Don't emit "field expressions may not have generic arguments" if it's a method call without ()

Author: Orion Gonzalez

Diff for: compiler/rustc_errors/src/lib.rs

@@ -576,6 +576,10 @@ pub enum StashKey {
     UndeterminedMacroResolution,
     /// Used by `Parser::maybe_recover_trailing_expr`
     ExprInPat,
+    /// If in the parser we detect a field expr with turbofish generic params it's possible that
+    /// it's a method call without parens. If later on in `hir_typeck` we find out that this is
+    /// the case we suppress this message and we give a better suggestion.
+    GenericInFieldExpr,
 }
 
 fn default_track_diagnostic<R>(diag: DiagInner, f: &mut dyn FnMut(DiagInner) -> R) -> R {

Diff for: compiler/rustc_hir_typeck/src/expr.rs

@@ -3055,7 +3055,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             err.help("methods are immutable and cannot be assigned to");
         }
 
-        err.emit()
+        // See `StashKey::GenericInFieldExpr` for more info
+        self.dcx().try_steal_replace_and_emit_err(field.span, StashKey::GenericInFieldExpr, err)
     }
 
     fn point_at_param_definition(&self, err: &mut Diag<'_>, param: ty::ParamTy) {

Diff for: compiler/rustc_parse/src/parser/expr.rs

@@ -1369,11 +1369,14 @@ impl<'a> Parser<'a> {
             ))
         } else {
             // Field access `expr.f`
+            let span = lo.to(self.prev_token.span);
             if let Some(args) = seg.args {
-                self.dcx().emit_err(errors::FieldExpressionWithGeneric(args.span()));
+                // See `StashKey::GenericInFieldExpr` for more info on why we stash this.
+                self.dcx()
+                    .create_err(errors::FieldExpressionWithGeneric(args.span()))
+                    .stash(seg.ident.span, StashKey::GenericInFieldExpr);
             }
 
-            let span = lo.to(self.prev_token.span);
             Ok(self.mk_expr(span, ExprKind::Field(self_arg, seg.ident)))
         }
     }

Diff for: tests/ui/parser/bad-name.stderr

@@ -1,9 +1,3 @@
-error: field expressions cannot have generic arguments
-  --> $DIR/bad-name.rs:2:12
-   |
-LL |   let x.y::<isize>.z foo;
-   |            ^^^^^^^
-
 error: expected a pattern, found an expression
   --> $DIR/bad-name.rs:2:7
    |
@@ -18,5 +12,11 @@ error: expected one of `(`, `.`, `::`, `:`, `;`, `=`, `?`, `|`, or an operator,
 LL |   let x.y::<isize>.z foo;
    |                      ^^^ expected one of 9 possible tokens
 
+error: field expressions cannot have generic arguments
+  --> $DIR/bad-name.rs:2:12
+   |
+LL |   let x.y::<isize>.z foo;
+   |            ^^^^^^^
+
 error: aborting due to 3 previous errors
 

Diff for: tests/ui/suggestions/method-missing-parentheses.rs

@@ -1,5 +1,4 @@
 fn main() {
     let _ = vec![].into_iter().collect::<usize>;
     //~^ ERROR attempted to take value of method `collect` on type `std::vec::IntoIter<_>`
-    //~| ERROR field expressions cannot have generic arguments
 }

Diff for: tests/ui/suggestions/method-missing-parentheses.stderr

@@ -1,9 +1,3 @@
-error: field expressions cannot have generic arguments
-  --> $DIR/method-missing-parentheses.rs:2:41
-   |
-LL |     let _ = vec![].into_iter().collect::<usize>;
-   |                                         ^^^^^^^
-
 error[E0615]: attempted to take value of method `collect` on type `std::vec::IntoIter<_>`
   --> $DIR/method-missing-parentheses.rs:2:32
    |
@@ -15,6 +9,6 @@ help: use parentheses to call the method
 LL |     let _ = vec![].into_iter().collect::<usize>();
    |                                                ++
 
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
 For more information about this error, try `rustc --explain E0615`.