Do not suggest use over extern crate w/ alias.

This commit stops `unused_extern_crates` lints from occuring on `extern
crate` statements that alias the crate as the suggestion to change to a
`use` statement would result in the aliased name no longer being added
to the prelude, thereby causing compilation errors if other imports
expected this to be the case.

11 files changed, 41 insertions, 66 deletions

diff --git a/src/librustc_typeck/check_unused.rs b/src/librustc_typeck/check_unused.rs
index cbb6d9b29f5..3c3509f24ce 100644
--- a/src/librustc_typeck/check_unused.rs
+++ b/src/librustc_typeck/check_unused.rs
@@ -158,6 +158,13 @@ fn unused_crates_lint<'tcx>(tcx: TyCtxt<'_, 'tcx, 'tcx>) {
             continue;
         }
 
+        // If the extern crate is renamed, then we cannot suggest replacing it with a use as this
+        // would not insert the new name into the prelude, where other imports in the crate may be
+        // expecting it.
+        if extern_crate.orig_name.is_some() {
+            continue;
+        }
+
         // If the extern crate has any attributes, they may have funky
         // semantics we can't faithfully represent using `use` (most
         // notably `#[macro_use]`). Ignore it.
diff --git a/src/test/ui/imports/extern-crate-used.rs b/src/test/ui/imports/extern-crate-used.rs
index 2d91cbc00f2..26150c7d4a1 100644
--- a/src/test/ui/imports/extern-crate-used.rs
+++ b/src/test/ui/imports/extern-crate-used.rs
@@ -5,10 +5,14 @@
 
 #![deny(unused_extern_crates)]
 
-extern crate core as iso1; //~ ERROR `extern crate` is not idiomatic in the new edition
-extern crate core as iso2; //~ ERROR `extern crate` is not idiomatic in the new edition
-extern crate core as iso3; //~ ERROR `extern crate` is not idiomatic in the new edition
-extern crate core as iso4; //~ ERROR `extern crate` is not idiomatic in the new edition
+// Shouldn't suggest changing to `use`, as new name
+// would no longer be added to the prelude which could cause
+// compilation errors for imports that use the new name in
+// other modules. See #57672.
+extern crate core as iso1;
+extern crate core as iso2;
+extern crate core as iso3;
+extern crate core as iso4;
 
 // Doesn't introduce its extern prelude entry, so it's still considered unused.
 extern crate core; //~ ERROR unused extern crate
diff --git a/src/test/ui/imports/extern-crate-used.stderr b/src/test/ui/imports/extern-crate-used.stderr
index c0783feb794..397bfa02902 100644
--- a/src/test/ui/imports/extern-crate-used.stderr
+++ b/src/test/ui/imports/extern-crate-used.stderr
@@ -1,8 +1,8 @@
-error: `extern crate` is not idiomatic in the new edition
-  --> $DIR/extern-crate-used.rs:8:1
+error: unused extern crate
+  --> $DIR/extern-crate-used.rs:18:1
    |
-LL | extern crate core as iso1;
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: convert it to a `use`
+LL | extern crate core;
+   | ^^^^^^^^^^^^^^^^^^ help: remove it
    |
 note: lint level defined here
   --> $DIR/extern-crate-used.rs:6:9
@@ -10,29 +10,5 @@ note: lint level defined here
 LL | #![deny(unused_extern_crates)]
    |         ^^^^^^^^^^^^^^^^^^^^
 
-error: `extern crate` is not idiomatic in the new edition
-  --> $DIR/extern-crate-used.rs:9:1
-   |
-LL | extern crate core as iso2;
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: convert it to a `use`
-
-error: `extern crate` is not idiomatic in the new edition
-  --> $DIR/extern-crate-used.rs:10:1
-   |
-LL | extern crate core as iso3;
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: convert it to a `use`
-
-error: `extern crate` is not idiomatic in the new edition
-  --> $DIR/extern-crate-used.rs:11:1
-   |
-LL | extern crate core as iso4;
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: convert it to a `use`
-
-error: unused extern crate
-  --> $DIR/extern-crate-used.rs:14:1
-   |
-LL | extern crate core;
-   | ^^^^^^^^^^^^^^^^^^ help: remove it
-
-error: aborting due to 5 previous errors
+error: aborting due to previous error
 
diff --git a/src/test/ui/rust-2018/extern-crate-idiomatic-in-2018.fixed b/src/test/ui/rust-2018/extern-crate-idiomatic-in-2018.fixed
index ddd9abbd50e..e51ce5d1d5b 100644
--- a/src/test/ui/rust-2018/extern-crate-idiomatic-in-2018.fixed
+++ b/src/test/ui/rust-2018/extern-crate-idiomatic-in-2018.fixed
@@ -12,8 +12,11 @@
 
 //~^ ERROR unused extern crate
 
-use edition_lint_paths as bar;
-//~^ ERROR `extern crate` is not idiomatic in the new edition
+// Shouldn't suggest changing to `use`, as `bar`
+// would no longer be added to the prelude which could cause
+// compilation errors for imports that use `bar` in other
+// modules. See #57672.
+extern crate edition_lint_paths as bar;
 
 fn main() {
     // This is not considered to *use* the `extern crate` in Rust 2018:
diff --git a/src/test/ui/rust-2018/extern-crate-idiomatic-in-2018.rs b/src/test/ui/rust-2018/extern-crate-idiomatic-in-2018.rs
index 47674bc19fc..debbf085d61 100644
--- a/src/test/ui/rust-2018/extern-crate-idiomatic-in-2018.rs
+++ b/src/test/ui/rust-2018/extern-crate-idiomatic-in-2018.rs
@@ -12,8 +12,11 @@
 extern crate edition_lint_paths;
 //~^ ERROR unused extern crate
 
+// Shouldn't suggest changing to `use`, as `bar`
+// would no longer be added to the prelude which could cause
+// compilation errors for imports that use `bar` in other
+// modules. See #57672.
 extern crate edition_lint_paths as bar;
-//~^ ERROR `extern crate` is not idiomatic in the new edition
 
 fn main() {
     // This is not considered to *use* the `extern crate` in Rust 2018:
diff --git a/src/test/ui/rust-2018/extern-crate-idiomatic-in-2018.stderr b/src/test/ui/rust-2018/extern-crate-idiomatic-in-2018.stderr
index f3f91939486..13980c70a82 100644
--- a/src/test/ui/rust-2018/extern-crate-idiomatic-in-2018.stderr
+++ b/src/test/ui/rust-2018/extern-crate-idiomatic-in-2018.stderr
@@ -11,11 +11,5 @@ LL | #![deny(rust_2018_idioms)]
    |         ^^^^^^^^^^^^^^^^
    = note: #[deny(unused_extern_crates)] implied by #[deny(rust_2018_idioms)]
 
-error: `extern crate` is not idiomatic in the new edition
-  --> $DIR/extern-crate-idiomatic-in-2018.rs:15:1
-   |
-LL | extern crate edition_lint_paths as bar;
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: convert it to a `use`
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
diff --git a/src/test/ui/rust-2018/remove-extern-crate.fixed b/src/test/ui/rust-2018/remove-extern-crate.fixed
index 17449caf84f..14575d18c23 100644
--- a/src/test/ui/rust-2018/remove-extern-crate.fixed
+++ b/src/test/ui/rust-2018/remove-extern-crate.fixed
@@ -7,7 +7,11 @@
 #![warn(rust_2018_idioms)]
 
 
-use core as another_name;
+// Shouldn't suggest changing to `use`, as `another_name`
+// would no longer be added to the prelude which could cause
+// compilation errors for imports that use `another_name` in other
+// modules. See #57672.
+extern crate core as another_name;
 use remove_extern_crate;
 #[macro_use]
 extern crate remove_extern_crate as something_else;
diff --git a/src/test/ui/rust-2018/remove-extern-crate.rs b/src/test/ui/rust-2018/remove-extern-crate.rs
index fb2217df000..0ee85f34e40 100644
--- a/src/test/ui/rust-2018/remove-extern-crate.rs
+++ b/src/test/ui/rust-2018/remove-extern-crate.rs
@@ -7,6 +7,10 @@
 #![warn(rust_2018_idioms)]
 
 extern crate core;
+// Shouldn't suggest changing to `use`, as `another_name`
+// would no longer be added to the prelude which could cause
+// compilation errors for imports that use `another_name` in other
+// modules. See #57672.
 extern crate core as another_name;
 use remove_extern_crate;
 #[macro_use]
diff --git a/src/test/ui/rust-2018/remove-extern-crate.stderr b/src/test/ui/rust-2018/remove-extern-crate.stderr
index 549693201b7..5de0dfe9613 100644
--- a/src/test/ui/rust-2018/remove-extern-crate.stderr
+++ b/src/test/ui/rust-2018/remove-extern-crate.stderr
@@ -12,13 +12,7 @@ LL | #![warn(rust_2018_idioms)]
    = note: #[warn(unused_extern_crates)] implied by #[warn(rust_2018_idioms)]
 
 warning: `extern crate` is not idiomatic in the new edition
-  --> $DIR/remove-extern-crate.rs:10:1
-   |
-LL | extern crate core as another_name;
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: convert it to a `use`
-
-warning: `extern crate` is not idiomatic in the new edition
-  --> $DIR/remove-extern-crate.rs:28:5
+  --> $DIR/remove-extern-crate.rs:32:5
    |
 LL |     extern crate core;
    |     ^^^^^^^^^^^^^^^^^^ help: convert it to a `use`
diff --git a/src/test/ui/suggestions/issue-57672.rs b/src/test/ui/suggestions/issue-57672.rs
index c8ea6e59582..1773f72fc74 100644
--- a/src/test/ui/suggestions/issue-57672.rs
+++ b/src/test/ui/suggestions/issue-57672.rs
@@ -1,11 +1,11 @@
 // aux-build:foo.rs
 // compile-flags:--extern foo
+// compile-pass
 // edition:2018
 
 #![deny(unused_extern_crates)]
 
 extern crate foo as foo_renamed;
-//~^ ERROR `extern crate` is not idiomatic in the new edition
 
 pub mod m {
     pub use foo_renamed::Foo;
diff --git a/src/test/ui/suggestions/issue-57672.stderr b/src/test/ui/suggestions/issue-57672.stderr
deleted file mode 100644
index c9e4c373799..00000000000
--- a/src/test/ui/suggestions/issue-57672.stderr
+++ /dev/null
@@ -1,14 +0,0 @@
-error: `extern crate` is not idiomatic in the new edition
-  --> $DIR/issue-57672.rs:7:1
-   |
-LL | extern crate foo as foo_renamed;
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: convert it to a `use`
-   |
-note: lint level defined here
-  --> $DIR/issue-57672.rs:5:9
-   |
-LL | #![deny(unused_extern_crates)]
-   |         ^^^^^^^^^^^^^^^^^^^^
-
-error: aborting due to previous error
-