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Infallible promotion #3027

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Merged
KodrAus merged 8 commits into from
Jan 13, 2021
Merged

Infallible promotion #3027

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dfcde45
make implicit promotion infallible
RalfJung Nov 15, 2020
5f08ece
add some info that was in the MCP but not here; link to MCP
RalfJung Nov 15, 2020
575bfac
incorporate some feedback
RalfJung Nov 28, 2020
4592892
use bulleted lists and add example for CheckedAdd promotion
RalfJung Nov 28, 2020
49b6d56
add MIR overflow check reference; mention bitwise and shift operators
RalfJung Dec 2, 2020
508ae47
add note on the interaction of promotion and const validity checks
RalfJung Dec 20, 2020
4d747ee
fix CheckedAdd example
RalfJung Dec 21, 2020
cb78c70
RFC 3027: Infallible Promition
KodrAus Jan 13, 2021
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22 changes: 19 additions & 3 deletions text/0000-infallible-promotion.md
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Expand Up @@ -97,9 +97,25 @@ Inside a function body's block:
it into a static memory location and give the resulting reference a
`'static` lifetime.

Operations that definitely succeed include literals of any kind, constructors (struct/enum/union/tuple), struct/tuple field accesses, `+`/`-`/`*`.
(Checked arithmetic is not a problem: an addition in debug mode is compiled to a `CheckedAdd` MIR operation that never fails, which returns an `(<int>, bool)`, and is followed by a check of said `bool` to possibly raise a panic. We only ever promote the `CheckedAdd`, so evaluation of the promoted will never fail, even if the operation overflows.)
Operations that might fail include `/`/`%`, `panic!` (including the assertion that follows `Checked*` arithmetic to ensure that no overflow happened), array/slice indexing, any unsafe operation, and `const fn` calls (as they might do any of the above).
Operations that definitely succeed include:
- literals of any kind
- constructors (struct/enum/union/tuple)
- struct/tuple field accesses
- arithmetic that does not involve division: `+`/`-`/`*`

Note that arithmetic overflow is not a problem: an addition in debug mode is compiled to a `CheckedAdd` MIR operation that never fails, which returns an `(<int>, bool)`, and is followed by a check of said `bool` to possibly raise a panic. We only ever promote the `CheckedAdd`, so evaluation of the promoted will never fail, even if the operation overflows. For example, `&(1 + u32::MAX)` turns into something like:
```rust
const C: (u32, bool) = CheckedAdd(1, u32::MAX); // evaluates to (1, true).
assert!(C.1 == false);
&C.0
```

Operations that might fail include:
- `/`/`%`
- `panic!` (including the assertion that follows `Checked*` arithmetic to ensure that no overflow happened)
- array/slice indexing
- any unsafe operation
- `const fn` calls (as they might do any of the above)

# Reference-level explanation
[reference-level-explanation]: #reference-level-explanation
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