Auto merge of #50760 - petrochenkov:legimp, r=nikomatsakis

Turn deprecation lint `legacy_imports` into a hard error

Closes https://github.com/rust-lang/rust/issues/38260

The lint was introduced in Dec 2016, then made deny-by-default in Jun 2017 when crater run found 0 regressions caused by it.

This lint requires some not entirely trivial amount of import resolution logic that (surprisingly or not) interacts with `feature(use_extern_macros)` (https://github.com/rust-lang/rust/issues/35896), so it would be desirable to remove it before stabilizing `use_extern_macros`.
In particular, this PR fixes the failing example in https://github.com/rust-lang/rust/issues/50725 (but not the whole issue, `use std::panic::{self}` still can cause other undesirable errors when `use_extern_macros` is enabled).

src/test/compile-fail/issue-38293.rs

@@ -10,23 +10,17 @@
 
 // Test that `fn foo::bar::{self}` only imports `bar` in the type namespace.
 
-#![allow(unused)]
-
 mod foo {
     pub fn f() { }
 }
-use foo::f::{self};
-//~^ ERROR `self` no longer imports values
-//~| WARN hard error
+use foo::f::{self}; //~ ERROR unresolved import `foo::f`
 
 mod bar {
     pub fn baz() {}
     pub mod baz {}
 }
 use bar::baz::{self};
-//~^ ERROR `self` no longer imports values
-//~| WARN hard error
 
 fn main() {
-    baz();
+    baz(); //~ ERROR expected function, found module `baz`
 }

src/test/ui/imports/rfc-1560-warning-cycle.rs

@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![allow(unused)]
-
 pub struct Foo;
 
 mod bar {
@@ -18,9 +16,7 @@ mod bar {
     mod baz {
         use *;
         use bar::*;
-        fn f(_: Foo) {}
-        //~^ ERROR `Foo` is ambiguous
-        //~| WARN hard error in a future release
+        fn f(_: Foo) {} //~ ERROR `Foo` is ambiguous
     }
 }
 

src/test/ui/imports/rfc-1560-warning-cycle.stderr

@@ -1,16 +1,21 @@
-error: `Foo` is ambiguous
-  --> $DIR/rfc-1560-warning-cycle.rs:21:17
+error[E0659]: `Foo` is ambiguous
+  --> $DIR/rfc-1560-warning-cycle.rs:19:17
    |
-LL |         use *;
-   |             - `Foo` could refer to the name imported here
-LL |         use bar::*;
-   |             ------ `Foo` could also refer to the name imported here
-LL |         fn f(_: Foo) {}
+LL |         fn f(_: Foo) {} //~ ERROR `Foo` is ambiguous
    |                 ^^^
    |
-   = note: #[deny(legacy_imports)] on by default
-   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #38260 <https://github.com/rust-lang/rust/issues/38260>
+note: `Foo` could refer to the name imported here
+  --> $DIR/rfc-1560-warning-cycle.rs:17:13
+   |
+LL |         use *;
+   |             ^
+note: `Foo` could also refer to the name imported here
+  --> $DIR/rfc-1560-warning-cycle.rs:18:13
+   |
+LL |         use bar::*;
+   |             ^^^^^^
+   = note: consider adding an explicit import of `Foo` to disambiguate
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0659`.