Detect missing `=>` after match guard during parsing

```
error: expected one of `,`, `:`, or `}`, found `.`
  --> $DIR/missing-fat-arrow.rs:25:14
   |
LL |         Some(a) if a.value == b {
   |                               - while parsing this struct
LL |             a.value = 1;
   |             -^ expected one of `,`, `:`, or `}`
   |             |
   |             while parsing this struct field
   |
help: try naming a field
   |
LL |             a: a.value = 1;
   |             ++
help: you might have meant to start a match arm after the match guard
   |
LL |         Some(a) if a.value == b => {
   |                                 ++
```

Fix #78585.

13 files changed, 229 insertions, 41 deletions

diff --git a/compiler/rustc_parse/messages.ftl b/compiler/rustc_parse/messages.ftl
index 05b6c406206..452c24efff3 100644
--- a/compiler/rustc_parse/messages.ftl
+++ b/compiler/rustc_parse/messages.ftl
@@ -195,6 +195,10 @@ parse_expected_else_block = expected `{"{"}`, found {$first_tok}
     .label = expected an `if` or a block after this `else`
     .suggestion = add an `if` if this is the condition of a chained `else if` statement
 
+parse_expected_struct_field = expected one of `,`, `:`, or `{"}"}`, found `{$token}`
+    .label = expected one of `,`, `:`, or `{"}"}`
+    .ident_label = while parsing this struct field
+
 parse_expected_expression_found_let = expected expression, found `let` statement
     .note = only supported directly in conditions of `if` and `while` expressions
     .not_supported_or = `||` operators are not supported in let chain expressions
diff --git a/compiler/rustc_parse/src/errors.rs b/compiler/rustc_parse/src/errors.rs
index 7c75e440aaa..aeb4fd0a304 100644
--- a/compiler/rustc_parse/src/errors.rs
+++ b/compiler/rustc_parse/src/errors.rs
@@ -431,6 +431,17 @@ pub(crate) struct ExpectedElseBlock {
 }
 
 #[derive(Diagnostic)]
+#[diag(parse_expected_struct_field)]
+pub(crate) struct ExpectedStructField {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+    pub token: Token,
+    #[label(parse_ident_label)]
+    pub ident_span: Span,
+}
+
+#[derive(Diagnostic)]
 #[diag(parse_outer_attribute_not_allowed_on_if_else)]
 pub(crate) struct OuterAttributeNotAllowedOnIfElse {
     #[primary_span]
diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index f4cee3a661e..2f8e3bb6497 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -2834,7 +2834,7 @@ impl<'a> Parser<'a> {
             )?;
             let guard = if this.eat_keyword(kw::If) {
                 let if_span = this.prev_token.span;
-                let mut cond = this.parse_expr_res(Restrictions::ALLOW_LET, None)?;
+                let mut cond = this.parse_match_guard_condition()?;
 
                 CondChecker { parser: this, forbid_let_reason: None }.visit_expr(&mut cond);
 
@@ -2860,9 +2860,9 @@ impl<'a> Parser<'a> {
                 {
                     err.span_suggestion(
                         this.token.span,
-                        "try using a fat arrow here",
+                        "use a fat arrow to start a match arm",
                         "=>",
-                        Applicability::MaybeIncorrect,
+                        Applicability::MachineApplicable,
                     );
                     err.emit();
                     this.bump();
@@ -2979,6 +2979,34 @@ impl<'a> Parser<'a> {
         })
     }
 
+    fn parse_match_guard_condition(&mut self) -> PResult<'a, P<Expr>> {
+        self.parse_expr_res(Restrictions::ALLOW_LET | Restrictions::IN_IF_GUARD, None).map_err(
+            |mut err| {
+                if self.prev_token == token::OpenDelim(Delimiter::Brace) {
+                    let sugg_sp = self.prev_token.span.shrink_to_lo();
+                    // Consume everything within the braces, let's avoid further parse
+                    // errors.
+                    self.recover_stmt_(SemiColonMode::Ignore, BlockMode::Ignore);
+                    let msg = "you might have meant to start a match arm after the match guard";
+                    if self.eat(&token::CloseDelim(Delimiter::Brace)) {
+                        let applicability = if self.token.kind != token::FatArrow {
+                            // We have high confidence that we indeed didn't have a struct
+                            // literal in the match guard, but rather we had some operation
+                            // that ended in a path, immediately followed by a block that was
+                            // meant to be the match arm.
+                            Applicability::MachineApplicable
+                        } else {
+                            Applicability::MaybeIncorrect
+                        };
+                        // self.recover_stmt_(SemiColonMode::Ignore, BlockMode::Ignore);
+                        err.span_suggestion_verbose(sugg_sp, msg, "=> ".to_string(), applicability);
+                    }
+                }
+                err
+            },
+        )
+    }
+
     pub(crate) fn is_builtin(&self) -> bool {
         self.token.is_keyword(kw::Builtin) && self.look_ahead(1, |t| *t == token::Pound)
     }
@@ -3049,9 +3077,10 @@ impl<'a> Parser<'a> {
                     || self.look_ahead(2, |t| t == &token::Colon)
                         && (
                             // `{ ident: token, ` cannot start a block.
-                            self.look_ahead(4, |t| t == &token::Comma) ||
-                // `{ ident: ` cannot start a block unless it's a type ascription `ident: Type`.
-                self.look_ahead(3, |t| !t.can_begin_type())
+                            self.look_ahead(4, |t| t == &token::Comma)
+                                // `{ ident: ` cannot start a block unless it's a type ascription
+                                // `ident: Type`.
+                                || self.look_ahead(3, |t| !t.can_begin_type())
                         )
             )
     }
@@ -3091,6 +3120,7 @@ impl<'a> Parser<'a> {
         let mut fields = ThinVec::new();
         let mut base = ast::StructRest::None;
         let mut recover_async = false;
+        let in_if_guard = self.restrictions.contains(Restrictions::IN_IF_GUARD);
 
         let mut async_block_err = |e: &mut Diagnostic, span: Span| {
             recover_async = true;
@@ -3128,6 +3158,26 @@ impl<'a> Parser<'a> {
                         e.span_label(pth.span, "while parsing this struct");
                     }
 
+                    if let Some((ident, _)) = self.token.ident()
+                        && !self.token.is_reserved_ident()
+                        && self.look_ahead(1, |t| {
+                            AssocOp::from_token(&t).is_some()
+                                || matches!(t.kind, token::OpenDelim(_))
+                                || t.kind == token::Dot
+                        })
+                    {
+                        // Looks like they tried to write a shorthand, complex expression.
+                        e.span_suggestion_verbose(
+                            self.token.span.shrink_to_lo(),
+                            "try naming a field",
+                            &format!("{ident}: ", ),
+                            Applicability::HasPlaceholders,
+                        );
+                    }
+                    if in_if_guard && close_delim == Delimiter::Brace {
+                        return Err(e);
+                    }
+
                     if !recover {
                         return Err(e);
                     }
@@ -3173,19 +3223,6 @@ impl<'a> Parser<'a> {
                                 ",",
                                 Applicability::MachineApplicable,
                             );
-                        } else if is_shorthand
-                            && (AssocOp::from_token(&self.token).is_some()
-                                || matches!(&self.token.kind, token::OpenDelim(_))
-                                || self.token.kind == token::Dot)
-                        {
-                            // Looks like they tried to write a shorthand, complex expression.
-                            let ident = parsed_field.expect("is_shorthand implies Some").ident;
-                            e.span_suggestion(
-                                ident.span.shrink_to_lo(),
-                                "try naming a field",
-                                &format!("{ident}: "),
-                                Applicability::HasPlaceholders,
-                            );
                         }
                     }
                     if !recover {
@@ -3288,6 +3325,24 @@ impl<'a> Parser<'a> {
 
             // Check if a colon exists one ahead. This means we're parsing a fieldname.
             let is_shorthand = !this.look_ahead(1, |t| t == &token::Colon || t == &token::Eq);
+            // Proactively check whether parsing the field will be correct.
+            let is_wrong = this.token.is_ident()
+                && !this.token.is_reserved_ident()
+                && !this.look_ahead(1, |t| {
+                    t == &token::Colon
+                        || t == &token::Eq
+                        || t == &token::Comma
+                        || t == &token::CloseDelim(Delimiter::Brace)
+                        || t == &token::CloseDelim(Delimiter::Parenthesis)
+                });
+            if is_wrong {
+                return Err(errors::ExpectedStructField {
+                    span: this.look_ahead(1, |t| t.span),
+                    ident_span: this.token.span,
+                    token: this.look_ahead(1, |t| t.clone()),
+                }
+                .into_diagnostic(&self.sess.span_diagnostic));
+            }
             let (ident, expr) = if is_shorthand {
                 // Mimic `x: x` for the `x` field shorthand.
                 let ident = this.parse_ident_common(false)?;
diff --git a/compiler/rustc_parse/src/parser/mod.rs b/compiler/rustc_parse/src/parser/mod.rs
index e84d8f5b358..6c24646f39a 100644
--- a/compiler/rustc_parse/src/parser/mod.rs
+++ b/compiler/rustc_parse/src/parser/mod.rs
@@ -52,6 +52,7 @@ bitflags::bitflags! {
         const NO_STRUCT_LITERAL = 1 << 1;
         const CONST_EXPR        = 1 << 2;
         const ALLOW_LET         = 1 << 3;
+        const IN_IF_GUARD       = 1 << 4;
     }
 }
 
diff --git a/tests/ui/parser/issues/issue-15980.rs b/tests/ui/parser/issues/issue-15980.rs
index 87faa7d5ff1..eb7b6ca8296 100644
--- a/tests/ui/parser/issues/issue-15980.rs
+++ b/tests/ui/parser/issues/issue-15980.rs
@@ -9,9 +9,6 @@ fn main(){
             //~^ ERROR expected identifier, found keyword `return`
             //~| NOTE expected identifier, found keyword
         }
-        //~^ NOTE expected one of `.`, `=>`, `?`, or an operator
         _ => {}
-        //~^ ERROR expected one of `.`, `=>`, `?`, or an operator, found reserved identifier `_`
-        //~| NOTE unexpected token
     }
 }
diff --git a/tests/ui/parser/issues/issue-15980.stderr b/tests/ui/parser/issues/issue-15980.stderr
index c59c811199e..cf8d0114787 100644
--- a/tests/ui/parser/issues/issue-15980.stderr
+++ b/tests/ui/parser/issues/issue-15980.stderr
@@ -11,15 +11,10 @@ help: escape `return` to use it as an identifier
    |
 LL |             r#return
    |             ++
-
-error: expected one of `.`, `=>`, `?`, or an operator, found reserved identifier `_`
-  --> $DIR/issue-15980.rs:13:9
+help: you might have meant to start a match arm after the match guard
    |
-LL |         }
-   |          - expected one of `.`, `=>`, `?`, or an operator
-LL |
-LL |         _ => {}
-   |         ^ unexpected token
+LL |         Err(ref e) if e.kind == io::EndOfFile => {
+   |                                               ++
 
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
diff --git a/tests/ui/parser/issues/issue-52496.stderr b/tests/ui/parser/issues/issue-52496.stderr
index 77335c64c21..78c81bf5b0d 100644
--- a/tests/ui/parser/issues/issue-52496.stderr
+++ b/tests/ui/parser/issues/issue-52496.stderr
@@ -8,10 +8,15 @@ error: expected one of `,`, `:`, or `}`, found `.`
   --> $DIR/issue-52496.rs:8:22
    |
 LL |     let _ = Foo { bar.into(), bat: -1, . };
-   |             ---   -  ^ expected one of `,`, `:`, or `}`
+   |             ---   ---^ expected one of `,`, `:`, or `}`
    |             |     |
-   |             |     help: try naming a field: `bar:`
+   |             |     while parsing this struct field
    |             while parsing this struct
+   |
+help: try naming a field
+   |
+LL |     let _ = Foo { bar: bar.into(), bat: -1, . };
+   |                   ++++
 
 error: expected identifier, found `.`
   --> $DIR/issue-52496.rs:8:40
diff --git a/tests/ui/parser/issues/issue-89396.fixed b/tests/ui/parser/issues/issue-89396.fixed
index 823ad8cd1f8..0c040ddea44 100644
--- a/tests/ui/parser/issues/issue-89396.fixed
+++ b/tests/ui/parser/issues/issue-89396.fixed
@@ -8,9 +8,9 @@ fn main() {
     let _ = match opt {
         Some(_) => true,
         //~^ ERROR: expected one of
-        //~| HELP: try using a fat arrow here
+        //~| HELP: use a fat arrow to start a match arm
         None => false,
         //~^ ERROR: expected one of
-        //~| HELP: try using a fat arrow here
+        //~| HELP: use a fat arrow to start a match arm
     };
 }
diff --git a/tests/ui/parser/issues/issue-89396.rs b/tests/ui/parser/issues/issue-89396.rs
index f1d9efa524f..d95f666d797 100644
--- a/tests/ui/parser/issues/issue-89396.rs
+++ b/tests/ui/parser/issues/issue-89396.rs
@@ -8,9 +8,9 @@ fn main() {
     let _ = match opt {
         Some(_) = true,
         //~^ ERROR: expected one of
-        //~| HELP: try using a fat arrow here
+        //~| HELP: use a fat arrow to start a match arm
         None -> false,
         //~^ ERROR: expected one of
-        //~| HELP: try using a fat arrow here
+        //~| HELP: use a fat arrow to start a match arm
     };
 }
diff --git a/tests/ui/parser/issues/issue-89396.stderr b/tests/ui/parser/issues/issue-89396.stderr
index 504420574e2..41ce0705074 100644
--- a/tests/ui/parser/issues/issue-89396.stderr
+++ b/tests/ui/parser/issues/issue-89396.stderr
@@ -5,7 +5,7 @@ LL |         Some(_) = true,
    |                 ^
    |                 |
    |                 expected one of `=>`, `if`, or `|`
-   |                 help: try using a fat arrow here: `=>`
+   |                 help: use a fat arrow to start a match arm: `=>`
 
 error: expected one of `=>`, `@`, `if`, or `|`, found `->`
   --> $DIR/issue-89396.rs:12:14
@@ -14,7 +14,7 @@ LL |         None -> false,
    |              ^^
    |              |
    |              expected one of `=>`, `@`, `if`, or `|`
-   |              help: try using a fat arrow here: `=>`
+   |              help: use a fat arrow to start a match arm: `=>`
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/parser/missing-fat-arrow.rs b/tests/ui/parser/missing-fat-arrow.rs
new file mode 100644
index 00000000000..fef16129cd0
--- /dev/null
+++ b/tests/ui/parser/missing-fat-arrow.rs
@@ -0,0 +1,41 @@
+fn main() {
+    let x = 1;
+    let y = 2;
+    let value = 3;
+
+    match value {
+        Some(x) if x == y {
+            self.next_token()?; //~ ERROR expected identifier, found keyword `self`
+            Ok(true)
+        },
+        _ => {
+            Ok(false)
+        }
+    }
+    let _: i32 = (); //~ ERROR mismatched types
+}
+
+struct Foo {
+    value: usize
+}
+
+fn foo(a: Option<&mut Foo>, b: usize) {
+    match a {
+        Some(a) if a.value == b {
+            a.value = 1; //~ ERROR expected one of `,`, `:`, or `}`, found `.`
+        },
+        _ => {}
+    }
+    let _: i32 = (); //~ ERROR mismatched types
+}
+
+fn bar(a: Option<&mut Foo>, b: usize) {
+    match a {
+        Some(a) if a.value == b {
+            a.value, //~ ERROR expected one of `,`, `:`, or `}`, found `.`
+        } => {
+        }
+        _ => {}
+    }
+    let _: i32 = (); //~ ERROR mismatched types
+}
diff --git a/tests/ui/parser/missing-fat-arrow.stderr b/tests/ui/parser/missing-fat-arrow.stderr
new file mode 100644
index 00000000000..abfed7a5683
--- /dev/null
+++ b/tests/ui/parser/missing-fat-arrow.stderr
@@ -0,0 +1,78 @@
+error: expected identifier, found keyword `self`
+  --> $DIR/missing-fat-arrow.rs:8:13
+   |
+LL |         Some(x) if x == y {
+   |                         - while parsing this struct
+LL |             self.next_token()?;
+   |             ^^^^ expected identifier, found keyword
+   |
+help: you might have meant to start a match arm after the match guard
+   |
+LL |         Some(x) if x == y => {
+   |                           ++
+
+error: expected one of `,`, `:`, or `}`, found `.`
+  --> $DIR/missing-fat-arrow.rs:25:14
+   |
+LL |         Some(a) if a.value == b {
+   |                               - while parsing this struct
+LL |             a.value = 1;
+   |             -^ expected one of `,`, `:`, or `}`
+   |             |
+   |             while parsing this struct field
+   |
+help: try naming a field
+   |
+LL |             a: a.value = 1;
+   |             ++
+help: you might have meant to start a match arm after the match guard
+   |
+LL |         Some(a) if a.value == b => {
+   |                                 ++
+
+error: expected one of `,`, `:`, or `}`, found `.`
+  --> $DIR/missing-fat-arrow.rs:35:14
+   |
+LL |         Some(a) if a.value == b {
+   |                               - while parsing this struct
+LL |             a.value,
+   |             -^ expected one of `,`, `:`, or `}`
+   |             |
+   |             while parsing this struct field
+   |
+help: try naming a field
+   |
+LL |             a: a.value,
+   |             ++
+help: you might have meant to start a match arm after the match guard
+   |
+LL |         Some(a) if a.value == b => {
+   |                                 ++
+
+error[E0308]: mismatched types
+  --> $DIR/missing-fat-arrow.rs:15:18
+   |
+LL |     let _: i32 = ();
+   |            ---   ^^ expected `i32`, found `()`
+   |            |
+   |            expected due to this
+
+error[E0308]: mismatched types
+  --> $DIR/missing-fat-arrow.rs:29:18
+   |
+LL |     let _: i32 = ();
+   |            ---   ^^ expected `i32`, found `()`
+   |            |
+   |            expected due to this
+
+error[E0308]: mismatched types
+  --> $DIR/missing-fat-arrow.rs:40:18
+   |
+LL |     let _: i32 = ();
+   |            ---   ^^ expected `i32`, found `()`
+   |            |
+   |            expected due to this
+
+error: aborting due to 6 previous errors
+
+For more information about this error, try `rustc --explain E0308`.
diff --git a/tests/ui/parser/removed-syntax-with-2.stderr b/tests/ui/parser/removed-syntax-with-2.stderr
index c6ae1ce674f..e75c5bcd643 100644
--- a/tests/ui/parser/removed-syntax-with-2.stderr
+++ b/tests/ui/parser/removed-syntax-with-2.stderr
@@ -2,8 +2,9 @@ error: expected one of `,`, `:`, or `}`, found `a`
   --> $DIR/removed-syntax-with-2.rs:8:31
    |
 LL |     let b = S { foo: (), with a };
-   |             -                 ^ expected one of `,`, `:`, or `}`
-   |             |
+   |             -            ---- ^ expected one of `,`, `:`, or `}`
+   |             |            |
+   |             |            while parsing this struct field
    |             while parsing this struct
 
 error[E0063]: missing field `bar` in initializer of `S`