Extend documentation of rustc_on_unimplemented

src/doc/unstable-book/src/language-features/on-unimplemented.md

@@ -8,7 +8,9 @@ The tracking issue for this feature is: [#29628]
 
 The `on_unimplemented` feature provides the `#[rustc_on_unimplemented]`
 attribute, which allows trait definitions to add specialized notes to error
-messages when an implementation was expected but not found.
+messages when an implementation was expected but not found. You can refer
+to the trait's generic arguments by name and to the resolved type using
+`Self`.
 
 For example:
 
@@ -41,7 +43,98 @@ error[E0277]: the trait bound `&[{integer}]: MyIterator<char>` is not satisfied
    |
    = help: the trait `MyIterator<char>` is not implemented for `&[{integer}]`
    = note: required by `iterate_chars`
+```
+
+`on_unimplemented` also supports advanced filtering for better targeting
+of messages, as well as modifying specific parts of the error message. You
+target the text of:
+
+ - the main error message (`message`)
+ - the label (`label`)
+ - an extra note (`note`)
+
+For example, the following attribute
+
+```rust,compile_fail
+#[rustc_on_unimplemented(
+    message="message",
+    label="label",
+    note="note"
+)]
+trait MyIterator<A> {
+    fn next(&mut self) -> A;
+}
+```
+
+Would generate the following output:
+
+```text
+error[E0277]: message
+  --> <anon>:14:5
+   |
+14 |     iterate_chars(&[1, 2, 3][..]);
+   |     ^^^^^^^^^^^^^ label
+   |
+   = note: note
+   = help: the trait `MyIterator<char>` is not implemented for `&[{integer}]`
+   = note: required by `iterate_chars`
+```
+
+To allow more targeted error messages, it is possible to filter the
+application of these fields based on a variety of attributes when using
+`on`:
 
-error: aborting due to previous error
+ - `crate_local`: whether the code causing the trait bound to not be
+   fulfilled is part of the user's crate. This is used to avoid suggesting
+   code changes that would require modifying a dependency.
+ - Any of the generic arguments that can be substituted in the text can be
+   referred by name as well for filtering, like `Rhs="i32"`, except for
+   `Self`.
+ - `_Self`: to filter only on a particular calculated trait resolution, like
+   `Self="std::iter::Iterator<char>"`. This is needed because `Self` is a
+   keyword which cannot appear in attributes.
+ - `direct`: user-specified rather than derived obligation.
+ - `from_method`: usable both as boolean (whether the flag is present, like
+   `crate_local`) or matching against a particular method. Currently used
+   for `try`.
+ - `from_desugaring`: usable both as boolean (whether the flag is present)
+   or matching against a particular desugaring.
+
+For example, the `Iterator` trait can be annotated in the following way:
+
+```rust,compile_fail
+#[rustc_on_unimplemented(
+    on(
+        _Self="&str",
+        note="call `.chars()` or `.as_bytes()` on `{Self}"
+    ),
+    message="`{Self}` is not an iterator",
+    label="`{Self}` is not an iterator",
+    note="maybe try calling `.iter()` or a similar method"
+)]
+pub trait Iterator {}
 ```
 
+Which would produce the following outputs:
+
+```text
+error[E0277]: `Foo` is not an iterator
+ --> src/main.rs:4:16
+  |
+4 |     for foo in Foo {}
+  |                ^^^ `Foo` is not an iterator
+  |
+  = note: maybe try calling `.iter()` or a similar method
+  = help: the trait `std::iter::Iterator` is not implemented for `Foo`
+  = note: required by `std::iter::IntoIterator::into_iter`
+
+error[E0277]: `&str` is not an iterator
+ --> src/main.rs:5:16
+  |
+5 |     for foo in "" {}
+  |                ^^ `&str` is not an iterator
+  |
+  = note: call `.chars()` or `.bytes() on `&str`
+  = help: the trait `std::iter::Iterator` is not implemented for `&str`
+  = note: required by `std::iter::IntoIterator::into_iter`
+```