Reflection MVP
I am opening this PR for discussion about the general design we should start out with, as there are various options (that are not too hard to transition between each other, so we should totally just pick one and go with it and reiterate later)
r? @scottmcm and @joshtriplett
project goal issue: https://github.com/rust-lang/rust-project-goals/issues/406
tracking issue: https://github.com/rust-lang/rust/issues/146922
The design currently implemented by this PR is
* `TypeId::info` (method, usually used as `id.info()` returns a `Type` struct
* the `Type` struct has fields that contain information about the type
* the most notable field is `kind`, which is a non-exhaustive enum over all possible type kinds and their specific information. So it has a `Tuple(Tuple)` variant, where the only field is a `Tuple` struct type that contains more information (The list of type ids that make up the tuple).
* To get nested type information (like the type of fields) you need to call `TypeId::info` again.
* There is only one language intrinsic to go from `TypeId` to `Type`, and it does all the work
An alternative design could be
* Lots of small methods (each backed by an intrinsic) on `TypeId` that return all the individual information pieces (size, align, number of fields, number of variants, ...)
* This is how C++ does it (see https://lemire.me/blog/2025/06/22/c26-will-include-compile-time-reflection-why-should-you-care/ and https://isocpp.org/files/papers/P2996R13.html#member-queries)
* Advantage: you only get the information you ask for, so it's probably cheaper if you get just one piece of information for lots of types (e.g. reimplementing size_of in terms of `TypeId::info` is likely expensive and wasteful)
* Disadvantage: lots of method calling (and `Option` return types, or "general" methods like `num_fields` returning 0 for primitives) instead of matching and field accesses
* a crates.io crate could implement `TypeId::info` in terms of this design
The backing implementation is modular enough that switching from one to the other is probably not an issue, and the alternative design could be easier for the CTFE engine's implementation, just not as nice to use for end users (without crates wrapping the logic)
One wart of this design that I'm fixing in separate branches is that `TypeId::info` will panic if used at runtime, while it should be uncallable
cleaned up cast-enum-const.rs
cleaned up ufcs-trait-object-format.rs
add comment to cast-to-box-arr.rs
add comment to shadow-primitives.rs
add comment to associated-type-const-nomalization.rs
add comment to fn-trait-explicit-call.rs
add comment to call-unit-struct-impl-fn-once.rs
add comment to cast-to-char-compare.rs
add comment to self-in-method-body-resolves.rs
add comment to derive-debug-newtype-unsized-slice.rs
add comment to tuple-ref-order-distinct-impls.rs
add comment to derive-debug-generic-with-lifetime.rs
add comment to recursive-trait-bound-on-type-param.rs
cleaned up const-static-ref-to-closure.rs
add comment to const-iter-no-conflict-for-loop.rs
Tidying up tests/ui/issues 15 tests [6/N]
> [!NOTE]
> Intermediate commits are intended to help review, but will be squashed add comment commit prior to merge.
part of rust-lang/rust#133895
r? Kivooeo
add comment to closure-move-use-after-move-diagnostic.rs
add comment to missing-operator-after-float.rs
add comment to closure-array-break-length.rs
add comment to box-lifetime-argument-not-allowed.rs
add comment to const-return-outside-fn.rs
add comment to drop-conflicting-impls.rs
add comment to unbalanced-doublequote-2.rs
add comment to borrow-immutable-deref-box.rs
add comment to for-in-const-eval.rs
add comment to borrowck-annotated-static-lifetime.rs
cleaned up cast-rfc0401.rs
add comment to nll-anon-to-static.rs
add comment to cast-to-dyn-any.rs
add comment to missing-associated-items.rs
add comment to enum-discriminant-missing-variant.rs
Tidying up tests/ui/issues 33 tests [4/N]
> [!NOTE]
> Intermediate commits are intended to help review, but will be squashed add comment commit prior to merge.
part of rust-lang/rust#133895
`tests/ui/compile-flags` split it into `tests/ui/compile-flags/invalid/` and `tests/ui/compile-flags/run-pass/`
r? Kivooeo
```
error[E0308]: mismatched types
--> $DIR/macro-span-caller-replacement.rs:5:17
|
LL | s = format!("{arg}");
| ^^^^^^^^^^^^^^^^ expected `&str`, found `String`
...
LL | macro_with_format!();
| -------------------- in this macro invocation
|
= note: this error originates in the macro `format` which comes from the expansion of the macro `macro_with_format` (in Nightly builds, run with -Z macro-backtrace for more info)
```
Merged tests/ui/typeck/non-function-call-error-2 with
tests/ui/typeck/non-function-call-error
Add comment to
tests/ui/traits/normalize-associated-type-in-where-clause.rs
Merged tests/ui/privacy/private-item-simple-2.rs with
tests/ui/privacy/private-item-simple.rs
Merged tests/ui/str/str-add-operator-2.rs with
tests/ui/str/str-add-operator.rs
Add comment to tests/ui/imports/duplicate-empty-imports.rs
Add comment to tests/ui/for-loop-while/nested-loop-break-unit.rs
Add comment to tests/ui/match/match-ref-option-pattern.rs
Add comment to tests/ui/closures/simple-capture-and-call.rs
Add comment to tests/ui/type/never-type-inference-fail.rs
Add comment to tests/ui/match/match-stack-overflow-72933.rs
Point at trait and associated item when that associated item is used in a const context. Suggest making the trait `#[const_trait]`.
```
error[E0015]: cannot call non-const method `<() as Trait>::foo` in constant functions
--> $DIR/inline-incorrect-early-bound-in-ctfe.rs:26:8
|
LL | ().foo();
| ^^^^^
|
note: method `foo` is not const because trait `Trait` is not const
--> $DIR/inline-incorrect-early-bound-in-ctfe.rs:13:1
|
LL | trait Trait {
| ^^^^^^^^^^^ this trait is not const
LL | fn foo(self);
| ------------- this method is not const
= note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
help: consider making trait `Trait` const
|
LL + #[const_trait]
LL | trait Trait {
|
```
```
error[E0277]: the trait bound `[[u16; 3]; 2]: Bar` is not satisfied
--> $DIR/issue-67185-2.rs:21:6
|
LL | impl Foo for FooImpl {}
| ^^^ the trait `Bar` is not implemented for `[[u16; 3]; 2]`
|
help: the following other types implement trait `Bar`
--> $DIR/issue-67185-2.rs:9:1
|
LL | impl Bar for [u16; 4] {}
| ^^^^^^^^^^^^^^^^^^^^^ `[u16; 4]`
LL | impl Bar for [[u16; 3]; 3] {}
| ^^^^^^^^^^^^^^^^^^^^^^^^^^ `[[u16; 3]; 3]`
note: required by a bound in `Foo`
--> $DIR/issue-67185-2.rs:14:30
|
LL | trait Foo
| --- required by a bound in this trait
LL | where
LL | [<u8 as Baz>::Quaks; 2]: Bar,
| ^^^ required by this bound in `Foo`
```
```
error[E0277]: the trait bound `u32: Trait` is not satisfied
--> $DIR/trait_objects_fail.rs:26:9
|
LL | foo(&10_u32);
| ^^^^^^^ the trait `Trait` is not implemented for `u32`
|
help: the trait `Trait<12>` is not implemented for `u32`
but trait `Trait<2>` is implemented for it
--> $DIR/trait_objects_fail.rs:7:1
|
LL | impl Trait<2> for u32 {}
| ^^^^^^^^^^^^^^^^^^^^^
= note: required for the cast from `&u32` to `&dyn Trait`
```
Pointing at the `impl` definition that *could* apply given a different self type is particularly useful when it has a blanket self type, as it might not be obvious and is not trivially greppable:
```
error[E0277]: the trait bound `RawImpl<_>: Raw<_>` is not satisfied
--> $DIR/issue-62742.rs:4:5
|
LL | WrongImpl::foo(0i32);
| ^^^^^^^^^ unsatisfied trait bound
|
help: the trait `Raw<_>` is not implemented for `RawImpl<_>`
but trait `Raw<[_]>` is implemented for it
--> $DIR/issue-62742.rs:29:1
|
LL | impl<T> Raw<[T]> for RawImpl<T> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: required by a bound in `SafeImpl`
--> $DIR/issue-62742.rs:33:35
|
LL | pub struct SafeImpl<T: ?Sized, A: Raw<T>>(PhantomData<(A, T)>);
| ^^^^^^ required by this bound in `SafeImpl`
```
Rehome 26 `tests/ui/issues/` tests to other subdirectories under `tests/ui/` [#5 of Batch #2]
Part of rust-lang/rust#133895
Methodology:
1. Refer to the previously written `tests/ui/SUMMARY.md`
2. Find an appropriate category for the test, using the original issue thread and the test contents.
3. Add the issue URL at the bottom (not at the top, as that would mess up stderr line numbers)
4. Rename the tests to make their purpose clearer
Inspired by the methodology that Kivooeo was using.
r? ```@jieyouxu```
Rehome 30 `tests/ui/issues/` tests to other subdirectories under `tests/ui/` [#4 of Batch #2]
Part of rust-lang/rust#133895
Methodology:
1. Refer to the previously written `tests/ui/SUMMARY.md`
2. Find an appropriate category for the test, using the original issue thread and the test contents.
3. Add the issue URL at the bottom (not at the top, as that would mess up stderr line numbers)
4. Rename the tests to make their purpose clearer
Inspired by the methodology that `@Kivooeo` was using.
r? `@jieyouxu`
We'll still error due to the `opt_bad_ty` of `method_autoderef_steps`.
This slightly worsens the span of `infer_var.method()` which is now the
same as for `Box::new(infer_var).method()`.
Unlike `structurally_resolve_type`, `probe_op` does not check whether
the infcx is already tainted, so this results in 2 previously not emitted
errors.
