Detect missing . in method chain in let bindings and statements

compiler/rustc_parse/src/parser/stmt.rs

@@ -745,6 +745,51 @@ impl<'a> Parser<'a> {
         Ok(self.mk_block(stmts, s, lo.to(self.prev_token.span)))
     }
 
+    fn recover_missing_dot(&mut self, err: &mut Diag<'_>) {
+        let Some((ident, _)) = self.token.ident() else {
+            return;
+        };
+        if let Some(c) = ident.name.as_str().chars().next()
+            && c.is_uppercase()
+        {
+            return;
+        }
+        if self.token.is_reserved_ident() && !self.token.is_ident_named(kw::Await) {
+            return;
+        }
+        if self.prev_token.is_reserved_ident() && self.prev_token.is_ident_named(kw::Await) {
+            // Likely `foo.await bar`
+        } else if !self.prev_token.is_reserved_ident() && self.prev_token.is_ident() {
+            // Likely `foo bar`
+        } else if self.prev_token.kind == token::Question {
+            // `foo? bar`
+        } else if self.prev_token.kind == token::CloseDelim(Delimiter::Parenthesis) {
+            // `foo() bar`
+        } else {
+            return;
+        }
+        if self.token.span == self.prev_token.span {
+            // Account for syntax errors in proc-macros.
+            return;
+        }
+        if self.look_ahead(1, |t| [token::Semi, token::Question, token::Dot].contains(&t.kind)) {
+            err.span_suggestion_verbose(
+                self.prev_token.span.between(self.token.span),
+                "you might have meant to write a field access",
+                ".".to_string(),
+                Applicability::MaybeIncorrect,
+            );
+        }
+        if self.look_ahead(1, |t| t.kind == token::OpenDelim(Delimiter::Parenthesis)) {
+            err.span_suggestion_verbose(
+                self.prev_token.span.between(self.token.span),
+                "you might have meant to write a method call",
+                ".".to_string(),
+                Applicability::MaybeIncorrect,
+            );
+        }
+    }
+
     /// Parses a statement, including the trailing semicolon.
     pub fn parse_full_stmt(
         &mut self,
@@ -851,7 +896,8 @@ impl<'a> Parser<'a> {
                             Some(if recover.no() {
                                 res?
                             } else {
-                                res.unwrap_or_else(|e| {
+                                res.unwrap_or_else(|mut e| {
+                                    self.recover_missing_dot(&mut e);
                                     let guar = e.emit();
                                     self.recover_stmt();
                                     guar
@@ -872,7 +918,12 @@ impl<'a> Parser<'a> {
                 // We might be at the `,` in `let x = foo<bar, baz>;`. Try to recover.
                 match &mut local.kind {
                     LocalKind::Init(expr) | LocalKind::InitElse(expr, _) => {
-                        self.check_mistyped_turbofish_with_multiple_type_params(e, expr)?;
+                        self.check_mistyped_turbofish_with_multiple_type_params(e, expr).map_err(
+                            |mut e| {
+                                self.recover_missing_dot(&mut e);
+                                e
+                            },
+                        )?;
                         // We found `foo<bar, baz>`, have we fully recovered?
                         self.expect_semi()?;
                     }

tests/ui/parser/recover/missing-dot-on-statement-expression.fixed

@@ -0,0 +1,28 @@
+//@ run-rustfix
+#![allow(unused_must_use, dead_code)]
+struct S {
+    field: (),
+}
+fn main() {
+    let _ = [1, 2, 3].iter().map(|x| x); //~ ERROR expected one of `.`, `;`, `?`, `else`, or an operator, found `map`
+    //~^ HELP you might have meant to write a method call
+}
+fn foo() {
+    let baz = S {
+        field: ()
+    };
+    let _ = baz.field; //~ ERROR expected one of `!`, `.`, `::`, `;`, `?`, `else`, `{`, or an operator, found `field`
+    //~^ HELP you might have meant to write a field
+}
+
+fn bar() {
+    [1, 2, 3].iter().map(|x| x); //~ ERROR expected one of `.`, `;`, `?`, `}`, or an operator, found `map`
+    //~^ HELP you might have meant to write a method call
+}
+fn baz() {
+    let baz = S {
+        field: ()
+    };
+    baz.field; //~ ERROR expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `field`
+    //~^ HELP you might have meant to write a field
+}

tests/ui/parser/recover/missing-dot-on-statement-expression.rs

@@ -0,0 +1,28 @@
+//@ run-rustfix
+#![allow(unused_must_use, dead_code)]
+struct S {
+    field: (),
+}
+fn main() {
+    let _ = [1, 2, 3].iter()map(|x| x); //~ ERROR expected one of `.`, `;`, `?`, `else`, or an operator, found `map`
+    //~^ HELP you might have meant to write a method call
+}
+fn foo() {
+    let baz = S {
+        field: ()
+    };
+    let _ = baz field; //~ ERROR expected one of `!`, `.`, `::`, `;`, `?`, `else`, `{`, or an operator, found `field`
+    //~^ HELP you might have meant to write a field
+}
+
+fn bar() {
+    [1, 2, 3].iter()map(|x| x); //~ ERROR expected one of `.`, `;`, `?`, `}`, or an operator, found `map`
+    //~^ HELP you might have meant to write a method call
+}
+fn baz() {
+    let baz = S {
+        field: ()
+    };
+    baz field; //~ ERROR expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `field`
+    //~^ HELP you might have meant to write a field
+}

tests/ui/parser/recover/missing-dot-on-statement-expression.stderr

@@ -0,0 +1,46 @@
+error: expected one of `.`, `;`, `?`, `else`, or an operator, found `map`
+  --> $DIR/missing-dot-on-statement-expression.rs:7:29
+   |
+LL |     let _ = [1, 2, 3].iter()map(|x| x);
+   |                             ^^^ expected one of `.`, `;`, `?`, `else`, or an operator
+   |
+help: you might have meant to write a method call
+   |
+LL |     let _ = [1, 2, 3].iter().map(|x| x);
+   |                             +
+
+error: expected one of `!`, `.`, `::`, `;`, `?`, `else`, `{`, or an operator, found `field`
+  --> $DIR/missing-dot-on-statement-expression.rs:14:17
+   |
+LL |     let _ = baz field;
+   |                 ^^^^^ expected one of 8 possible tokens
+   |
+help: you might have meant to write a field access
+   |
+LL |     let _ = baz.field;
+   |                +
+
+error: expected one of `.`, `;`, `?`, `}`, or an operator, found `map`
+  --> $DIR/missing-dot-on-statement-expression.rs:19:21
+   |
+LL |     [1, 2, 3].iter()map(|x| x);
+   |                     ^^^ expected one of `.`, `;`, `?`, `}`, or an operator
+   |
+help: you might have meant to write a method call
+   |
+LL |     [1, 2, 3].iter().map(|x| x);
+   |                     +
+
+error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `field`
+  --> $DIR/missing-dot-on-statement-expression.rs:26:9
+   |
+LL |     baz field;
+   |         ^^^^^ expected one of 8 possible tokens
+   |
+help: you might have meant to write a field access
+   |
+LL |     baz.field;
+   |        +
+
+error: aborting due to 4 previous errors
+

tests/ui/suggestions/type-ascription-and-other-error.stderr

@@ -3,6 +3,11 @@ error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found
    |
 LL |     not rust;
    |         ^^^^ expected one of 8 possible tokens
+   |
+help: you might have meant to write a field access
+   |
+LL |     not.rust;
+   |        +
 
 error: aborting due to 1 previous error