Auto merge of #29620 - petrochenkov:reachable2, r=alexcrichton

Handle them in `middle::reachable` instead (no optimizations so far, just drop all trait impl items into the reachable set, as before). Addresses the concerns from https://github.com/rust-lang/rust/pull/29291#discussion_r43672413
\+ In `middle::reachable` don't treat impls of `Drop` specially, they are subsumed by the general impl treatment.
\+ Add some tests checking reachability of trait methods written in UFCS form
\+ Minor refactoring in the second commit

r? @alexcrichton

src/test/auxiliary/issue-11225-1.rs

@@ -11,13 +11,18 @@
 mod inner {
     pub trait Trait {
         fn f(&self) { f(); }
+        fn f_ufcs(&self) { f_ufcs(); }
     }
 
     impl Trait for isize {}
 
     fn f() {}
+    fn f_ufcs() {}
 }
 
 pub fn foo<T: inner::Trait>(t: T) {
     t.f();
 }
+pub fn foo_ufcs<T: inner::Trait>(t: T) {
+    T::f_ufcs(&t);
+}

src/test/auxiliary/issue-11225-2.rs

@@ -14,15 +14,18 @@ mod inner {
     pub struct Foo;
     pub trait Trait {
         fn f(&self);
+        fn f_ufcs(&self);
     }
 
     impl Trait for Foo {
         fn f(&self) { }
+        fn f_ufcs(&self) { }
     }
 }
 
 pub trait Outer {
     fn foo<T: Trait>(&self, t: T) { t.f(); }
+    fn foo_ufcs<T: Trait>(&self, t: T) { T::f(&t); }
 }
 
 impl Outer for isize {}
@@ -30,3 +33,6 @@ impl Outer for isize {}
 pub fn foo<T: Outer>(t: T) {
     t.foo(inner::Foo);
 }
+pub fn foo_ufcs<T: Outer>(t: T) {
+    T::foo_ufcs(&t, inner::Foo)
+}

src/test/auxiliary/issue-11225-3.rs

@@ -10,20 +10,29 @@
 
 trait PrivateTrait {
     fn private_trait_method(&self);
+    fn private_trait_method_ufcs(&self);
 }
 
 struct PrivateStruct;
 
 impl PrivateStruct {
     fn private_inherent_method(&self) { }
+    fn private_inherent_method_ufcs(&self) { }
 }
 
 impl PrivateTrait for PrivateStruct {
     fn private_trait_method(&self) { }
+    fn private_trait_method_ufcs(&self) { }
 }
 
 #[inline]
-pub fn public_generic_function() {
+pub fn public_inlinable_function() {
     PrivateStruct.private_trait_method();
     PrivateStruct.private_inherent_method();
 }
+
+#[inline]
+pub fn public_inlinable_function_ufcs() {
+    PrivateStruct::private_trait_method(&PrivateStruct);
+    PrivateStruct::private_inherent_method(&PrivateStruct);
+}

src/test/run-pass/issue-11225-1.rs

@@ -16,4 +16,5 @@ extern crate issue_11225_1 as foo;
 
 pub fn main() {
     foo::foo(1);
+    foo::foo_ufcs(1);
 }

src/test/run-pass/issue-11225-2.rs

@@ -16,4 +16,5 @@ extern crate issue_11225_2 as foo;
 
 pub fn main() {
     foo::foo(1);
+    foo::foo_ufcs(1);
 }

src/test/run-pass/issue-11225-3.rs

@@ -15,5 +15,6 @@
 extern crate issue_11225_3;
 
 pub fn main() {
-    issue_11225_3::public_generic_function();
+    issue_11225_3::public_inlinable_function();
+    issue_11225_3::public_inlinable_function_ufcs();
 }