Auto merge of #83729 - JohnTitor:issue-43913, r=estebank

Add a suggestion when using a type alias instead of trait alias

Fixes #43913

r? `@estebank`

src/test/ui/codemap_tests/two_files.stderr

@@ -5,10 +5,9 @@ LL | impl Bar for Baz { }
    |      ^^^ type aliases cannot be used as traits
    |
 help: you might have meant to use `#![feature(trait_alias)]` instead of a `type` alias
-  --> $DIR/two_files_data.rs:5:1
    |
-LL | type Bar = dyn Foo;
-   | ^^^^^^^^^^^^^^^^^^^
+LL | trait Bar = dyn Foo;
+   |
 
 error: aborting due to previous error
 

src/test/ui/resolve/issue-3907.stderr

@@ -5,10 +5,9 @@ LL | impl Foo for S {
    |      ^^^ type aliases cannot be used as traits
    |
 help: you might have meant to use `#![feature(trait_alias)]` instead of a `type` alias
-  --> $DIR/issue-3907.rs:5:1
    |
-LL | type Foo = dyn issue_3907::Foo;
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | trait Foo = dyn issue_3907::Foo;
+   |
 help: consider importing this trait instead
    |
 LL | use issue_3907::Foo;

src/test/ui/resolve/issue-5035.stderr

@@ -11,16 +11,16 @@ LL | trait I {}
    | ------- similarly named trait `I` defined here
 LL | type K = dyn I;
 LL | impl K for isize {}
-   |      ^
-   |      |
-   |      type aliases cannot be used as traits
-   |      help: a trait with a similar name exists: `I`
+   |      ^ type aliases cannot be used as traits
    |
 help: you might have meant to use `#![feature(trait_alias)]` instead of a `type` alias
-  --> $DIR/issue-5035.rs:2:1
    |
-LL | type K = dyn I;
-   | ^^^^^^^^^^^^^^^
+LL | trait K = dyn I;
+   |
+help: a trait with a similar name exists
+   |
+LL | impl I for isize {}
+   |      ^
 
 error: aborting due to 2 previous errors
 

src/test/ui/resolve/unboxed-closure-sugar-nonexistent-trait.stderr

@@ -11,10 +11,9 @@ LL | fn g<F:Typedef(isize) -> isize>(x: F) {}
    |        ^^^^^^^^^^^^^^^^^^^^^^^ type aliases cannot be used as traits
    |
 help: you might have meant to use `#![feature(trait_alias)]` instead of a `type` alias
-  --> $DIR/unboxed-closure-sugar-nonexistent-trait.rs:4:1
    |
-LL | type Typedef = isize;
-   | ^^^^^^^^^^^^^^^^^^^^^
+LL | trait Typedef = isize;
+   |
 
 error: aborting due to 2 previous errors
 

src/test/ui/traits/alias/suggest-trait-alias-instead-of-type.fixed

@@ -0,0 +1,13 @@
+// Regression test of #43913.
+
+// run-rustfix
+
+#![feature(trait_alias)]
+#![allow(bare_trait_objects, dead_code)]
+
+trait Strings = Iterator<Item=String>;
+
+struct Struct<S: Strings>(S);
+//~^ ERROR: expected trait, found type alias `Strings`
+
+fn main() {}

src/test/ui/traits/alias/suggest-trait-alias-instead-of-type.rs

@@ -0,0 +1,13 @@
+// Regression test of #43913.
+
+// run-rustfix
+
+#![feature(trait_alias)]
+#![allow(bare_trait_objects, dead_code)]
+
+type Strings = Iterator<Item=String>;
+
+struct Struct<S: Strings>(S);
+//~^ ERROR: expected trait, found type alias `Strings`
+
+fn main() {}

src/test/ui/traits/alias/suggest-trait-alias-instead-of-type.stderr

@@ -0,0 +1,14 @@
+error[E0404]: expected trait, found type alias `Strings`
+  --> $DIR/suggest-trait-alias-instead-of-type.rs:10:18
+   |
+LL | struct Struct<S: Strings>(S);
+   |                  ^^^^^^^ type aliases cannot be used as traits
+   |
+help: you might have meant to use `#![feature(trait_alias)]` instead of a `type` alias
+   |
+LL | trait Strings = Iterator<Item=String>;
+   |
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0404`.