Make Cow covariant w.r.t ToOwned::Owned

[ohadravid]

This PR makes Cow covariant w.r.t ToOwned::Owned, by using the trick from #57440 (moving the associated type to parameter with a default):

// Before:
pub enum Cow<'a, B: ?Sized + 'a>
where B: ToOwned { ... }

// After:                        ........... new .........
pub enum Cow<'a, B: ?Sized + 'a, O = <B as ToOwned>::Owned>
where B: ToOwned { ... }

So, for example, this will now compile:

fn cow_cow<'new>(c: Cow<'static, Cow<'static, str>>) -> Cow<'new, Cow<'new, str>> { c }

I'm not really sure this is a good idea: it makes Cow's definition more complex,
it's a terrible hack, and it's not really clear to me if bad things can happen if someone tries to "use" the new generic parameter, and it's possible someone is relying on the fact that Cow is invariant w.r.t Owned.

However, I ran into this years ago when working on a crate which tries to minimize copying,
and got super confused by scary lifetimes errors.
This is usually caused by something like struct S1<'a> { c: Cow<'a, str> }; struct S2<'a> { c: Cow<'a, S1<'a>> } (Link to playground).

(This was previously impossible because of #79224)

PS
See #21726 (comment) and #59875 (comment) on why this is probably not true in the general case for associated types.

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[rust-log-analyzer]

The job x86_64-gnu-llvm-12 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

---- [ui] src/test/ui/associated-types/issue-85103.rs stdout ----
diff of stderr:

- error: layout error: NormalizationFailure(<[E] as std::borrow::ToOwned>::Owned, Type(<[E] as std::borrow::ToOwned>::Owned))
+ error: layout error: NormalizationFailure(std::borrow::Cow<'a, [E]>, Type(std::borrow::Cow<[E]>))
3    |
3    |
4 LL | type Edges<'a, E> = Cow<'a, [E]>;

The actual stderr differed from the expected stderr.
Actual stderr saved to /checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/associated-types/issue-85103/issue-85103.stderr
To update references, rerun the tests and pass the `--bless` flag
To update references, rerun the tests and pass the `--bless` flag
To only update this specific test, also pass `--test-args associated-types/issue-85103.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui/associated-types/issue-85103.rs" "-Zthreads=1" "--target=x86_64-unknown-linux-gnu" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/associated-types/issue-85103" "-A" "unused" "-Crpath" "-O" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/associated-types/issue-85103/auxiliary"
Some tests failed in compiletest suite=ui mode=ui host=x86_64-unknown-linux-gnu target=x86_64-unknown-linux-gnu
stdout: none
--- stderr -------------------------------
error: layout error: NormalizationFailure(std::borrow::Cow<'a, [E]>, Type(std::borrow::Cow<[E]>))
   |
   |
LL | type Edges<'a, E> = Cow<'a, [E]>;

error: aborting due to previous error
------------------------------------------



---- [ui] src/test/ui/issues/issue-22629.rs stdout ----

error: test compilation failed although it shouldn't!
status: exit status: 1
command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui/issues/issue-22629.rs" "-Zthreads=1" "--target=x86_64-unknown-linux-gnu" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-C" "prefer-dynamic" "-o" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/issues/issue-22629/a" "-Crpath" "-O" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/issues/issue-22629/auxiliary"
stdout: none
--- stderr -------------------------------
  --> /checkout/src/test/ui/issues/issue-22629.rs:13:17
   |
   |
LL |     assert_send(Cow::Borrowed("foo"));
   |                 ^^^^^^^^^^^^^ cannot infer type for type parameter `O` declared on the enum `Cow`
error: aborting due to previous error

For more information about this error, try `rustc --explain E0282`.
------------------------------------------
------------------------------------------


---- [ui] src/test/ui/lexer/lex-bad-char-literals-6.rs stdout ----
diff of stderr:

41    = help: the following other types implement trait `PartialEq<Rhs>`:
42              <&'a str as PartialEq<OsString>>
43              <&'a str as PartialEq<String>>
-              <&'b str as PartialEq<Cow<'a, str>>>
+              <&'b str as PartialEq<Cow<'a, str, String>>>
45              <String as PartialEq<&'a str>>
-              <String as PartialEq<Cow<'a, str>>>
+              <String as PartialEq<Cow<'a, str, String>>>
47              <String as PartialEq<str>>
48              <String as PartialEq>
-              <str as PartialEq<Cow<'a, str>>>
+              <str as PartialEq<Cow<'a, str, String>>>
51 
52 error[E0308]: mismatched types


67    = help: the following other types implement trait `PartialEq<Rhs>`:
68              <&'a str as PartialEq<OsString>>
69              <&'a str as PartialEq<String>>
-              <&'b str as PartialEq<Cow<'a, str>>>
+              <&'b str as PartialEq<Cow<'a, str, String>>>
71              <String as PartialEq<&'a str>>
-              <String as PartialEq<Cow<'a, str>>>
+              <String as PartialEq<Cow<'a, str, String>>>
73              <String as PartialEq<str>>
74              <String as PartialEq>
-              <str as PartialEq<Cow<'a, str>>>
+              <str as PartialEq<Cow<'a, str, String>>>
77 
78 error: aborting due to 6 previous errors



The actual stderr differed from the expected stderr.
Actual stderr saved to /checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/lexer/lex-bad-char-literals-6/lex-bad-char-literals-6.stderr
To only update this specific test, also pass `--test-args lexer/lex-bad-char-literals-6.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
status: exit status: 1
command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui/lexer/lex-bad-char-literals-6.rs" "-Zthreads=1" "--target=x86_64-unknown-linux-gnu" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/lexer/lex-bad-char-literals-6" "-A" "unused" "-Crpath" "-O" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/lexer/lex-bad-char-literals-6/auxiliary"
stdout: none
--- stderr -------------------------------
error: character literal may only contain one codepoint
   |
   |
LL |     let x: &str = 'ab';
   |
   |
help: if you meant to write a `str` literal, use double quotes
   |
LL |     let x: &str = "ab";


error: character literal may only contain one codepoint
   |
   |
LL |     let y: char = 'cd';
   |
   |
help: if you meant to write a `str` literal, use double quotes
   |
LL |     let y: char = "cd";


error: character literal may only contain one codepoint
   |
   |
LL |     let z = 'ef';
   |
   |
help: if you meant to write a `str` literal, use double quotes
   |
LL |     let z = "ef";


error[E0277]: can't compare `&str` with `char`
   |
   |
LL |     if x == y {}
   |          ^^ no implementation for `&str == char`
   |
   = help: the trait `PartialEq<char>` is not implemented for `&str`
   = help: the following other types implement trait `PartialEq<Rhs>`:
             <&'a str as PartialEq<OsString>>
             <&'a str as PartialEq<String>>
             <&'b str as PartialEq<Cow<'a, str, String>>>
             <String as PartialEq<&'a str>>
             <String as PartialEq<Cow<'a, str, String>>>
             <String as PartialEq<str>>
             <String as PartialEq>
             <str as PartialEq<Cow<'a, str, String>>>

error[E0308]: mismatched types
  --> /checkout/src/test/ui/lexer/lex-bad-char-literals-6.rs:15:20
   |
   |
LL |     let a: usize = "";
   |            -----   ^^ expected `usize`, found `&str`
   |            expected due to this


error[E0277]: can't compare `&str` with `char`
   |
   |
LL |     if x == z {}
   |          ^^ no implementation for `&str == char`
   |
   = help: the trait `PartialEq<char>` is not implemented for `&str`
   = help: the following other types implement trait `PartialEq<Rhs>`:
             <&'a str as PartialEq<OsString>>
             <&'a str as PartialEq<String>>
             <&'b str as PartialEq<Cow<'a, str, String>>>
             <String as PartialEq<&'a str>>
             <String as PartialEq<Cow<'a, str, String>>>
             <String as PartialEq<str>>
             <String as PartialEq>
             <str as PartialEq<Cow<'a, str, String>>>

error: aborting due to 6 previous errors

Some errors have detailed explanations: E0277, E0308.
Some errors have detailed explanations: E0277, E0308.
For more information about an error, try `rustc --explain E0277`.
------------------------------------------


---- [ui] src/test/ui/suggestions/into-str.rs stdout ----
diff of stderr:

12              <String as From<&mut str>>
13              <String as From<&str>>
14              <String as From<Box<str>>>
-              <String as From<Cow<'a, str>>>
+              <String as From<Cow<'a, str, String>>>
16              <String as From<char>>
17    = note: required because of the requirements on the impl of `Into<&str>` for `String`
18 note: required by a bound in `foo`

The actual stderr differed from the expected stderr.
Actual stderr saved to /checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/suggestions/into-str/into-str.stderr
To update references, rerun the tests and pass the `--bless` flag
To update references, rerun the tests and pass the `--bless` flag
To only update this specific test, also pass `--test-args suggestions/into-str.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui/suggestions/into-str.rs" "-Zthreads=1" "--target=x86_64-unknown-linux-gnu" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/suggestions/into-str" "-A" "unused" "-Crpath" "-O" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/suggestions/into-str/auxiliary"
stdout: none
--- stderr -------------------------------
error[E0277]: the trait bound `&str: From<String>` is not satisfied
   |
LL |     foo(String::new());
LL |     foo(String::new());
   |     --- ^^^^^^^^^^^^^ the trait `From<String>` is not implemented for `&str`
   |     required by a bound introduced by this call
   |
   |
   = note: to coerce a `String` into a `&str`, use `&*` as a prefix
   = help: the following other types implement trait `From<T>`:
             <String as From<&String>>
             <String as From<&mut str>>
             <String as From<&str>>
             <String as From<Box<str>>>
             <String as From<Cow<'a, str, String>>>
             <String as From<char>>
   = note: required because of the requirements on the impl of `Into<&str>` for `String`
note: required by a bound in `foo`
   |
   |
LL | fn foo<'a, T>(_t: T) where T: Into<&'a str> {}
   |                               ^^^^^^^^^^^^^ required by this bound in `foo`
error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.
------------------------------------------
------------------------------------------


---- [ui] src/test/ui/typeck/issue-90101.rs stdout ----
diff of stderr:

- error[E0277]: the trait bound `PathBuf: From<Cow<'_, str>>` is not satisfied
+ error[E0277]: the trait bound `PathBuf: From<Cow<'_, str, String>>` is not satisfied
3    |
3    |
4 LL |     func(Path::new("hello").to_path_buf().to_string_lossy(), "world")

-    |     ---- ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `From<Cow<'_, str>>` is not implemented for `PathBuf`
+    |     ---- ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `From<Cow<'_, str, String>>` is not implemented for `PathBuf`
7    |     required by a bound introduced by this call
8    |


9    = help: the following other types implement trait `From<T>`:
10              <PathBuf as From<&T>>
11              <PathBuf as From<Box<Path>>>
-              <PathBuf as From<Cow<'a, Path>>>
+              <PathBuf as From<Cow<'a, Path, PathBuf>>>
13              <PathBuf as From<OsString>>
14              <PathBuf as From<String>>
-    = note: required because of the requirements on the impl of `Into<PathBuf>` for `Cow<'_, str>`
+    = note: required because of the requirements on the impl of `Into<PathBuf>` for `Cow<'_, str, String>`
16 note: required by a bound in `func`
18    |


The actual stderr differed from the expected stderr.
The actual stderr differed from the expected stderr.
Actual stderr saved to /checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/typeck/issue-90101/issue-90101.stderr
To update references, rerun the tests and pass the `--bless` flag
To only update this specific test, also pass `--test-args typeck/issue-90101.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui/typeck/issue-90101.rs" "-Zthreads=1" "--target=x86_64-unknown-linux-gnu" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/typeck/issue-90101" "-A" "unused" "-Crpath" "-O" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/typeck/issue-90101/auxiliary"
stdout: none
--- stderr -------------------------------
error[E0277]: the trait bound `PathBuf: From<Cow<'_, str, String>>` is not satisfied
   |
   |
LL |     func(Path::new("hello").to_path_buf().to_string_lossy(), "world")
   |     ---- ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `From<Cow<'_, str, String>>` is not implemented for `PathBuf`
   |     required by a bound introduced by this call
   |
   |
   = help: the following other types implement trait `From<T>`:
             <PathBuf as From<&T>>
             <PathBuf as From<Box<Path>>>
             <PathBuf as From<Cow<'a, Path, PathBuf>>>
             <PathBuf as From<OsString>>
             <PathBuf as From<String>>
   = note: required because of the requirements on the impl of `Into<PathBuf>` for `Cow<'_, str, String>`
note: required by a bound in `func`
   |
   |
LL | fn func(path: impl Into<PathBuf>, code: impl Into<String>) {}
   |                    ^^^^^^^^^^^^^ required by this bound in `func`
error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.
------------------------------------------

[ohadravid]

Oh no! Most failures are changed error messages, but also this:

error[E0282]: type annotations needed
  --> /.../rust/src/test/ui/issues/issue-22629.rs:13:17
LL |     assert_send(Cow::Borrowed("foo"));
   |                 ^^^^^^^^^^^^^ cannot infer type for type parameter `O` declared on the enum `Cow`

Seems like this relates to #27336

one of the biggest use cases is adding a new type parameter to an existing type, without breaking semver compatibility

#27336 (comment)

and

https://gankra.github.io/blah/defaults-affect-inference