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Add suggestion to .to_owned() used on Cow when borrowing #144925

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Add suggestion to .to_owned() used on Cow when borrowing #144925

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18 changes: 18 additions & 0 deletions compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -3411,6 +3411,24 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
Applicability::MaybeIncorrect,
);
}

if let Some(cow) = tcx.get_diagnostic_item(sym::Cow)

@davidtwco davidtwco Aug 13, 2025

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Suggested change
if let Some(cow) = tcx.get_diagnostic_item(sym::Cow)
if let Some(cow_did) = tcx.get_diagnostic_item(sym::Cow)

nit: likewise, more typical naming convention

&& let ty::Adt(adtdef, _) = return_ty.kind()

@davidtwco davidtwco Aug 13, 2025

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Suggested change
&& let ty::Adt(adtdef, _) = return_ty.kind()
&& let ty::Adt(adt_def, _) = return_ty.kind()

nit: more common name for variables of AdtDef

&& adtdef.did() == cow
{
if let Ok(snippet) = tcx.sess.source_map().span_to_snippet(return_span) {
if let Some(pos) = snippet.rfind(".to_owned") {

@Periodic1911 Periodic1911 Aug 4, 2025

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I'm not happy about detecting the use of .to_owned by way of string comparison, but since this check is in the MIR I don't know how to do it better. Constructive criticism would be greatly appreciated!

@davidtwco davidtwco Aug 13, 2025

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There are other diagnostics code here that does similar things, so happy to call it pre-existing until there's a better solution for this part of the compiler

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)
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7 changes: 7 additions & 0 deletions tests/ui/errors/cow-to-owned.rs
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@Kivooeo Kivooeo Aug 4, 2025

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I'd personally move this test to suggestions/ or borrowck/ and about name... I think name is fine but we can do better right? So i'd suggest something like cow-into-owned-suggestion.rs

Original file line number Diff line number Diff line change
@@ -0,0 +1,7 @@
// issue #144792

@Kivooeo Kivooeo Aug 4, 2025

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It's better to use full links and special comment //! Regression test for: https://github.com/rust-lang/rust/issues/144792 just because it will be easy to follow with just copy past link


fn main() {
_ = std::env::var_os("RUST_LOG").map_or("warn".into(), |x| x.to_string_lossy().to_owned());

@Kivooeo Kivooeo Aug 4, 2025

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It might be fine but I don't like it's env dependable, can we have more interesting test like below and with more coverage

// More robust - doesn't depend on environment
fn test_cow_suggestion() -> String {
    let os_string = std::ffi::OsString::from("test");
    os_string.to_string_lossy().to_owned()
}

// Test multiple Cow scenarios
fn test_cow_from_str() -> String {
    let s = "hello";
    let cow = std::borrow::Cow::from(s);
    cow.to_owned() // Should suggest into_owned()
}

// Test with different Cow types
fn test_cow_bytes() -> Vec<u8> {
    let bytes = b"hello";
    let cow = std::borrow::Cow::from(&bytes[..]);
    cow.to_owned() // Should suggest into_owned()
}

@davidtwco davidtwco Aug 13, 2025

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I'd adopt this as the test

//~^ ERROR cannot return value referencing function parameter
//~| HELP try using `.into_owned()`
}
17 changes: 17 additions & 0 deletions tests/ui/errors/cow-to-owned.stderr
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@@ -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`.
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