Replace Rvalue::NullaryOp by a variant in mir::Operand. #148766

cjgillot · 2025-11-09T20:19:43Z

This PR fully removes the MIR Rvalue::NullaryOp. After #148151, it was only useful for runtime checks like ub_checks, contract_checks and overflow_checks.

These are "runtime" checks, boolean constants that may only be true in codegen. It depends on a rustc flag passed to codegen, so we need to represent those flags cross-crate.

This PR replaces those runtime checks by special variants in MIR ConstValue. This allows code that expects constants to manipulate those as such, even if we may not always be able to evaluate them to actual scalars.

rustbot · 2025-11-09T20:19:47Z

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

⚠️ #[rustc_intrinsic_const_stable_indirect] controls whether intrinsics can be exposed to stable const
code; adding it needs t-lang approval.

cc @rust-lang/wg-const-eval

Some changes occurred in rustc_ty_utils::consts.rs

cc @BoxyUwU

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

This PR changes MIR

cc @oli-obk, @RalfJung, @JakobDegen, @vakaras

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a

Some changes occurred in match checking

cc @Nadrieril

Some changes occurred to constck

cc @fee1-dead

This PR changes a file inside tests/crashes. If a crash was fixed, please move into the corresponding ui subdir and add 'Fixes #' to the PR description to autoclose the issue upon merge.

rustbot · 2025-11-09T20:19:49Z

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RalfJung · 2025-11-10T10:56:06Z

compiler/rustc_middle/src/mir/consts.rs

+    /// Special constants whose value depends on the evaluation context. Their value depends on a
+    /// flag on the crate being codegenned.
+    RuntimeChecks(RuntimeChecks),


For const-eval execution, their value is fixed, right?

RalfJung · 2025-11-10T10:59:56Z

It seems slightly odd to call these things "constants" when they have a different value inside and outside of const blocks... I am not sure whether this actually breaks anything today, but it seems at least conceptually suboptimal and could lead to issues down the line. For instance, if a constant's body entirely consists just of one of these new constants, then replacing that outer constant by the inner one is not correct (or at least, it can change program behavior) -- so e.g. @saethlin's "trivial const" machinery might need a special case for this.

bors · 2025-11-11T20:36:08Z

☔ The latest upstream changes (presumably #148658) made this pull request unmergeable. Please resolve the merge conflicts.

cjgillot · 2025-11-11T21:39:01Z

I agree that's not entirely satisfactory.

Another design would put the variant inside mir::Const, but the conversion RuntimeChecks -> ConstValue would be done in mir::Const::eval, so without access to the interpreter hooks. This makes everything more complex, and I haven't managed to get the correct behavior yet.

RalfJung · 2025-11-11T21:44:16Z

That would be marginally better ("values" really should be unchanging), though still a footgun e.g. with the trivial const machinery I mentioned.

Making them a new variant of mir::Operand would be the conceptually cleanest option I think. Not sure how well it works in practice though.

cjgillot · 2025-11-17T23:29:16Z

Making them a new variant of mir::Operand would be the conceptually cleanest option I think. Not sure how well it works in practice though.

Latest commit does that. Cleaner than I expected.

rustbot · 2025-11-18T00:33:19Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

rust-log-analyzer · 2025-11-18T01:13:35Z

The job aarch64-gnu-llvm-20-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

---- [ui] tests/ui-fulldeps/rustc_public/check_transform.rs stdout ----

error: test compilation failed although it shouldn't!
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui-fulldeps/rustc_public/check_transform.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "-O" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "-C" "prefer-dynamic" "-o" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui-fulldeps/rustc_public/check_transform/a" "-A" "internal_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0"
stdout: none
--- stderr -------------------------------
error[E0004]: non-exhaustive patterns: `&rustc_public::mir::Operand::RuntimeChecks(_)` not covered
##[error]  --> /checkout/tests/ui-fulldeps/rustc_public/check_transform.rs:57:39
   |
LL |                 let msg_const = match &args[0] {
   |                                       ^^^^^^^^ pattern `&rustc_public::mir::Operand::RuntimeChecks(_)` not covered
   |
note: `rustc_public::mir::Operand` defined here
  --> /rustc/FAKE_PREFIX/compiler/rustc_public/src/mir/body.rs:672:0
  ::: /rustc/FAKE_PREFIX/compiler/rustc_public/src/mir/body.rs:676:4
   |
   = note: not covered
   = note: the matched value is of type `&rustc_public::mir::Operand`
help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
   |
LL ~                     },
LL +                     &rustc_public::mir::Operand::RuntimeChecks(_) => todo!()
   |

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0004`.

RalfJung · 2025-11-18T08:23:45Z

compiler/rustc_const_eval/src/const_eval/dummy_machine.rs

+        // We can't look at `tcx.sess` here as that can differ across crates, which can lead to
+        // unsound differences in evaluating the same constant at different instantiation sites.


This comment doesn't really make sense in the dummy machine.

RalfJung · 2025-11-18T08:24:54Z

compiler/rustc_middle/src/mir/pretty.rs

            Constant(ref a) => write!(fmt, "{a:?}"),
            Copy(ref place) => write!(fmt, "copy {place:?}"),
            Move(ref place) => write!(fmt, "move {place:?}"),
+            RuntimeChecks(checks) => write!(fmt, "const {checks:?}"),


"const" is an odd choice, given that we moved it out of the constants

RalfJung · 2025-11-18T08:26:10Z

compiler/rustc_middle/src/mir/syntax.rs

+    /// Special constants whose value depends on the evaluation context. Their value depends on a
+    /// flag on the crate being codegenned.


Suggested change

/// Special constants whose value depends on the evaluation context. Their value depends on a

/// flag on the crate being codegenned.

/// Query the compilation session of the current crate for a particular flag.

/// This is not quite a const since its value can differ across crates within a single crate graph.

RalfJung · 2025-11-18T08:32:01Z

compiler/rustc_middle/src/mir/statement.rs

                local_decls.local_decls()[l.local].source_info.span
            }
            Operand::Constant(c) => c.span,
+            Operand::RuntimeChecks(_) => DUMMY_SP,


I guess this is fine since user code won't contain this operand?

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-clippy Relevant to the Clippy team. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Nov 9, 2025

rustbot assigned lcnr Nov 9, 2025

rustbot added the T-libs Relevant to the library team, which will review and decide on the PR/issue. label Nov 9, 2025