From aba56ddd05d821b6f0a3e5fc05bc47311e09051c Mon Sep 17 00:00:00 2001
From: "Zack M. Davis" <code@zackmdavis.net>
Date: Sun, 19 Nov 2017 11:25:35 -0800
Subject: type error method suggestions use whitelisted identity-like
 conversions
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Previously, on a type mismatch (and if this wasn't preëmpted by a
higher-priority suggestion), we would look for argumentless methods
returning the expected type, and list them in a `help` note.

This had two major shortcomings. Firstly, a lot of the suggestions didn't
really make sense (if you used a &str where a String was expected,
`.to_ascii_uppercase()` is probably not the solution you were hoping
for). Secondly, we weren't generating suggestions from the most useful
traits!

We address the first problem with an internal
`#[rustc_conversion_suggestion]` attribute meant to mark methods that keep
the "same value" in the relevant sense, just converting the type. We
address the second problem by making `FnCtxt.probe_for_return_type` pass
the `ProbeScope::AllTraits` to `probe_op`: this would seem to be safe
because grep reveals no other callers of `probe_for_return_type`.

Also, structured suggestions are preferred (because they're pretty, but
also for RLS and friends).

Also also, we make the E0055 autoderef recursion limit error use the
one-time-diagnostics set, because we can potentially hit the limit a lot
during probing. (Without this,
test/ui/did_you_mean/recursion_limit_deref.rs would report "aborting due to
51 errors").

Unfortunately, the trait probing is still not all one would hope for: at a
minimum, we don't know how to rule out `into()` in cases where it wouldn't
actually work, and we don't know how to rule in `.to_owned()` where it
would. Issues #46459 and #46460 have been filed and are ref'd in a FIXME.

This is hoped to resolve #42929, #44672, and #45777.
---
 src/test/ui/span/coerce-suggestions.stderr | 8 ++++----
 src/test/ui/span/issue-34264.stderr        | 2 --
 2 files changed, 4 insertions(+), 6 deletions(-)

(limited to 'src/test/ui/span')

diff --git a/src/test/ui/span/coerce-suggestions.stderr b/src/test/ui/span/coerce-suggestions.stderr
index 07852619765..06f0e6ec228 100644
--- a/src/test/ui/span/coerce-suggestions.stderr
+++ b/src/test/ui/span/coerce-suggestions.stderr
@@ -6,9 +6,6 @@ error[E0308]: mismatched types
    |
    = note: expected type `usize`
               found type `std::string::String`
-   = help: here are some functions which might fulfill your needs:
-           - .capacity()
-           - .len()
 
 error[E0308]: mismatched types
   --> $DIR/coerce-suggestions.rs:19:19
@@ -44,7 +41,10 @@ error[E0308]: mismatched types
   --> $DIR/coerce-suggestions.rs:27:9
    |
 27 |     f = box f;
-   |         ^^^^^ cyclic type of infinite size
+   |         ^^^^^
+   |         |
+   |         cyclic type of infinite size
+   |         help: try using a conversion method: `box f.to_string()`
 
 error[E0308]: mismatched types
   --> $DIR/coerce-suggestions.rs:31:9
diff --git a/src/test/ui/span/issue-34264.stderr b/src/test/ui/span/issue-34264.stderr
index 3794d6ba2de..18860a7456e 100644
--- a/src/test/ui/span/issue-34264.stderr
+++ b/src/test/ui/span/issue-34264.stderr
@@ -33,8 +33,6 @@ error[E0308]: mismatched types
    |
    = note: expected type `usize`
               found type `&'static str`
-   = help: here are some functions which might fulfill your needs:
-           - .len()
 
 error[E0061]: this function takes 2 parameters but 3 parameters were supplied
   --> $DIR/issue-34264.rs:20:5
-- 
cgit 1.4.1-3-g733a5