2015-08-14.md

Subteam report: lang 2015-08-14

Highlights

The following RFCs were promoted to final comment period:

• RFC #1229, which specifies that when the compiler is doing constant evaluation as a form of optimization, it should not report compilation errors for overflows or other problems that are encountered (warnings are ok). Otherwise, improvements in the constant evaluator can become breaking changes.
• RFC #1234, which modifies the DST coercion rules to permit PhantomData. The discussion has primarily focused on whether we can indeed make the rules even more accepting than that; conclusion was to leave this for future work.

Full list of T-lang RFCs in FCP.

Some fine fellow (ahem) also started a scintillating discussion on the interaction of type parameter fallback with integral fallback started on internals, which would benefit from more eyeballs:

• Interaction of user-defined and integral fallbacks with inference

Last week, we had a number of RFCs enter FCP. Unfortunately, we did not reach final decisions on very many of them, due to Felix Klock being out for some well-earned R&R. However, we did merge a few noncontroversial items:

• RFC #1209 updates the follow set for types in macros to include ;.
• RFC #1189 fixes some typos.

We also decided to close several RFCs for prioritization reasons, even if the ideas espoused had merit:

• RFC 886, allowing "must use" on functions (by @huonw).
• RFC 890, custom preludes (by @aturon).
• RFC 1216, promote ! to a type (by @canndrew).

Dashboard

RFC phase

• PR #197: ManuallyDrop type gives precise control of dtors of inline data.
• PR #243: Trait-based exception handling
• PR #638: Specify unwinding
• PR #733: Amend macro future proofing RFC
• PR #1062: Constants that depend on type parameters in generic code
• PR #1129: IndexAssign: overloading the a[b] = c expression
• PR #1148: Mutually exclusive traits
• PR #1168: Make size an associated constant
• PR #1177: Anonymous/placeholder lifetime "'_".
• PR #1180: Propose Interior<T> data-type, to allow moves out of the dropped value during the drop hook.
• PR #1190: Style: function declarations
• PR #1196: RFC for allowing eliding more type parameters.
• PR #1199: SIMD groundwork
• PR #1210: RFC: impl specialization
• PR #1224: Update the RFC process with sub-teams, amongst other things.
• PR #1228: Place left arrow syntax (place <- expr)
• PR #1229: turn statically known erroneous code into a warning and continue normal code-generation
• PR #1234: specify that CoerceUnsized should ignore PhantomData fields
• PR #1237: clarify extreme operator behaviour
• PR #1238: Nonparametric dropck
• PR #1240: References into repr(packed) structs should be unsafe.
• PR #1245: Amend #911 const-fn to allow unsafe const functions
• PR #1248: Safe intrinsics RFC
• FCP PR #886: Allow #[must_use] on functions, rather than just types. Mark Result::{ok,err} #[must_use].
• FCP PR #890: RFC: Custom preludes
• FCP PR #953: overloaded assignment operations a += b
• FCP PR #1135: Implement raw fat pointer comparisons
• FCP PR #1189: 0401: misc improvements
• FCP PR #1192: RFC for inclusive ranges with ...
• FCP PR #1209: Update FOLLOW set for ty tokens

Implementation phase

• Issue #13231: opt-in built-in bounds traits RFC tracker
• Issue #15552: Implement lifetime elision
• Issue #15701: Add attributes to expressions, etc.
• Issue #16293: Implement UFCS (Tracking RFC 132)
• Issue #17307: Implement associated items
• Issue #18469: Implement revised coercion rules
• Issue #21939: Tracking issue for Finalize defaulted type parameters (RFC 213)
• Issue #22020: Tracking issue for Integer Overflow (RFC 560)
• Issue #22181: box and in expressions (tracking issue for RFC 809)
• Issue #23416: Type ascription (tracking issue for RFC 803)
• Issue #24111: const fn tracking issue (RFC 911)
• Issue #24263: discriminant_value intrinsic -- tracking issue for 639
• Issue #24266: Empty struct with braces tracking issue (RFC 218)
• Issue #26438: Tracking issue for Adjust default object bounds (RFC 1156)
• Issue #27336: Tracking Issue for RFC 213: Default Type Parameter Fallback

High priority issues

• Issue #24622: Mismatched assumptions for interaction between associated type and lifetime in trait
• Issue #25658: In macros, $($x:expr),* fragments can be used to bypass future-proofing restrictions
• Issue #25860: Implied bounds on nested references + variance = soundness hole
• Issue #26438: Tracking issue for Adjust default object bounds (RFC 1156)
• Issue #26739: In macros, expr followed by { is buggy
• Issue #26775: 'pub extern crate' is accepted and behaves badly
• Issue #26813: Regression on beta for trait bounds on supertrait's associated items
• Issue #27060: repr(packed) allows invalid unaligned loads

Needs decision

• Issue #27499: Use different numbers of #s when expanding documentation comments