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"nonexistent struct field" warning when type-punning in a const definition #48746

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sfackler opened this issue Mar 5, 2018 · 1 comment · Fixed by #46882
Closed

"nonexistent struct field" warning when type-punning in a const definition #48746

sfackler opened this issue Mar 5, 2018 · 1 comment · Fixed by #46882
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A-const-fn Area: const fn foo(..) {..}. Pure functions which can be applied at compile time. C-enhancement Category: An issue proposing an enhancement or a PR with one. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@sfackler
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sfackler commented Mar 5, 2018 

#[repr(C)]
union Foo {
    a: u32,
    b: i32,
}

const FOO: i32 = unsafe { Foo { a: 15 }.b };

fn main() {
    println!("{}", FOO);
}
warning: constant evaluation error: nonexistent struct field
 --> src/main.rs:8:18
  |
8 | const FOO: i32 = unsafe { Foo { a: 15 }.b };
  |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: #[warn(const_err)] on by default
@petrochenkov
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Should be fixed in #46882

@oli-obk oli-obk mentioned this issue Mar 5, 2018
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@pietroalbini pietroalbini added C-enhancement Category: An issue proposing an enhancement or a PR with one. A-const-fn Area: const fn foo(..) {..}. Pure functions which can be applied at compile time. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Mar 6, 2018
bors added a commit that referenced this issue Mar 7, 2018
@bors
Auto merge of #46882 - oli-obk:miri3, r=eddyb
48acf10 
Replace all const evaluation with miri

* error reporting in constants prints a stacktrace through all called const fns
* Trivial constant propagation and folding in MIR (always active, irrelevant of the optimization level)
* can now use floating constants in patterns (previously only floating point literals were allowed)
    * the future compat lint is still produced for both cases
* can index into constant arrays during const eval (previously feature gated)
* can create a constant union value with field `a` and read from field `b`
* can dereference references into constants
* can create references inside constants (`const X: &u32 = &22`)
* Tuple struct constructors can be used in constants
* regression in const eval errors spans (some of these need improvements in mir debug info)
* can cast floats to ints and vice versa (in constants, and even nan/inf constants)
* Mir dump prints false/true instead of 0u8/1u8
* `1i8 >> [8][0]` does not lint about exceeding bitshifts anymore.
    * Needs const propagation across projections
* `foo[I]` produces a const eval lint if `foo: [T; N]` and `N < I`
    * Essentially all builtin panics produce lints if they can be statically proven to trigger at runtime. This is on a best effort basis, so there might be some complex cases that don't trigger. (The runtime panic stays there, irrelevant of whether the lint is produced or not)
* can use `union`s to implement `transmute` for `Copy` types in constants without a feature gate. With all the greatness and nasal demons that come with this.
* can convert integers to `&'static T` in constants (useful for embedded)

fixes #34997 (stack overflow with many constants)
fixes #25574 (deref byte strings in patterns)
fixes #27918 (broken mir ICE)
fixes #46114 (ICE on struct constructors in patterns)
fixes #37448 (`SomeStruct { foo } as SomeStruct`)
fixes #43754 (`return` in const fn)
fixes #41898 (tuple struct constructors)
fixes #31364 (infinite recursion with const fn, fixed by miri's recursion limit)
closes #29947 (const indexing stabilization)
fixes #45044 (pattern matching repeat expressions)
fixes #47971 (ICE on const fn + references)
fixes #48081 (ICE on cyclic assoc const error)
fixes #48746 (nonhelpful error message with unions)

r? @eddyb

even though 1k loc are added in tests, this PR reduces the loc in this repository by 700
bors added a commit that referenced this issue Mar 8, 2018
@bors
Auto merge of #46882 - oli-obk:miri3, r=eddyb
c90f682 
Replace all const evaluation with miri

* error reporting in constants prints a stacktrace through all called const fns
* Trivial constant propagation and folding in MIR (always active, irrelevant of the optimization level)
* can now use floating constants in patterns (previously only floating point literals were allowed)
    * the future compat lint is still produced for both cases
* can index into constant arrays during const eval (previously feature gated)
* can create a constant union value with field `a` and read from field `b`
* can dereference references into constants
* can create references inside constants (`const X: &u32 = &22`)
* Tuple struct constructors can be used in constants
* regression in const eval errors spans (some of these need improvements in mir debug info)
* can cast floats to ints and vice versa (in constants, and even nan/inf constants)
* Mir dump prints false/true instead of 0u8/1u8
* `1i8 >> [8][0]` does not lint about exceeding bitshifts anymore.
    * Needs const propagation across projections
* `foo[I]` produces a const eval lint if `foo: [T; N]` and `N < I`
    * Essentially all builtin panics produce lints if they can be statically proven to trigger at runtime. This is on a best effort basis, so there might be some complex cases that don't trigger. (The runtime panic stays there, irrelevant of whether the lint is produced or not)
* can use `union`s to implement `transmute` for `Copy` types in constants without a feature gate. With all the greatness and nasal demons that come with this.
* can convert integers to `&'static T` in constants (useful for embedded)

fixes #34997 (stack overflow with many constants)
fixes #25574 (deref byte strings in patterns)
fixes #27918 (broken mir ICE)
fixes #46114 (ICE on struct constructors in patterns)
fixes #37448 (`SomeStruct { foo } as SomeStruct`)
fixes #43754 (`return` in const fn)
fixes #41898 (tuple struct constructors)
fixes #31364 (infinite recursion with const fn, fixed by miri's recursion limit)
closes #29947 (const indexing stabilization)
fixes #45044 (pattern matching repeat expressions)
fixes #47971 (ICE on const fn + references)
fixes #48081 (ICE on cyclic assoc const error)
fixes #48746 (nonhelpful error message with unions)

r? @eddyb

even though 1k loc are added in tests, this PR reduces the loc in this repository by 700
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A-const-fn Area: const fn foo(..) {..}. Pure functions which can be applied at compile time. C-enhancement Category: An issue proposing an enhancement or a PR with one. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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Replace all const evaluation with miri oli-obk/rust
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@sfackler @pietroalbini @petrochenkov