type error method suggestions use whitelisted identity-like conversions

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.

1 files changed, 7 insertions, 0 deletions

diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index e5ef9393e7b..d3e78b3be26 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -966,6 +966,13 @@ pub const BUILTIN_ATTRIBUTES: &'static [(&'static str, AttributeType, AttributeG
                                       never be stable",
                                      cfg_fn!(rustc_attrs))),
 
+    // whitelists "identity-like" conversion methods to suggest on type mismatch
+    ("rustc_conversion_suggestion", Whitelisted, Gated(Stability::Unstable,
+                                                       "rustc_attrs",
+                                                       "this is an internal attribute that will \
+                                                        never be stable",
+                                                       cfg_fn!(rustc_attrs))),
+
     ("wasm_import_memory", Whitelisted, Gated(Stability::Unstable,
                                  "wasm_import_memory",
                                  "wasm_import_memory attribute is currently unstable",