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Auto merge of #148182 - saethlin:trivial-consts-recursive, r=eholk

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Accept trivial consts based on trivial consts

This is an expansion of https://github.com/rust-lang/rust/pull/148040.

The previous implementation only accepted trivial consts that assign a literal. For example:
```rust
const A: usize = 0;
const B: usize = A;
```
Before this PR, only `A` was a trivial const. Now `B` is too.
This commit is contained in:
bors
2025-10-28 22:52:08 +00:00
parent 278a90913d 9cbfbb164e
commit 044d68c3cb
7 changed files with 71 additions and 55 deletions
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1
compiler/rustc_middle/src/mir/interpret/queries.rs
View File

@@ -134,6 +134,7 @@ impl<'tcx> TyCtxt<'tcx> {
// If we don't *only* FCW anon consts we can wind up incorrectly FCW'ing uses of assoc
// consts in pattern positions. #140447
&& self.def_kind(cid.instance.def_id()) == DefKind::AnonConst
&& !self.is_trivial_const(cid.instance.def_id())
{
let mir_body = self.mir_for_ctfe(cid.instance.def_id());
if mir_body.is_polymorphic {

31
compiler/rustc_mir_transform/src/trivial_const.rs
View File

@@ -3,8 +3,8 @@ use std::ops::Deref;
use rustc_hir::def::DefKind;
use rustc_hir::def_id::LocalDefId;
use rustc_middle::mir::{
Body, ConstValue, Operand, Place, RETURN_PLACE, Rvalue, START_BLOCK, StatementKind,
TerminatorKind,
Body, Const, ConstValue, Operand, Place, RETURN_PLACE, Rvalue, START_BLOCK, StatementKind,
TerminatorKind, UnevaluatedConst,
};
use rustc_middle::ty::{Ty, TyCtxt, TypeVisitableExt};
@@ -13,7 +13,9 @@ use rustc_middle::ty::{Ty, TyCtxt, TypeVisitableExt};
/// A "trivial const" is a const which can be easily proven to evaluate successfully, and the value
/// that it evaluates to can be easily found without going through the usual MIR phases for a const.
///
/// Currently the only form of trivial const that is supported is this:
/// Currently, we support two forms of trivial const.
///
/// The base case is this:
/// ```
/// const A: usize = 0;
/// ```
@@ -34,6 +36,13 @@ use rustc_middle::ty::{Ty, TyCtxt, TypeVisitableExt};
/// This scenario meets the required criteria because:
/// * Control flow cannot panic, we don't have any calls or assert terminators
/// * The value of the const is already computed, so it cannot fail
///
/// In addition to assignment of literals, assignments of trivial consts are also considered
/// trivial consts. In this case, both `A` and `B` are trivial:
/// ```
/// const A: usize = 0;
/// const B: usize = A;
/// ```
pub(crate) fn trivial_const<'a, 'tcx: 'a, F, B>(
tcx: TyCtxt<'tcx>,
def: LocalDefId,
@@ -74,13 +83,19 @@ where
return None;
}
if let Rvalue::Use(Operand::Constant(c)) = rvalue {
if let rustc_middle::mir::Const::Val(v, ty) = c.const_ {
return Some((v, ty));
let Rvalue::Use(Operand::Constant(c)) = rvalue else {
return None;
};
match c.const_ {
Const::Ty(..) => None,
Const::Unevaluated(UnevaluatedConst { def, args, .. }, _ty) => {
if !args.is_empty() {
return None;
}
tcx.trivial_const(def)
}
Const::Val(v, ty) => Some((v, ty)),
}
return None;
}
// The query provider is based on calling the free function trivial_const, which calls mir_built,

28
tests/ui/associated-consts/issue-24949-assoc-const-static-recursion-impl.stderr
View File

@@ -1,36 +1,30 @@
error[E0391]: cycle detected when simplifying constant for the type system `IMPL_REF_BAR`
error[E0391]: cycle detected when checking if `IMPL_REF_BAR` is a trivial const
--> $DIR/issue-24949-assoc-const-static-recursion-impl.rs:7:1
|
LL | const IMPL_REF_BAR: u32 = GlobalImplRef::BAR;
| ^^^^^^^^^^^^^^^^^^^^^^^
|
note: ...which requires const-evaluating + checking `IMPL_REF_BAR`...
--> $DIR/issue-24949-assoc-const-static-recursion-impl.rs:7:27
note: ...which requires building MIR for `IMPL_REF_BAR`...
--> $DIR/issue-24949-assoc-const-static-recursion-impl.rs:7:1
|
LL | const IMPL_REF_BAR: u32 = GlobalImplRef::BAR;
| ^^^^^^^^^^^^^^^^^^
note: ...which requires simplifying constant for the type system `<impl at $DIR/issue-24949-assoc-const-static-recursion-impl.rs:11:1: 11:19>::BAR`...
| ^^^^^^^^^^^^^^^^^^^^^^^
note: ...which requires checking if `<impl at $DIR/issue-24949-assoc-const-static-recursion-impl.rs:11:1: 11:19>::BAR` is a trivial const...
--> $DIR/issue-24949-assoc-const-static-recursion-impl.rs:12:5
|
LL | const BAR: u32 = IMPL_REF_BAR;
| ^^^^^^^^^^^^^^
note: ...which requires const-evaluating + checking `<impl at $DIR/issue-24949-assoc-const-static-recursion-impl.rs:11:1: 11:19>::BAR`...
note: ...which requires building MIR for `<impl at $DIR/issue-24949-assoc-const-static-recursion-impl.rs:11:1: 11:19>::BAR`...
--> $DIR/issue-24949-assoc-const-static-recursion-impl.rs:12:5
|
LL | const BAR: u32 = IMPL_REF_BAR;
| ^^^^^^^^^^^^^^
note: ...which requires caching mir of `<impl at $DIR/issue-24949-assoc-const-static-recursion-impl.rs:11:1: 11:19>::BAR` for CTFE...
--> $DIR/issue-24949-assoc-const-static-recursion-impl.rs:12:5
= note: ...which again requires checking if `IMPL_REF_BAR` is a trivial const, completing the cycle
note: cycle used when simplifying constant for the type system `IMPL_REF_BAR`
--> $DIR/issue-24949-assoc-const-static-recursion-impl.rs:7:1
|
LL | const BAR: u32 = IMPL_REF_BAR;
| ^^^^^^^^^^^^^^
note: ...which requires elaborating drops for `<impl at $DIR/issue-24949-assoc-const-static-recursion-impl.rs:11:1: 11:19>::BAR`...
--> $DIR/issue-24949-assoc-const-static-recursion-impl.rs:12:22
|
LL | const BAR: u32 = IMPL_REF_BAR;
| ^^^^^^^^^^^^
= note: ...which again requires simplifying constant for the type system `IMPL_REF_BAR`, completing the cycle
= note: cycle used when running analysis passes on this crate
LL | const IMPL_REF_BAR: u32 = GlobalImplRef::BAR;
| ^^^^^^^^^^^^^^^^^^^^^^^
= note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
error: aborting due to 1 previous error

32
tests/ui/issues/issue-17252.stderr
View File

@@ -1,31 +1,39 @@
error[E0391]: cycle detected when simplifying constant for the type system `FOO`
error[E0391]: cycle detected when checking if `FOO` is a trivial const
--> $DIR/issue-17252.rs:1:1
|
LL | const FOO: usize = FOO;
| ^^^^^^^^^^^^^^^^
|
note: ...which requires const-evaluating + checking `FOO`...
--> $DIR/issue-17252.rs:1:20
note: ...which requires building MIR for `FOO`...
--> $DIR/issue-17252.rs:1:1
|
LL | const FOO: usize = FOO;
| ^^^
= note: ...which again requires simplifying constant for the type system `FOO`, completing the cycle
= note: cycle used when running analysis passes on this crate
| ^^^^^^^^^^^^^^^^
= note: ...which again requires checking if `FOO` is a trivial const, completing the cycle
note: cycle used when simplifying constant for the type system `FOO`
--> $DIR/issue-17252.rs:1:1
|
LL | const FOO: usize = FOO;
| ^^^^^^^^^^^^^^^^
= note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
error[E0391]: cycle detected when simplifying constant for the type system `main::BAR`
error[E0391]: cycle detected when checking if `main::BAR` is a trivial const
--> $DIR/issue-17252.rs:6:9
|
LL | const BAR: usize = BAR;
| ^^^^^^^^^^^^^^^^
|
note: ...which requires const-evaluating + checking `main::BAR`...
--> $DIR/issue-17252.rs:6:28
note: ...which requires building MIR for `main::BAR`...
--> $DIR/issue-17252.rs:6:9
|
LL | const BAR: usize = BAR;
| ^^^
= note: ...which again requires simplifying constant for the type system `main::BAR`, completing the cycle
= note: cycle used when running analysis passes on this crate
| ^^^^^^^^^^^^^^^^
= note: ...which again requires checking if `main::BAR` is a trivial const, completing the cycle
note: cycle used when simplifying constant for the type system `main::BAR`
--> $DIR/issue-17252.rs:6:9
|
LL | const BAR: usize = BAR;
| ^^^^^^^^^^^^^^^^
= note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
error: aborting due to 2 previous errors

2
tests/ui/parallel-rustc/cycle_crash-issue-135870.rs
View File

@@ -3,6 +3,6 @@
//@ compile-flags: -Z threads=2
//@ compare-output-by-lines
const FOO: usize = FOO; //~ ERROR cycle detected when simplifying constant for the type system `FOO`
const FOO: usize = FOO; //~ ERROR cycle detected
fn main() {}

6
tests/ui/parallel-rustc/cycle_crash-issue-135870.stderr
View File

@@ -1,16 +1,10 @@
error[E0391]: cycle detected when simplifying constant for the type system `FOO`
--> $DIR/cycle_crash-issue-135870.rs:6:1
|
LL | const FOO: usize = FOO;
| ^^^^^^^^^^^^^^^^
|
note: ...which requires const-evaluating + checking `FOO`...
--> $DIR/cycle_crash-issue-135870.rs:6:20
|
LL | const FOO: usize = FOO;
| ^^^
= note: ...which again requires simplifying constant for the type system `FOO`, completing the cycle
= note: cycle used when running analysis passes on this crate
= note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
error: aborting due to 1 previous error

24
tests/ui/recursion/issue-23302-3.stderr
View File

@@ -1,26 +1,30 @@
error[E0391]: cycle detected when simplifying constant for the type system `A`
error[E0391]: cycle detected when checking if `A` is a trivial const
--> $DIR/issue-23302-3.rs:1:1
|
LL | const A: i32 = B;
| ^^^^^^^^^^^^
|
note: ...which requires const-evaluating + checking `A`...
--> $DIR/issue-23302-3.rs:1:16
note: ...which requires building MIR for `A`...
--> $DIR/issue-23302-3.rs:1:1
|
LL | const A: i32 = B;
| ^
note: ...which requires simplifying constant for the type system `B`...
| ^^^^^^^^^^^^
note: ...which requires checking if `B` is a trivial const...
--> $DIR/issue-23302-3.rs:3:1
|
LL | const B: i32 = A;
| ^^^^^^^^^^^^
note: ...which requires const-evaluating + checking `B`...
--> $DIR/issue-23302-3.rs:3:16
note: ...which requires building MIR for `B`...
--> $DIR/issue-23302-3.rs:3:1
|
LL | const B: i32 = A;
| ^
= note: ...which again requires simplifying constant for the type system `A`, completing the cycle
= note: cycle used when running analysis passes on this crate
| ^^^^^^^^^^^^
= note: ...which again requires checking if `A` is a trivial const, completing the cycle
note: cycle used when simplifying constant for the type system `A`
--> $DIR/issue-23302-3.rs:1:1
|
LL | const A: i32 = B;
| ^^^^^^^^^^^^
= note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
error: aborting due to 1 previous error