Tweak wording when encountering fn call in pattern

Author: estebank

src/librustc_typeck/check/_match.rs

@@ -1080,8 +1080,18 @@ https://doc.rust-lang.org/reference/types.html#trait-objects");
             let msg = format!("expected tuple struct/variant, found {} `{}`",
                               res.descr(),
                               hir::print::to_string(tcx.hir(), |s| s.print_qpath(qpath, false)));
-            struct_span_err!(tcx.sess, pat.span, E0164, "{}", msg)
-                .span_label(pat.span, "not a tuple variant or struct").emit();
+            let mut err = struct_span_err!(tcx.sess, pat.span, E0164, "{}", msg);
+            match (res, &pat.node) {
+                (Res::Def(DefKind::Fn, _), _) | (Res::Def(DefKind::Method, _), _) => {
+                    err.span_label(pat.span, "`fn` calls are not allowed in patterns");
+                    err.help("for more information, visit \
+                              https://doc.rust-lang.org/book/ch18-00-patterns.html");
+                }
+                _ => {
+                    err.span_label(pat.span, "not a tuple variant or struct");
+                }
+            }
+            err.emit();
             on_error();
         };
 

src/test/ui/fn-in-pat.stderr

@@ -2,7 +2,9 @@ error[E0164]: expected tuple struct/variant, found method `<A>::new`
   --> $DIR/fn-in-pat.rs:11:9
    |
 LL |         A::new() => (),
-   |         ^^^^^^^^ not a tuple variant or struct
+   |         ^^^^^^^^ `fn` calls are not allowed in patterns
+   |
+   = help: for more information, visit https://doc.rust-lang.org/book/ch18-00-patterns.html
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-55587.stderr

@@ -2,7 +2,9 @@ error[E0164]: expected tuple struct/variant, found method `<Path>::new`
   --> $DIR/issue-55587.rs:4:9
    |
 LL |     let Path::new();
-   |         ^^^^^^^^^^^ not a tuple variant or struct
+   |         ^^^^^^^^^^^ `fn` calls are not allowed in patterns
+   |
+   = help: for more information, visit https://doc.rust-lang.org/book/ch18-00-patterns.html
 
 error: aborting due to previous error
 

src/test/ui/match/match-fn-call.stderr

@@ -2,13 +2,17 @@ error[E0164]: expected tuple struct/variant, found method `<Path>::new`
   --> $DIR/match-fn-call.rs:6:9
    |
 LL |         Path::new("foo") => println!("foo"),
-   |         ^^^^^^^^^^^^^^^^ not a tuple variant or struct
+   |         ^^^^^^^^^^^^^^^^ `fn` calls are not allowed in patterns
+   |
+   = help: for more information, visit https://doc.rust-lang.org/book/ch18-00-patterns.html
 
 error[E0164]: expected tuple struct/variant, found method `<Path>::new`
   --> $DIR/match-fn-call.rs:8:9
    |
 LL |         Path::new("bar") => println!("bar"),
-   |         ^^^^^^^^^^^^^^^^ not a tuple variant or struct
+   |         ^^^^^^^^^^^^^^^^ `fn` calls are not allowed in patterns
+   |
+   = help: for more information, visit https://doc.rust-lang.org/book/ch18-00-patterns.html
 
 error: aborting due to 2 previous errors