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

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -3530,6 +3530,24 @@ 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`.