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

Author: Periodic1911

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -3530,6 +3530,24 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                     Applicability::MaybeIncorrect,
                 );
             }
+
+            if let Some(cow) = tcx.get_diagnostic_item(sym::Cow)
+                && let ty::Adt(adtdef, _) = return_ty.kind()
+                && adtdef.did() == cow
+            {
+                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)

tests/ui/errors/cow-to-owned.rs

@@ -0,0 +1,7 @@
+// issue #144792
+
+fn main() {
+    _ = std::env::var_os("RUST_LOG").map_or("warn".into(), |x| x.to_string_lossy().to_owned());
+    //~^ ERROR cannot return value referencing function parameter
+    //~| HELP try using `.into_owned()`
+}

tests/ui/errors/cow-to-owned.stderr

@@ -0,0 +1,17 @@
+error[E0515]: cannot return value referencing function parameter `x`
+  --> $DIR/cow-to-owned.rs:4:64
+   |
+LL |     _ = std::env::var_os("RUST_LOG").map_or("warn".into(), |x| x.to_string_lossy().to_owned());
+   |                                                                -^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |                                                                |
+   |                                                                returns a value referencing data owned by the current function
+   |                                                                `x` is borrowed here
+   |
+help: try using `.into_owned()` if you meant to convert a `Cow<'_, T>` to an owned `T`
+   |
+LL |     _ = std::env::var_os("RUST_LOG").map_or("warn".into(), |x| x.to_string_lossy().into_owned());
+   |                                                                                    ++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0515`.