Provide context when `?` can't be called because of `Result<_, E>` #116496

estebank · 2023-10-06T20:52:57Z

When a method chain ending in ? causes an E0277 because the expression's Result::Err variant doesn't have a type that can be converted to the Result<_, E> type parameter in the return type, provide additional context of which parts of the chain can and can't support the ? operator.

error[E0277]: `?` couldn't convert the error to `String`
  --> $DIR/question-mark-result-err-mismatch.rs:27:25
   |
LL | fn bar() -> Result<(), String> {
   |             ------------------ expected `String` because of this
LL |     let x = foo();
   |             ----- this has type `Result<_, String>`
...
LL |         .map_err(|_| ())?;
   |          ---------------^ the trait `From<()>` is not implemented for `String`
   |          |
   |          this can't be annotated with `?` because it has type `Result<_, ()>`
   |
   = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
   = help: the following other types implement trait `From<T>`:
             <String as From<char>>
             <String as From<Box<str>>>
             <String as From<Cow<'a, str>>>
             <String as From<&str>>
             <String as From<&mut str>>
             <String as From<&String>>
   = note: required for `Result<(), String>` to implement `FromResidual<Result<Infallible, ()>>`

Fix #72124.

rustbot · 2023-10-06T20:53:05Z

r? @wesleywiser

(rustbot has picked a reviewer for you, use r? to override)

compiler-errors · 2023-10-06T21:00:18Z

tests/ui/traits/question-mark-result-err-mismatch.stderr

+LL |           .map(|s| ())
+   |            ----------- this can be annotated with `?` because it has type `Result<(), &str>`
+LL |           .map_err(|_| ())
+   |            --------------- this can't be annotated with `?` because it has type `Result<(), ()>`
+LL |           .map(|()| "")?;
+   |            ------------^ the trait `From<()>` is not implemented for `String`
+   |            |
+   |            this can't be annotated with `?` because it has type `Result<&str, ()>`


this seems really heavy for long chains -- do i really care about ?-ability 4 .maps down?

Probably not. I would like to add an extra check to only point at places where the E changed (like we do for assoc types in method call chains), but wanted to keep the code down to a reviewable size.

I'd rather just put a hard limit on the number of method chain to walk backwards. Maybe 2 is sensible.

Let me know what you think of the new output.

compiler-errors · 2023-10-06T21:03:13Z

Also, this doesn't really fix #72124, does it? The problem in that issue is a shortcoming of closure inference causing us to forgo suggesting removing ; in a closure body?

estebank · 2023-10-06T21:05:36Z

Also, this doesn't really fix #72124, does it?

I'm adding some checks for the specific methods that take closures and that can change the type of Result<_, E>, but we still need the general context for any other cases that have underlying reasons beyond map_err calls.

compiler-errors · 2023-10-06T21:07:41Z

I'm adding some checks for the specific methods that take closures

For what it's worth, I think those checks should be implemented in a separate PR to separate concerns.

I'm mostly just noting that the PR shouldn't close (via fixes ### in the PR summary) that issue since there's additional work that needs to be done on that case specifically.

compiler-errors · 2023-12-05T16:43:09Z

@bors r+ rollup

bors · 2023-12-05T16:43:12Z

📌 Commit cce82d8 has been approved by compiler-errors

It is now in the queue for this repository.

…text, r=compiler-errors Provide context when `?` can't be called because of `Result<_, E>` When a method chain ending in `?` causes an E0277 because the expression's `Result::Err` variant doesn't have a type that can be converted to the `Result<_, E>` type parameter in the return type, provide additional context of which parts of the chain can and can't support the `?` operator. ``` error[E0277]: `?` couldn't convert the error to `String` --> $DIR/question-mark-result-err-mismatch.rs:27:25 | LL | fn bar() -> Result<(), String> { | ------------------ expected `String` because of this LL | let x = foo(); | ----- this has type `Result<_, String>` ... LL | .map_err(|_| ())?; | ---------------^ the trait `From<()>` is not implemented for `String` | | | this can't be annotated with `?` because it has type `Result<_, ()>` | = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait = help: the following other types implement trait `From<T>`: <String as From<char>> <String as From<Box<str>>> <String as From<Cow<'a, str>>> <String as From<&str>> <String as From<&mut str>> <String as From<&String>> = note: required for `Result<(), String>` to implement `FromResidual<Result<Infallible, ()>>` ``` Fix rust-lang#72124.

compiler-errors · 2023-12-05T19:34:06Z

compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs

+                hir::intravisit::walk_expr(self, ex);
+            }
+        }
+        let hir_id = self.tcx.hir().local_def_id_to_hir_id(obligation.cause.body_id);


This needs a rebase.

compiler-errors

please amend commit 2faaea9 after rebasing because local_def_id_to_hir_id has moved onto TyCtxt.

compiler-errors · 2023-12-05T19:34:59Z

@bors r-

r=me after fixing up that first commit

compiler-errors · 2023-12-05T19:36:11Z

(also if you're already changing this, pls squash the formatting commits down into more relevant ones)

When a method chain ending in `?` causes an E0277 because the expression's `Result::Err` variant doesn't have a type that can be converted to the `Result<_, E>` type parameter in the return type, provide additional context of which parts of the chain can and can't support the `?` operator. ``` error[E0277]: `?` couldn't convert the error to `String` --> $DIR/question-mark-result-err-mismatch.rs:28:25 | LL | fn bar() -> Result<(), String> { | ------------------ expected `String` because of this LL | let x = foo(); | ----- this can be annotated with `?` because it has type `Result<String, String>` LL | let one = x LL | .map(|s| ()) | ----------- this can be annotated with `?` because it has type `Result<(), String>` LL | .map_err(|_| ())?; | ---------------^ the trait `From<()>` is not implemented for `String` | | | this can't be annotated with `?` because it has type `Result<(), ()>` | = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait = help: the following other types implement trait `From<T>`: <String as From<char>> <String as From<Box<str>>> <String as From<Cow<'a, str>>> <String as From<&str>> <String as From<&mut str>> <String as From<&String>> = note: required for `Result<(), String>` to implement `FromResidual<Result<Infallible, ()>>` ``` Fix rust-lang#72124.

compiler-errors · 2023-12-05T22:23:13Z

Thanks!

@bors r+

bors · 2023-12-05T22:23:16Z

📌 Commit 23fa94f has been approved by compiler-errors

It is now in the queue for this repository.

Fix rust-lang#72124.

estebank · 2023-12-05T22:26:21Z

Pushed again (squashed the comment adding commit too)

@bors r=compiler-errors

bors · 2023-12-05T22:26:23Z

📌 Commit 70fe624 has been approved by compiler-errors

It is now in the queue for this repository.

…iaskrgr Rollup of 7 pull requests Successful merges: - rust-lang#116496 (Provide context when `?` can't be called because of `Result<_, E>`) - rust-lang#117563 (docs: clarify explicitly freeing heap allocated memory) - rust-lang#117874 (`riscv32` platform support) - rust-lang#118516 (Add ADT variant infomation to StableMIR and finish implementing TyKind::internal()) - rust-lang#118650 (add comment about keeping flags in sync between bootstrap.py and bootstrap.rs) - rust-lang#118664 (docs: remove rust-lang#110800 from release notes) - rust-lang#118669 (library: fix comment about const assert in win api) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of rust-lang#116496 - estebank:question-method-chain-context, r=compiler-errors Provide context when `?` can't be called because of `Result<_, E>` When a method chain ending in `?` causes an E0277 because the expression's `Result::Err` variant doesn't have a type that can be converted to the `Result<_, E>` type parameter in the return type, provide additional context of which parts of the chain can and can't support the `?` operator. ``` error[E0277]: `?` couldn't convert the error to `String` --> $DIR/question-mark-result-err-mismatch.rs:27:25 | LL | fn bar() -> Result<(), String> { | ------------------ expected `String` because of this LL | let x = foo(); | ----- this has type `Result<_, String>` ... LL | .map_err(|_| ())?; | ---------------^ the trait `From<()>` is not implemented for `String` | | | this can't be annotated with `?` because it has type `Result<_, ()>` | = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait = help: the following other types implement trait `From<T>`: <String as From<char>> <String as From<Box<str>>> <String as From<Cow<'a, str>>> <String as From<&str>> <String as From<&mut str>> <String as From<&String>> = note: required for `Result<(), String>` to implement `FromResidual<Result<Infallible, ()>>` ``` Fix rust-lang#72124.

rustbot assigned wesleywiser Oct 6, 2023

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compiler-errors assigned compiler-errors and unassigned wesleywiser Nov 30, 2023

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Rollup of 7 pull requests #118651

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estebank added 2 commits December 5, 2023 22:22

Point at fewer methods in the chain, only those that change the E type

5381796

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estebank added 2 commits December 5, 2023 22:24

Detect incorrect ; in Option::ok_or_else and Result::map_err

98e5317

Fix rust-lang#72124.

Reduce verbosity of error

70fe624

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Rollup of 7 pull requests #118679

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Provide context when `?` can't be called because of `Result<_, E>` #116496

Provide context when `?` can't be called because of `Result<_, E>` #116496

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Provide context when ? can't be called because of Result<_, E> #116496

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Provide context when `?` can't be called because of `Result<_, E>` #116496

Provide context when `?` can't be called because of `Result<_, E>` #116496

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