Add more rules about when expressions diverge#2186
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ehuss wants to merge 1 commit intorust-lang:masterfrom
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Add more rules about when expressions diverge#2186ehuss wants to merge 1 commit intorust-lang:masterfrom
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rust-lang#2067 added a chapter on divergence along with rules for some of the more subtle expressions like `if` or `match`. This adds rules for almost all the rest of the expressions. Most of these are pretty straightforward propagation rules which could be left implicit. It may seem a little excessive to have a separate rule for each one, but I think the consistency is worth it because there are various subtleties. This also may help us be clear about these things when adding new expressions to the language to ensure we think about the divergence rules. This does not cover const expressions (const blocks, static, const, array repeat, etc.) because I still do not yet fully know how those should be documented (see rust-lang#2153). I'm not entirely excited by having the long list of rules in the divergence chapter, mostly because of the length. However, I think it is helpful to cross-index these kinds of things. There are some interesting subtleties that I noticed: - `while` loops cannot diverge. It's a little surprising to me (I would expect the condition to allow it to diverge). I suspect this is due to the desugaring to a loop expression using `break`, which does not diverge. - Updated details for let-chains. - I couldn't think of a way for a `path` expression to refer to a never type on stable, so I left a comment in there to update it when never type is stabilized. I am writing this as-if the uninhabited-static lint is an error and not supported. References for the implementation: - Place expression with never must be read: https://github.com/rust-lang/rust/blob/59fd4ef94daa991e6797b5aa6127e824f3067def/compiler/rustc_hir_typeck/src/expr.rs#L318-L326 - `break` is never: https://github.com/rust-lang/rust/blob/59fd4ef94daa991e6797b5aa6127e824f3067def/compiler/rustc_hir_typeck/src/expr.rs#L819-L820 - `continue` is never: https://github.com/rust-lang/rust/blob/0376d43d443cba463a0b6a6ec9140ea17d7b7130/compiler/rustc_hir_typeck/src/expr.rs#L861 - `return` is never: https://github.com/rust-lang/rust/blob/0376d43d443cba463a0b6a6ec9140ea17d7b7130/compiler/rustc_hir_typeck/src/expr.rs#L921 - `if` divergence: https://github.com/rust-lang/rust/blob/59fd4ef94daa991e6797b5aa6127e824f3067def/compiler/rustc_hir_typeck/src/expr.rs#L1236-L1242 - `loop`: https://github.com/rust-lang/rust/blob/59fd4ef94daa991e6797b5aa6127e824f3067def/compiler/rustc_hir_typeck/src/expr.rs#L1450-L1456 - `asm!`: https://github.com/rust-lang/rust/blob/59fd4ef94daa991e6797b5aa6127e824f3067def/compiler/rustc_hir_typeck/src/expr.rs#L3684 - `match`: https://github.com/rust-lang/rust/blob/59fd4ef94daa991e6797b5aa6127e824f3067def/compiler/rustc_hir_typeck/src/_match.rs#L19-L178 - block break: https://github.com/rust-lang/rust/blob/59fd4ef94daa991e6797b5aa6127e824f3067def/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs#L1167-L1171 - lazy bool: https://github.com/rust-lang/rust/blob/0376d43d443cba463a0b6a6ec9140ea17d7b7130/compiler/rustc_hir_typeck/src/op.rs#L111 Closes rust-lang#2152
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Seems good to me. Definitely the expression list in divergence.md is pretty long now.
| > The future type that rustc generates is roughly equivalent to an enum with one variant per `await` point, where each variant stores the data needed to resume from its corresponding point. | ||
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| r[expr.block.async.diverging] | ||
| An async block expression does not itself [diverge], but evaluating the future (such as through `await`) diverges if the output type is the [never type]. |
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The latter half is covered by expr.await.diverging. Is the duplication worth it?
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#2067 added a chapter on divergence along with rules for some of the more subtle expressions like
iformatch. This adds rules for almost all the rest of the expressions.Most of these are pretty straightforward propagation rules which could be left implicit. It may seem a little excessive to have a separate rule for each one, but I think the consistency is worth it because there are various subtleties. This also may help us be clear about these things when adding new expressions to the language to ensure we think about the divergence rules.
This does not cover const expressions (const blocks, static, const, array repeat, etc.) because I still do not yet fully know how those should be documented (see #2153).
I'm not entirely excited by having the long list of rules in the divergence chapter, mostly because of the length. However, I think it is helpful to cross-index these kinds of things.
There are some interesting subtleties that I noticed:
whileloops cannot diverge. It's a little surprising to me (I would expect the condition to allow it to diverge). I suspect this is due to the desugaring to a loop expression usingbreak, which does not diverge.pathexpression to refer to a never type on stable, so I left a comment in there to update it when never type is stabilized. I am writing this as-if the uninhabited-static lint is an error and not supported.References for the implementation:
breakis never: https://github.com/rust-lang/rust/blob/59fd4ef94daa991e6797b5aa6127e824f3067def/compiler/rustc_hir_typeck/src/expr.rs#L819-L820continueis never: https://github.com/rust-lang/rust/blob/0376d43d443cba463a0b6a6ec9140ea17d7b7130/compiler/rustc_hir_typeck/src/expr.rs#L861returnis never: https://github.com/rust-lang/rust/blob/0376d43d443cba463a0b6a6ec9140ea17d7b7130/compiler/rustc_hir_typeck/src/expr.rs#L921ifdivergence: https://github.com/rust-lang/rust/blob/59fd4ef94daa991e6797b5aa6127e824f3067def/compiler/rustc_hir_typeck/src/expr.rs#L1236-L1242loop: https://github.com/rust-lang/rust/blob/59fd4ef94daa991e6797b5aa6127e824f3067def/compiler/rustc_hir_typeck/src/expr.rs#L1450-L1456asm!: https://github.com/rust-lang/rust/blob/59fd4ef94daa991e6797b5aa6127e824f3067def/compiler/rustc_hir_typeck/src/expr.rs#L3684match: https://github.com/rust-lang/rust/blob/59fd4ef94daa991e6797b5aa6127e824f3067def/compiler/rustc_hir_typeck/src/_match.rs#L19-L178Closes #2152