Rollup merge of #121326 - fmease:detect-empty-leading-where-clauses-on-ty-aliases, r=compiler-errors

Detect empty leading where clauses on type aliases

1. commit: refactor the AST of type alias where clauses
   * I could no longer bear the look of `.0.1` and `.1.0`
   * Arguably moving `split` out of `TyAlias` into a substruct might not make that much sense from a semantic standpoint since it reprs an index into `TyAlias.predicates` but it's alright and it cleans up the usage sites of `TyAlias`
2. commit: fix an oversight: An empty leading where clause is still a leading where clause
   * semantically reject empty leading where clauses on lazy type aliases
     * e.g., on `#![feature(lazy_type_alias)] type X where = ();`
   * make empty leading where clauses on assoc types trigger lint `deprecated_where_clause_location`
     * e.g., `impl Trait for () { type X where = (); }`

tests/ui/lazy-type-alias/leading-where-clause.fixed

@@ -2,14 +2,22 @@
 
 #![feature(lazy_type_alias)]
 #![allow(incomplete_features)]
+#![crate_type = "lib"]
 
 // Check that we *reject* leading where-clauses on lazy type aliases.
 
-type Alias<T>
+pub type Leading0<T>
 
 = T where String: From<T>;
-//~^^^ ERROR where clauses are not allowed before the type for type aliases
 
-fn main() {
-    let _: Alias<&str>;
-}
+pub type Leading1<T, U>
+
+= (T, U)
+where
+    U: Copy, String: From<T>;
+
+pub type EmptyLeading0  = () where;
+//~^ ERROR where clauses are not allowed before the type for type aliases
+
+pub type EmptyLeading1<T>  = T where T: Copy;
+//~^ ERROR where clauses are not allowed before the type for type aliases

tests/ui/lazy-type-alias/leading-where-clause.rs

@@ -2,15 +2,24 @@
 
 #![feature(lazy_type_alias)]
 #![allow(incomplete_features)]
+#![crate_type = "lib"]
 
 // Check that we *reject* leading where-clauses on lazy type aliases.
 
-type Alias<T>
-where
+pub type Leading0<T>
+where //~ ERROR where clauses are not allowed before the type for type aliases
     String: From<T>,
 = T;
-//~^^^ ERROR where clauses are not allowed before the type for type aliases
 
-fn main() {
-    let _: Alias<&str>;
-}
+pub type Leading1<T, U>
+where //~ ERROR where clauses are not allowed before the type for type aliases
+    String: From<T>,
+= (T, U)
+where
+    U: Copy;
+
+pub type EmptyLeading0 where = ();
+//~^ ERROR where clauses are not allowed before the type for type aliases
+
+pub type EmptyLeading1<T> where = T where T: Copy;
+//~^ ERROR where clauses are not allowed before the type for type aliases

tests/ui/lazy-type-alias/leading-where-clause.stderr

@@ -1,5 +1,5 @@
 error: where clauses are not allowed before the type for type aliases
-  --> $DIR/leading-where-clause.rs:9:1
+  --> $DIR/leading-where-clause.rs:10:1
    |
 LL | / where
 LL | |     String: From<T>,
@@ -12,5 +12,42 @@ LL +
 LL ~ = T where String: From<T>;
    |
 
-error: aborting due to 1 previous error
+error: where clauses are not allowed before the type for type aliases
+  --> $DIR/leading-where-clause.rs:15:1
+   |
+LL | / where
+LL | |     String: From<T>,
+   | |____________________^
+   |
+   = note: see issue #89122 <https://github.com/rust-lang/rust/issues/89122> for more information
+help: move it to the end of the type declaration
+   |
+LL + 
+LL | = (T, U)
+LL | where
+LL ~     U: Copy, String: From<T>;
+   |
+
+error: where clauses are not allowed before the type for type aliases
+  --> $DIR/leading-where-clause.rs:21:24
+   |
+LL | pub type EmptyLeading0 where = ();
+   |                        ^^^^^
+   |
+   = note: see issue #89122 <https://github.com/rust-lang/rust/issues/89122> for more information
+help: move it to the end of the type declaration
+   |
+LL - pub type EmptyLeading0 where = ();
+LL + pub type EmptyLeading0  = () where;
+   |
+
+error: where clauses are not allowed before the type for type aliases
+  --> $DIR/leading-where-clause.rs:24:27
+   |
+LL | pub type EmptyLeading1<T> where = T where T: Copy;
+   |                           ^^^^^ help: remove this `where`
+   |
+   = note: see issue #89122 <https://github.com/rust-lang/rust/issues/89122> for more information
+
+error: aborting due to 4 previous errors
 

tests/ui/where-clauses/where-clause-placement-assoc-type-in-impl.fixed

@@ -8,6 +8,8 @@ trait Trait {
     type Assoc where u32: Copy;
     // Fine.
     type Assoc2 where u32: Copy, i32: Copy;
+    //
+    type Assoc3;
 }
 
 impl Trait for u32 {
@@ -17,6 +19,8 @@ impl Trait for u32 {
     // Not fine, suggests moving `u32: Copy`
     type Assoc2  = () where i32: Copy, u32: Copy;
     //~^ WARNING where clause not allowed here
+    type Assoc3  = () where;
+    //~^ WARNING where clause not allowed here
 }
 
 impl Trait for i32 {
@@ -25,6 +29,8 @@ impl Trait for i32 {
     // Not fine, suggests moving both.
     type Assoc2  = () where u32: Copy, i32: Copy;
     //~^ WARNING where clause not allowed here
+    type Assoc3  = () where;
+    //~^ WARNING where clause not allowed here
 }
 
 fn main() {}

tests/ui/where-clauses/where-clause-placement-assoc-type-in-impl.rs

@@ -8,6 +8,8 @@ trait Trait {
     type Assoc where u32: Copy;
     // Fine.
     type Assoc2 where u32: Copy, i32: Copy;
+    //
+    type Assoc3;
 }
 
 impl Trait for u32 {
@@ -17,6 +19,8 @@ impl Trait for u32 {
     // Not fine, suggests moving `u32: Copy`
     type Assoc2 where u32: Copy = () where i32: Copy;
     //~^ WARNING where clause not allowed here
+    type Assoc3 where = ();
+    //~^ WARNING where clause not allowed here
 }
 
 impl Trait for i32 {
@@ -25,6 +29,8 @@ impl Trait for i32 {
     // Not fine, suggests moving both.
     type Assoc2 where u32: Copy, i32: Copy = ();
     //~^ WARNING where clause not allowed here
+    type Assoc3 where = () where;
+    //~^ WARNING where clause not allowed here
 }
 
 fn main() {}

tests/ui/where-clauses/where-clause-placement-assoc-type-in-impl.stderr

@@ -1,5 +1,5 @@
 warning: where clause not allowed here
-  --> $DIR/where-clause-placement-assoc-type-in-impl.rs:15:16
+  --> $DIR/where-clause-placement-assoc-type-in-impl.rs:17:16
    |
 LL |     type Assoc where u32: Copy = ();
    |                ^^^^^^^^^^^^^^^
@@ -13,7 +13,7 @@ LL +     type Assoc  = () where u32: Copy;
    |
 
 warning: where clause not allowed here
-  --> $DIR/where-clause-placement-assoc-type-in-impl.rs:18:17
+  --> $DIR/where-clause-placement-assoc-type-in-impl.rs:20:17
    |
 LL |     type Assoc2 where u32: Copy = () where i32: Copy;
    |                 ^^^^^^^^^^^^^^^
@@ -26,7 +26,20 @@ LL +     type Assoc2  = () where i32: Copy, u32: Copy;
    |
 
 warning: where clause not allowed here
-  --> $DIR/where-clause-placement-assoc-type-in-impl.rs:26:17
+  --> $DIR/where-clause-placement-assoc-type-in-impl.rs:22:17
+   |
+LL |     type Assoc3 where = ();
+   |                 ^^^^^
+   |
+   = note: see issue #89122 <https://github.com/rust-lang/rust/issues/89122> for more information
+help: move it to the end of the type declaration
+   |
+LL -     type Assoc3 where = ();
+LL +     type Assoc3  = () where;
+   |
+
+warning: where clause not allowed here
+  --> $DIR/where-clause-placement-assoc-type-in-impl.rs:30:17
    |
 LL |     type Assoc2 where u32: Copy, i32: Copy = ();
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -38,5 +51,13 @@ LL -     type Assoc2 where u32: Copy, i32: Copy = ();
 LL +     type Assoc2  = () where u32: Copy, i32: Copy;
    |
 
-warning: 3 warnings emitted
+warning: where clause not allowed here
+  --> $DIR/where-clause-placement-assoc-type-in-impl.rs:32:17
+   |
+LL |     type Assoc3 where = () where;
+   |                 ^^^^^ help: remove this `where`
+   |
+   = note: see issue #89122 <https://github.com/rust-lang/rust/issues/89122> for more information
+
+warning: 5 warnings emitted