Unrolled build for #154646

Rollup merge of #154646 - m4rch3n1ng:144792-cow-diag, r=davidtwco

Add suggestion to `.to_owned()` used on `Cow` when borrowing

fixes #144792
supersedes #144925 with the review comments addressed

the tests suggested from @Kivooeo from #144925 (comment) didn't work entirely, because these tests failed due to error `[E0308]` mismatched types, which actually already provides a suggestion, that actually makes the code compile:

```
$ cargo check
error[E0308]: mismatched types
 --> src/main.rs:3:5
  |
1 | fn test_cow_suggestion() -> String {
  |                             ------ expected `std::string::String` because of return type
2 |     let os_string = std::ffi::OsString::from("test");
3 |     os_string.to_string_lossy().to_owned()
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `String`, found `Cow<'_, str>`
  |
  = note: expected struct `std::string::String`
               found enum `std::borrow::Cow<'_, str>`
help: try using a conversion method
  |
3 |     os_string.to_string_lossy().to_owned().to_string()
  |                                           ++++++++++++
```

now this suggestion is of course not good or efficient code, but via clippy with `-Wclippy::nursery` lint group you can actually get to the correct code, so i don't think this is too much of an issue:

<details>
<summary>the clippy suggestions</summary>

```
$ cargo clippy -- -Wclippy::nursery
warning: this `to_owned` call clones the `Cow<'_, str>` itself and does not cause its contents to become owned
 --> src/main.rs:3:5
  |
3 |     os_string.to_string_lossy().to_owned().to_string()
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/beta/index.html#suspicious_to_owned
  = note: `#[warn(clippy::suspicious_to_owned)]` on by default
help: depending on intent, either make the `Cow` an `Owned` variant
  |
3 |     os_string.to_string_lossy().into_owned().to_string()
  |                                 ++
help: or clone the `Cow` itself
  |
3 -     os_string.to_string_lossy().to_owned().to_string()
3 +     os_string.to_string_lossy().clone().to_string()
  |
$ # apply first suggestion
$ cargo c -- -Wclippy::nursery
warning: redundant clone
 --> src/main.rs:3:45
  |
3 |     os_string.to_string_lossy().into_owned().to_string()
  |                                             ^^^^^^^^^^^^ help: remove this
  |
note: this value is dropped without further use
 --> src/main.rs:3:5
  |
3 |     os_string.to_string_lossy().into_owned().to_string()
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = help: for further information visit https://rust-lang.github.io/rust-clippy/beta/index.html#redundant_clone
  = note: `-W clippy::redundant-clone` implied by `-W clippy::nursery`
  = help: to override `-W clippy::nursery` add `#[allow(clippy::redundant_clone)]`
```

</details>

the actual error that we are looking for is error `[E0515]`, cannot return value referencing local variable, which was only present in the original issue due to automatic type inference assuming you want to return a cloned `Cow<'_, str>`, which is of course not possible. this is why i took the original test functions and turned them into closures, where it's less obvious that it's trying to return the wrong type.

r? davidtwco

(because you reviewed the last attempt)
(also, let me know if i should squash this down to one commit and add myself as the co-contributor)

Author: rust-timer

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -3538,6 +3538,24 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                     Applicability::MaybeIncorrect,
                 );
             }
+
+            if let Some(cow_did) = tcx.get_diagnostic_item(sym::Cow)
+                && let ty::Adt(adt_def, _) = return_ty.kind()
+                && adt_def.did() == cow_did
+            {
+                if let Ok(snippet) = tcx.sess.source_map().span_to_snippet(return_span) {
+                    if let Some(pos) = snippet.rfind(".to_owned") {
+                        let byte_pos = BytePos(pos as u32 + 1u32);
+                        let to_owned_span = return_span.with_hi(return_span.lo() + byte_pos);
+                        err.span_suggestion_short(
+                            to_owned_span.shrink_to_hi(),
+                            "try using `.into_owned()` if you meant to convert a `Cow<'_, T>` to an owned `T`",
+                            "in",
+                            Applicability::MaybeIncorrect,
+                        );
+                    }
+                }
+            }
         }
 
         Err(err)

compiler/rustc_span/src/symbol.rs

@@ -196,6 +196,7 @@ symbols! {
         Continue,
         ControlFlow,
         Copy,
+        Cow,
         Debug,
         Default,
         Deref,

src/tools/clippy/clippy_utils/src/sym.rs

@@ -50,7 +50,6 @@ generate! {
     Cargo_toml: "Cargo.toml",
     Child,
     Command,
-    Cow,
     Current,
     DOUBLE_QUOTE: "\"",
     DebugStruct,

tests/ui/suggestions/cow-into-owned-suggestion.rs

@@ -0,0 +1,23 @@
+//! Regression test for: https://github.com/rust-lang/rust/issues/144792
+
+fn main() {
+    let _ = || {
+        let os_string = std::ffi::OsString::from("test");
+        os_string.to_string_lossy().to_owned()
+        //~^ ERROR: cannot return value referencing local variable `os_string` [E0515]
+    };
+
+    let _ = || {
+        let s = "hello".to_owned();
+        let cow = std::borrow::Cow::from(&s);
+        cow.to_owned()
+        //~^ ERROR: cannot return value referencing local variable `s` [E0515]
+    };
+
+    let _ = || {
+        let bytes = b"hello".to_owned();
+        let cow = std::borrow::Cow::from(&bytes[..]);
+        cow.to_owned()
+        //~^ ERROR: cannot return value referencing local variable `bytes` [E0515]
+    };
+}

tests/ui/suggestions/cow-into-owned-suggestion.stderr

@@ -0,0 +1,43 @@
+error[E0515]: cannot return value referencing local variable `os_string`
+  --> $DIR/cow-into-owned-suggestion.rs:6:9
+   |
+LL |         os_string.to_string_lossy().to_owned()
+   |         ---------^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |         |
+   |         returns a value referencing data owned by the current function
+   |         `os_string` is borrowed here
+   |
+help: try using `.into_owned()` if you meant to convert a `Cow<'_, T>` to an owned `T`
+   |
+LL |         os_string.to_string_lossy().into_owned()
+   |                                     ++
+
+error[E0515]: cannot return value referencing local variable `s`
+  --> $DIR/cow-into-owned-suggestion.rs:13:9
+   |
+LL |         let cow = std::borrow::Cow::from(&s);
+   |                                          -- `s` is borrowed here
+LL |         cow.to_owned()
+   |         ^^^^^^^^^^^^^^ returns a value referencing data owned by the current function
+   |
+help: try using `.into_owned()` if you meant to convert a `Cow<'_, T>` to an owned `T`
+   |
+LL |         cow.into_owned()
+   |             ++
+
+error[E0515]: cannot return value referencing local variable `bytes`
+  --> $DIR/cow-into-owned-suggestion.rs:20:9
+   |
+LL |         let cow = std::borrow::Cow::from(&bytes[..]);
+   |                                           ----- `bytes` is borrowed here
+LL |         cow.to_owned()
+   |         ^^^^^^^^^^^^^^ returns a value referencing data owned by the current function
+   |
+help: try using `.into_owned()` if you meant to convert a `Cow<'_, T>` to an owned `T`
+   |
+LL |         cow.into_owned()
+   |             ++
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0515`.