplicable,
+                                );
+                            }
+                        }
                         return Err(err);
                     }
                 }
diff --git a/src/libsyntax/util/parser.rs b/src/libsyntax/util/parser.rs
index 5f15ede7b0b..d76dede8155 100644
--- a/src/libsyntax/util/parser.rs
+++ b/src/libsyntax/util/parser.rs
@@ -207,6 +207,28 @@ impl AssocOp {
             ObsoleteInPlace | Assign | AssignOp(_) | As | DotDot | DotDotEq | Colon => None
         }
     }
+
+    pub fn can_continue_expr_unambiguously(&self) -> bool {
+        use AssocOp::*;
+        match self {
+            BitXor | // `{ 42 } ^ 3`
+            Assign | // `{ 42 } = { 42 }`
+            Divide | // `{ 42 } / 42`
+            Modulus | // `{ 42 } % 2`
+            ShiftRight | // `{ 42 } >> 2`
+            LessEqual | // `{ 42 } <= 3`
+            Greater | // `{ 42 } > 3`
+            GreaterEqual | // `{ 42 } >= 3`
+            AssignOp(_) | // `{ 42 } +=`
+            LAnd | // `{ 42 } &&foo`
+            As | // `{ 42 } as usize`
+            // Equal | // `{ 42 } == { 42 }`    Accepting these here would regress incorrect
+            // NotEqual | // `{ 42 } != { 42 }  struct literals parser recovery.
+            Colon => true, // `{ 42 }: usize`
+            _ => false,
+        }
+
+    }
 }
 
 pub const PREC_RESET: i8 = -100;
diff --git a/src/test/ui/parser/expr-as-stmt.fixed b/src/test/ui/parser/expr-as-stmt.fixed
new file mode 100644
index 00000000000..a0abd00a15c
--- /dev/null
+++ b/src/test/ui/parser/expr-as-stmt.fixed
@@ -0,0 +1,34 @@
+// run-rustfix
+#![allow(unused_variables)]
+#![allow(dead_code)]
+#![allow(unused_must_use)]
+
+fn foo() -> i32 {
+    ({2}) + {2} //~ ERROR expected expression, found `+`
+    //~^ ERROR mismatched types
+}
+
+fn bar() -> i32 {
+    ({2}) + 2 //~ ERROR expected expression, found `+`
+    //~^ ERROR mismatched types
+}
+
+fn zul() -> u32 {
+    let foo = 3;
+    ({ 42 }) + foo; //~ ERROR expected expression, found `+`
+    //~^ ERROR mismatched types
+    32
+}
+
+fn baz() -> i32 {
+    ({ 3 }) * 3 //~ ERROR type `{integer}` cannot be dereferenced
+    //~^ ERROR mismatched types
+}
+
+fn qux(a: Option<u32>, b: Option<u32>) -> bool {
+    (if let Some(x) = a { true } else { false })
+    && //~ ERROR ambiguous parse
+    if let Some(y) = a { true } else { false }
+}
+
+fn main() {}
diff --git a/src/test/ui/parser/expr-as-stmt.rs b/src/test/ui/parser/expr-as-stmt.rs
new file mode 100644
index 00000000000..cf2e7266a4a
--- /dev/null
+++ b/src/test/ui/parser/expr-as-stmt.rs
@@ -0,0 +1,34 @@
+// run-rustfix
+#![allow(unused_variables)]
+#![allow(dead_code)]
+#![allow(unused_must_use)]
+
+fn foo() -> i32 {
+    {2} + {2} //~ ERROR expected expression, found `+`
+    //~^ ERROR mismatched types
+}
+
+fn bar() -> i32 {
+    {2} + 2 //~ ERROR expected expression, found `+`
+    //~^ ERROR mismatched types
+}
+
+fn zul() -> u32 {
+    let foo = 3;
+    { 42 } + foo; //~ ERROR expected expression, found `+`
+    //~^ ERROR mismatched types
+    32
+}
+
+fn baz() -> i32 {
+    { 3 } * 3 //~ ERROR type `{integer}` cannot be dereferenced
+    //~^ ERROR mismatched types
+}
+
+fn qux(a: Option<u32>, b: Option<u32>) -> bool {
+    if let Some(x) = a { true } else { false }
+    && //~ ERROR ambiguous parse
+    if let Some(y) = a { true } else { false }
+}
+
+fn main() {}
diff --git a/src/test/ui/parser/expr-as-stmt.stderr b/src/test/ui/parser/expr-as-stmt.stderr
new file mode 100644
index 00000000000..03119605432
--- /dev/null
+++ b/src/test/ui/parser/expr-as-stmt.stderr
@@ -0,0 +1,80 @@
+error: expected expression, found `+`
+  --> $DIR/expr-as-stmt.rs:7:9
+   |
+LL |     {2} + {2}
+   |     --- ^ expected expression
+   |     |
+   |     help: parenthesis are required to parse this as an expression: `({2})`
+
+error: expected expression, found `+`
+  --> $DIR/expr-as-stmt.rs:12:9
+   |
+LL |     {2} + 2
+   |     --- ^ expected expression
+   |     |
+   |     help: parenthesis are required to parse this as an expression: `({2})`
+
+error: expected expression, found `+`
+  --> $DIR/expr-as-stmt.rs:18:12
+   |
+LL |     { 42 } + foo;
+   |     ------ ^ expected expression
+   |     |
+   |     help: parenthesis are required to parse this as an expression: `({ 42 })`
+
+error: ambiguous parse
+  --> $DIR/expr-as-stmt.rs:30:5
+   |
+LL |     if let Some(x) = a { true } else { false }
+   |     ------------------------------------------ help: parenthesis are required to parse this as an expression: `(if let Some(x) = a { true } else { false })`
+LL |     &&
+   |     ^^
+
+error[E0308]: mismatched types
+  --> $DIR/expr-as-stmt.rs:7:6
+   |
+LL |     {2} + {2}
+   |      ^ expected (), found integer
+   |
+   = note: expected type `()`
+              found type `{integer}`
+
+error[E0308]: mismatched types
+  --> $DIR/expr-as-stmt.rs:12:6
+   |
+LL |     {2} + 2
+   |      ^ expected (), found integer
+   |
+   = note: expected type `()`
+              found type `{integer}`
+
+error[E0308]: mismatched types
+  --> $DIR/expr-as-stmt.rs:18:7
+   |
+LL |     { 42 } + foo;
+   |       ^^ expected (), found integer
+   |
+   = note: expected type `()`
+              found type `{integer}`
+
+error[E0308]: mismatched types
+  --> $DIR/expr-as-stmt.rs:24:7
+   |
+LL |     { 3 } * 3
+   |       ^ expected (), found integer
+   |
+   = note: expected type `()`
+              found type `{integer}`
+
+error[E0614]: type `{integer}` cannot be dereferenced
+  --> $DIR/expr-as-stmt.rs:24:11
+   |
+LL |     { 3 } * 3
+   |     ----- ^^^
+   |     |
+   |     help: parenthesis are required to parse this as an expression: `({ 3 })`
+
+error: aborting due to 9 previous errors
+
+Some errors have detailed explanations: E0308, E0614.
+For more information about an error, try `rustc --explain E0308`.
diff --git a/src/test/ui/parser/match-arrows-block-then-binop.rs b/src/test/ui/parser/match-arrows-block-then-binop.rs
index 1a40d674929..56c917c7462 100644
--- a/src/test/ui/parser/match-arrows-block-then-binop.rs
+++ b/src/test/ui/parser/match-arrows-block-then-binop.rs
@@ -1,7 +1,7 @@
 fn main() {
-
-    match 0 {
+    let _ = match 0 {
       0 => {
+        0
       } + 5 //~ ERROR expected pattern, found `+`
-    }
+    };
 }
diff --git a/src/test/ui/parser/match-arrows-block-then-binop.stderr b/src/test/ui/parser/match-arrows-block-then-binop.stderr
index a844cac189a..0d7f81645b4 100644
--- a/src/test/ui/parser/match-arrows-block-then-binop.stderr
+++ b/src/test/ui/parser/match-arrows-block-then-binop.stderr
@@ -3,6 +3,12 @@ error: expected pattern, found `+`
    |
 LL |       } + 5
    |         ^ expected pattern
+help: parenthesis are required to parse this as an expression
+   |
+LL |       0 => ({
+LL |         0
+LL |       }) + 5
+   |
 
 error: aborting due to previous error
generated by cgit-pink 1.4.1-3-g733a5 (git 2.37.1) at 2026-11-01 16:24:08 +0000