field does not exist error: note fields if Levenshtein suggestion fails

When trying to access or initialize a nonexistent field, if we can't infer what
field was meant (by virtue of the purported field in the source being a small
Levenshtein distance away from an actual field, suggestive of a typo), issue a
note listing all the available fields. To reduce terminal clutter, we don't
issue the note when we have a `find_best_match_for_name` Levenshtein
suggestion: the suggestion is probably right.

The third argument of the call to `find_best_match_for_name` is changed to
`None`, accepting the default maximum Levenshtein distance of one-third of the
identifier supplied for correction. The previous value of `Some(name.len())`
was overzealous, inappropriately very Levenshtein-distant suggestions when the
attempted field access could not plausibly be a mere typo. For example, if a
struct has fields `mule` and `phone`, but I type `.donkey`, I'd rather the
error have a note listing that the available fields are, in fact, `mule` and
`phone` (which is the behavior induced by this patch) rather than the error
asking "did you mean `phone`?" (which is the behavior on master). The "only
find fits with at least one matching letter" comment was accurate when it was
first introduced in 09d992471 (January 2015), but is a vicious lie in its
present context before a call to `find_best_match_for_name` and must be
destroyed (replacing every letter is a Levenshtein distance of name.len()).

The present author claims that this suffices to resolve #42599.

src/test/ui/did_you_mean/issue-36798_unknown_field.stderr

@@ -3,6 +3,8 @@ error[E0609]: no field `zz` on type `Foo`
    |
 17 |     f.zz;
    |       ^^ unknown field
+   |
+   = note: available fields are: bar
 
 error: aborting due to previous error
 

src/test/ui/did_you_mean/issue-42599_available_fields_note.rs

@@ -0,0 +1,39 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+mod submodule {
+
+    #[derive(Default)]
+    pub struct Demo {
+        pub favorite_integer: isize,
+        secret_integer: isize,
+        pub innocently_misspellable: ()
+    }
+
+    impl Demo {
+        fn new_with_secret_two() -> Self {
+            Self { secret_integer: 2, inocently_mispellable: () }
+        }
+
+        fn new_with_secret_three() -> Self {
+            Self { secret_integer: 3, egregiously_nonexistent_field: () }
+        }
+    }
+
+}
+
+fn main() {
+    use submodule::Demo;
+
+    let demo = Demo::default();
+    let innocent_field_misaccess = demo.inocently_mispellable;
+    // note shouldn't suggest private `secret_integer` field
+    let egregious_field_misaccess = demo.egregiously_nonexistent_field;
+}

src/test/ui/did_you_mean/issue-42599_available_fields_note.stderr

@@ -0,0 +1,30 @@
+error[E0560]: struct `submodule::Demo` has no field named `inocently_mispellable`
+  --> $DIR/issue-42599_available_fields_note.rs:22:39
+   |
+22 |             Self { secret_integer: 2, inocently_mispellable: () }
+   |                                       ^^^^^^^^^^^^^^^^^^^^^^ field does not exist - did you mean `innocently_misspellable`?
+
+error[E0560]: struct `submodule::Demo` has no field named `egregiously_nonexistent_field`
+  --> $DIR/issue-42599_available_fields_note.rs:26:39
+   |
+26 |             Self { secret_integer: 3, egregiously_nonexistent_field: () }
+   |                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `submodule::Demo` does not have this field
+   |
+   = note: available fields are: favorite_integer, secret_integer, innocently_misspellable
+
+error[E0609]: no field `inocently_mispellable` on type `submodule::Demo`
+  --> $DIR/issue-42599_available_fields_note.rs:36:41
+   |
+36 |     let innocent_field_misaccess = demo.inocently_mispellable;
+   |                                         ^^^^^^^^^^^^^^^^^^^^^ did you mean `innocently_misspellable`?
+
+error[E0609]: no field `egregiously_nonexistent_field` on type `submodule::Demo`
+  --> $DIR/issue-42599_available_fields_note.rs:38:42
+   |
+38 |     let egregious_field_misaccess = demo.egregiously_nonexistent_field;
+   |                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unknown field
+   |
+   = note: available fields are: favorite_integer, innocently_misspellable
+
+error: aborting due to 4 previous errors
+