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Silence unreachable code lint from await desugaring - #64930
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This commit silences the unreachable code lint when it originates from within a await desugaring. Signed-off-by: David Wood <david@davidtw.co>
rust-highfive
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Sep 30, 2019
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(rust_highfive has picked a reviewer for you, use r? to override) |
petrochenkov
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Sep 30, 2019
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@bors r+ rollup |
bors
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Sep 30, 2019
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📌 Commit 870b47f has been approved by |
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Oct 1, 2019
…it, r=petrochenkov Silence unreachable code lint from await desugaring Fixesrust-lang#61798. This PR silences the unreachable code lint when it originates from within an await desugaring.
Centril added a commit
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Oct 1, 2019
…it, r=petrochenkov Silence unreachable code lint from await desugaring Fixesrust-lang#61798. This PR silences the unreachable code lint when it originates from within an await desugaring.
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Oct 1, 2019
Rollup of 10 pull requests Successful merges: - #63674 (syntax: Support modern attribute syntax in the `meta` matcher) - #63931 (Stabilize macros in some more positions) - #64887 (syntax: recover trailing `|` in or-patterns) - #64895 (async/await: improve not-send errors) - #64896 (Remove legacy grammar) - #64907 (A small amount of tidying-up factored out from PR #64648) - #64928 (Add tests for some issues) - #64930 (Silence unreachable code lint from await desugaring) - #64935 (Improve code clarity) - #64937 (Deduplicate closure type errors) Failed merges: r? @ghost
pnkfelix
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Oct 3, 2019
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discussed at rustc mtg. accepted for beta backport. |
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Oct 24, 2019
[beta] backport rollup This includes a bunch of PRs: * Fix redundant semicolon lint interaction with proc macro attributes #64387 * Upgrade async/await to "used" keywords. #64875 * syntax: fix dropping of attribute on first param of non-method assocated fn #64894 * async/await: improve not-send errors #64895 * Silence unreachable code lint from await desugaring #64930 * Always mark rust and rust-call abi's as unwind #65020 * Account for macro invocation in `let mut $pat` diagnostic. #65123 * Ensure that associated `async fn`s have unique fresh param names #65142 * Add troubleshooting section to PGO chapter in rustc book. #65402 * Upgrade GCC to 8.3.0, glibc to 1.17.0 and crosstool-ng to 1.24.0 for dist-armv7-linux #65302 * Optimize `try_expand_impl_trait_type` #65293 * use precalculated dominators in explain_borrow #65172 * Fix ICE #64964#64989
bors added a commit
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Oct 26, 2019
[beta] backport rollup This includes a bunch of PRs: * Fix redundant semicolon lint interaction with proc macro attributes #64387 * Upgrade async/await to "used" keywords. #64875 * syntax: fix dropping of attribute on first param of non-method assocated fn #64894 * async/await: improve not-send errors #64895 * Silence unreachable code lint from await desugaring #64930 * Always mark rust and rust-call abi's as unwind #65020 * Account for macro invocation in `let mut $pat` diagnostic. #65123 * Ensure that associated `async fn`s have unique fresh param names #65142 * Add troubleshooting section to PGO chapter in rustc book. #65402 * Upgrade GCC to 8.3.0, glibc to 1.17.0 and crosstool-ng to 1.24.0 for dist-armv7-linux #65302 * Optimize `try_expand_impl_trait_type` #65293 * use precalculated dominators in explain_borrow #65172 * Fix ICE #64964#64989 * [beta] Revert "Auto merge of #62948 - matklad:failable-file-loading, r=petro… #65273 * save-analysis: Don't ICE when resolving qualified type paths in struct members #65353 * save-analysis: Nest tables when processing impl block definitions #65511 * Avoid ICE when checking `Destination` of `break` inside a closure #65518 * Avoid ICE when adjusting bad self ty #65755 * workaround msys2 bug #65762
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Jul 31, 2024
… r=fmease
Properly mark loop as diverging if it has no breaks
Due to specifics about the desugaring of the `.await` operator, HIR typeck doesn't recognize that `.await`ing an `impl Future<Output = !>` will diverge in the same way as calling a `fn() -> !`.
This is because the await operator desugars to approximately:
```rust
loop {
match future.poll(...) {
Poll::Ready(x) => break x,
Poll::Pending => {}
}
}
```
We know that the value of `x` is `!`, however since `break` is a coercion site, we coerce `!` to some `?0` (the type of the loop expression). Then since the type of the `loop {...}` expression is `?0`, we will not detect the loop as diverging like we do with other expressions that evaluate to `!`:
https://github.com/rust-lang/rust/blob/0b5eb7ba7bd796fb39c8bb6acd9ef6c140f28b65/compiler/rustc_hir_typeck/src/expr.rs#L240-L243
We can technically fix this in two ways:
1. Make coercion of loop exprs more eagerly result in a type of `!` when the only break expressions have type `!`.
2. Make loops understand that all of that if they have only diverging break values, then the loop diverges as well.
(1.) likely has negative effects on inference, and seems like a weird special case to drill into coercion. However, it turns out that (2.) is very easy to implement, we already record whether a loop has any break expressions, and when we do so, we actually skip over any break expressions with diverging values!:
https://github.com/rust-lang/rust/blob/0b5eb7ba7bd796fb39c8bb6acd9ef6c140f28b65/compiler/rustc_hir_typeck/src/expr.rs#L713-L716
Thus, we can consider the loop as diverging if we see that it has no breaks, which is the change implemented in this PR.
This is not usually a problem in regular code for two reasons:
1. In regular code, we already mark `break diverging()` as unreachable if `diverging()` is unreachable. We don't do this for `.await`, since we suppress unreachable errors within `.await` (rust-lang#64930). Un-suppressing this code will result in spurious unreachable expression errors pointing to internal await machinery.
3. In loops that truly have no breaks (e.g. `loop {}`), we already evaluate the type of the loop to `!`, so this special case is kinda moot. This only affects loops that have `break`s with values of type `!`.
Thus, this seems like a change that may affect more code than just `.await`, but it likely does not in meaningful ways; if it does, it's certainly correct to apply.
Fixesrust-lang#128434matthiaskrgr added a commit
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Jul 31, 2024
… r=fmease
Properly mark loop as diverging if it has no breaks
Due to specifics about the desugaring of the `.await` operator, HIR typeck doesn't recognize that `.await`ing an `impl Future<Output = !>` will diverge in the same way as calling a `fn() -> !`.
This is because the await operator desugars to approximately:
```rust
loop {
match future.poll(...) {
Poll::Ready(x) => break x,
Poll::Pending => {}
}
}
```
We know that the value of `x` is `!`, however since `break` is a coercion site, we coerce `!` to some `?0` (the type of the loop expression). Then since the type of the `loop {...}` expression is `?0`, we will not detect the loop as diverging like we do with other expressions that evaluate to `!`:
https://github.com/rust-lang/rust/blob/0b5eb7ba7bd796fb39c8bb6acd9ef6c140f28b65/compiler/rustc_hir_typeck/src/expr.rs#L240-L243
We can technically fix this in two ways:
1. Make coercion of loop exprs more eagerly result in a type of `!` when the only break expressions have type `!`.
2. Make loops understand that all of that if they have only diverging break values, then the loop diverges as well.
(1.) likely has negative effects on inference, and seems like a weird special case to drill into coercion. However, it turns out that (2.) is very easy to implement, we already record whether a loop has any break expressions, and when we do so, we actually skip over any break expressions with diverging values!:
https://github.com/rust-lang/rust/blob/0b5eb7ba7bd796fb39c8bb6acd9ef6c140f28b65/compiler/rustc_hir_typeck/src/expr.rs#L713-L716
Thus, we can consider the loop as diverging if we see that it has no breaks, which is the change implemented in this PR.
This is not usually a problem in regular code for two reasons:
1. In regular code, we already mark `break diverging()` as unreachable if `diverging()` is unreachable. We don't do this for `.await`, since we suppress unreachable errors within `.await` (rust-lang#64930). Un-suppressing this code will result in spurious unreachable expression errors pointing to internal await machinery.
3. In loops that truly have no breaks (e.g. `loop {}`), we already evaluate the type of the loop to `!`, so this special case is kinda moot. This only affects loops that have `break`s with values of type `!`.
Thus, this seems like a change that may affect more code than just `.await`, but it likely does not in meaningful ways; if it does, it's certainly correct to apply.
Fixesrust-lang#128434rust-timer added a commit
to rust-lang-ci/rust
that referenced
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Aug 1, 2024
Rollup merge of rust-lang#128443 - compiler-errors:async-unreachable, r=fmease Properly mark loop as diverging if it has no breaks Due to specifics about the desugaring of the `.await` operator, HIR typeck doesn't recognize that `.await`ing an `impl Future<Output = !>` will diverge in the same way as calling a `fn() -> !`. This is because the await operator desugars to approximately: ```rust loop { match future.poll(...) { Poll::Ready(x) => break x, Poll::Pending => {} } } ``` We know that the value of `x` is `!`, however since `break` is a coercion site, we coerce `!` to some `?0` (the type of the loop expression). Then since the type of the `loop {...}` expression is `?0`, we will not detect the loop as diverging like we do with other expressions that evaluate to `!`: https://github.com/rust-lang/rust/blob/0b5eb7ba7bd796fb39c8bb6acd9ef6c140f28b65/compiler/rustc_hir_typeck/src/expr.rs#L240-L243 We can technically fix this in two ways: 1. Make coercion of loop exprs more eagerly result in a type of `!` when the only break expressions have type `!`. 2. Make loops understand that all of that if they have only diverging break values, then the loop diverges as well. (1.) likely has negative effects on inference, and seems like a weird special case to drill into coercion. However, it turns out that (2.) is very easy to implement, we already record whether a loop has any break expressions, and when we do so, we actually skip over any break expressions with diverging values!: https://github.com/rust-lang/rust/blob/0b5eb7ba7bd796fb39c8bb6acd9ef6c140f28b65/compiler/rustc_hir_typeck/src/expr.rs#L713-L716 Thus, we can consider the loop as diverging if we see that it has no breaks, which is the change implemented in this PR. This is not usually a problem in regular code for two reasons: 1. In regular code, we already mark `break diverging()` as unreachable if `diverging()` is unreachable. We don't do this for `.await`, since we suppress unreachable errors within `.await` (rust-lang#64930). Un-suppressing this code will result in spurious unreachable expression errors pointing to internal await machinery. 3. In loops that truly have no breaks (e.g. `loop {}`), we already evaluate the type of the loop to `!`, so this special case is kinda moot. This only affects loops that have `break`s with values of type `!`. Thus, this seems like a change that may affect more code than just `.await`, but it likely does not in meaningful ways; if it does, it's certainly correct to apply. Fixesrust-lang#128434
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Fixes#61798.
This PR silences the unreachable code lint when it originates from within an await desugaring.