Rollup merge of #106931 - Ezrashaw:docs-e0208, r=compiler-errors

document + UI test `E0208` and make its output more user-friendly

Cleans up `E0208`'s output a lot. It could actually be useful for someone learning about variance now. I also added a UI test for it in `tests/ui/error-codes/` and wrote some docs for it.

r? `@GuillaumeGomez` another error code, can't be bothered to find the issue :P. Obviously there's some compiler stuff, so you'll have to hand it off.

Part of https://github.com/rust-lang/rust/issues/61137.

compiler/rustc_error_codes/src/error_codes/E0208.md

@@ -1 +1,46 @@
 #### This error code is internal to the compiler and will not be emitted with normal Rust code.
+#### Note: this error code is no longer emitted by the compiler.
+
+This error code shows the variance of a type's generic parameters.
+
+Erroneous code example:
+
+```compile_fail
+// NOTE: this feature is perma-unstable and should *only* be used for
+//       testing purposes.
+#![feature(rustc_attrs)]
+
+#[rustc_variance]
+struct Foo<'a, T> { // error: deliberate error to display type's variance
+    t: &'a mut T,
+}
+```
+
+which produces the following error:
+
+```text
+error: [-, o]
+ --> <anon>:4:1
+  |
+4 | struct Foo<'a, T> {
+  | ^^^^^^^^^^^^^^^^^
+```
+
+*Note that while `#[rustc_variance]` still exists and is used within the*
+*compiler, it no longer is marked as `E0208` and instead has no error code.*
+
+This error is deliberately triggered with the `#[rustc_variance]` attribute
+(`#![feature(rustc_attrs)]` must be enabled) and helps to show you the variance
+of the type's generic parameters. You can read more about variance and
+subtyping in [this section of the Rustnomicon]. For a more in depth look at
+variance (including a more complete list of common variances) see
+[this section of the Reference]. For information on how variance is implemented
+in the compiler, see [this section of `rustc-dev-guide`].
+
+This error can be easily fixed by removing the `#[rustc_variance]` attribute,
+the compiler's suggestion to comment it out can be applied automatically with
+`rustfix`.
+
+[this section of the Rustnomicon]: https://doc.rust-lang.org/nomicon/subtyping.html
+[this section of the Reference]: https://doc.rust-lang.org/reference/subtyping.html#variance
+[this section of `rustc-dev-guide`]: https://rustc-dev-guide.rust-lang.org/variance.html

compiler/rustc_hir_analysis/src/variance/test.rs

@@ -1,4 +1,3 @@
-use rustc_errors::struct_span_err;
 use rustc_middle::ty::TyCtxt;
 use rustc_span::symbol::sym;
 
@@ -8,8 +7,8 @@ pub fn test_variance(tcx: TyCtxt<'_>) {
     for id in tcx.hir().items() {
         if tcx.has_attr(id.owner_id.to_def_id(), sym::rustc_variance) {
             let variances_of = tcx.variances_of(id.owner_id);
-            struct_span_err!(tcx.sess, tcx.def_span(id.owner_id), E0208, "{:?}", variances_of)
-                .emit();
+
+            tcx.sess.struct_span_err(tcx.def_span(id.owner_id), format!("{variances_of:?}")).emit();
         }
     }
 }

src/tools/tidy/src/error_codes.rs

@@ -27,8 +27,7 @@ const ERROR_DOCS_PATH: &str = "compiler/rustc_error_codes/src/error_codes/";
 const ERROR_TESTS_PATH: &str = "tests/ui/error-codes/";
 
 // Error codes that (for some reason) can't have a doctest in their explanation. Error codes are still expected to provide a code example, even if untested.
-const IGNORE_DOCTEST_CHECK: &[&str] =
-    &["E0208", "E0464", "E0570", "E0601", "E0602", "E0640", "E0717"];
+const IGNORE_DOCTEST_CHECK: &[&str] = &["E0464", "E0570", "E0601", "E0602", "E0640", "E0717"];
 
 // Error codes that don't yet have a UI test. This list will eventually be removed.
 const IGNORE_UI_TEST_CHECK: &[&str] =

tests/ui/error-codes/E0208.rs

@@ -0,0 +1,8 @@
+#![feature(rustc_attrs)]
+
+#[rustc_variance]
+struct Foo<'a, T> { //~ ERROR [-, o]
+    t: &'a mut T,
+}
+
+fn main() {}

tests/ui/error-codes/E0208.stderr

@@ -0,0 +1,8 @@
+error: [-, o]
+  --> $DIR/E0208.rs:4:1
+   |
+LL | struct Foo<'a, T> {
+   | ^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+

tests/ui/variance/variance-associated-consts.stderr

@@ -1,4 +1,4 @@
-error[E0208]: [o]
+error: [o]
   --> $DIR/variance-associated-consts.rs:13:1
    |
 LL | struct Foo<T: Trait> {
@@ -6,4 +6,3 @@ LL | struct Foo<T: Trait> {
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0208`.

tests/ui/variance/variance-associated-types.stderr

@@ -1,10 +1,10 @@
-error[E0208]: [-, +]
+error: [-, +]
   --> $DIR/variance-associated-types.rs:13:1
    |
 LL | struct Foo<'a, T : Trait<'a>> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0208]: [o, o]
+error: [o, o]
   --> $DIR/variance-associated-types.rs:18:1
    |
 LL | struct Bar<'a, T : Trait<'a>> {
@@ -12,4 +12,3 @@ LL | struct Bar<'a, T : Trait<'a>> {
 
 error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0208`.

tests/ui/variance/variance-object-types.stderr

@@ -1,4 +1,4 @@
-error[E0208]: [o]
+error: [o]
   --> $DIR/variance-object-types.rs:7:1
    |
 LL | struct Foo<'a> {
@@ -6,4 +6,3 @@ LL | struct Foo<'a> {
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0208`.

tests/ui/variance/variance-regions-direct.stderr

@@ -1,40 +1,40 @@
-error[E0208]: [-, -, -]
+error: [-, -, -]
   --> $DIR/variance-regions-direct.rs:9:1
    |
 LL | struct Test2<'a, 'b, 'c> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0208]: [+, +, +]
+error: [+, +, +]
   --> $DIR/variance-regions-direct.rs:18:1
    |
 LL | struct Test3<'a, 'b, 'c> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0208]: [-, o]
+error: [-, o]
   --> $DIR/variance-regions-direct.rs:27:1
    |
 LL | struct Test4<'a, 'b:'a> {
    | ^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0208]: [+, o]
+error: [+, o]
   --> $DIR/variance-regions-direct.rs:35:1
    |
 LL | struct Test5<'a, 'b:'a> {
    | ^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0208]: [-, o]
+error: [-, o]
   --> $DIR/variance-regions-direct.rs:45:1
    |
 LL | struct Test6<'a, 'b:'a> {
    | ^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0208]: [*]
+error: [*]
   --> $DIR/variance-regions-direct.rs:52:1
    |
 LL | struct Test7<'a> {
    | ^^^^^^^^^^^^^^^^
 
-error[E0208]: [+, -, o]
+error: [+, -, o]
   --> $DIR/variance-regions-direct.rs:59:1
    |
 LL | enum Test8<'a, 'b, 'c:'b> {
@@ -42,4 +42,3 @@ LL | enum Test8<'a, 'b, 'c:'b> {
 
 error: aborting due to 7 previous errors
 
-For more information about this error, try `rustc --explain E0208`.

tests/ui/variance/variance-regions-indirect.stderr

@@ -1,28 +1,28 @@
-error[E0208]: [+, -, o, *]
+error: [+, -, o, *]
   --> $DIR/variance-regions-indirect.rs:8:1
    |
 LL | enum Base<'a, 'b, 'c:'b, 'd> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0208]: [*, o, -, +]
+error: [*, o, -, +]
   --> $DIR/variance-regions-indirect.rs:15:1
    |
 LL | struct Derived1<'w, 'x:'y, 'y, 'z> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0208]: [o, o, *]
+error: [o, o, *]
   --> $DIR/variance-regions-indirect.rs:20:1
    |
 LL | struct Derived2<'a, 'b:'a, 'c> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0208]: [o, -, *]
+error: [o, -, *]
   --> $DIR/variance-regions-indirect.rs:25:1
    |
 LL | struct Derived3<'a:'b, 'b, 'c> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0208]: [+, -, o]
+error: [+, -, o]
   --> $DIR/variance-regions-indirect.rs:30:1
    |
 LL | struct Derived4<'a, 'b, 'c:'b> {
@@ -30,4 +30,3 @@ LL | struct Derived4<'a, 'b, 'c:'b> {
 
 error: aborting due to 5 previous errors
 
-For more information about this error, try `rustc --explain E0208`.

tests/ui/variance/variance-trait-bounds.stderr

@@ -1,22 +1,22 @@
-error[E0208]: [+, +]
+error: [+, +]
   --> $DIR/variance-trait-bounds.rs:16:1
    |
 LL | struct TestStruct<U,T:Setter<U>> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0208]: [*, +]
+error: [*, +]
   --> $DIR/variance-trait-bounds.rs:21:1
    |
 LL | enum TestEnum<U,T:Setter<U>> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0208]: [*, +]
+error: [*, +]
   --> $DIR/variance-trait-bounds.rs:26:1
    |
 LL | struct TestContraStruct<U,T:Setter<U>> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0208]: [*, +]
+error: [*, +]
   --> $DIR/variance-trait-bounds.rs:31:1
    |
 LL | struct TestBox<U,T:Getter<U>+Setter<U>> {
@@ -24,4 +24,3 @@ LL | struct TestBox<U,T:Getter<U>+Setter<U>> {
 
 error: aborting due to 4 previous errors
 
-For more information about this error, try `rustc --explain E0208`.

tests/ui/variance/variance-trait-object-bound.stderr

@@ -1,4 +1,4 @@
-error[E0208]: [-]
+error: [-]
   --> $DIR/variance-trait-object-bound.rs:14:1
    |
 LL | struct TOption<'a> {
@@ -6,4 +6,3 @@ LL | struct TOption<'a> {
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0208`.

tests/ui/variance/variance-types-bounds.stderr

@@ -1,28 +1,28 @@
-error[E0208]: [+, +]
+error: [+, +]
   --> $DIR/variance-types-bounds.rs:7:1
    |
 LL | struct TestImm<A, B> {
    | ^^^^^^^^^^^^^^^^^^^^
 
-error[E0208]: [+, o]
+error: [+, o]
   --> $DIR/variance-types-bounds.rs:13:1
    |
 LL | struct TestMut<A, B:'static> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0208]: [+, o]
+error: [+, o]
   --> $DIR/variance-types-bounds.rs:19:1
    |
 LL | struct TestIndirect<A:'static, B:'static> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0208]: [o, o]
+error: [o, o]
   --> $DIR/variance-types-bounds.rs:24:1
    |
 LL | struct TestIndirect2<A:'static, B:'static> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0208]: [o, o]
+error: [o, o]
   --> $DIR/variance-types-bounds.rs:38:1
    |
 LL | struct TestObject<A, R> {
@@ -30,4 +30,3 @@ LL | struct TestObject<A, R> {
 
 error: aborting due to 5 previous errors
 
-For more information about this error, try `rustc --explain E0208`.

tests/ui/variance/variance-types.stderr

@@ -1,34 +1,34 @@
-error[E0208]: [-, o, o]
+error: [-, o, o]
   --> $DIR/variance-types.rs:10:1
    |
 LL | struct InvariantMut<'a,A:'a,B:'a> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0208]: [o]
+error: [o]
   --> $DIR/variance-types.rs:15:1
    |
 LL | struct InvariantCell<A> {
    | ^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0208]: [o]
+error: [o]
   --> $DIR/variance-types.rs:20:1
    |
 LL | struct InvariantIndirect<A> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0208]: [+]
+error: [+]
   --> $DIR/variance-types.rs:25:1
    |
 LL | struct Covariant<A> {
    | ^^^^^^^^^^^^^^^^^^^
 
-error[E0208]: [-]
+error: [-]
   --> $DIR/variance-types.rs:30:1
    |
 LL | struct Contravariant<A> {
    | ^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0208]: [+, -, o]
+error: [+, -, o]
   --> $DIR/variance-types.rs:35:1
    |
 LL | enum Enum<A,B,C> {
@@ -36,4 +36,3 @@ LL | enum Enum<A,B,C> {
 
 error: aborting due to 6 previous errors
 
-For more information about this error, try `rustc --explain E0208`.