Accept trivial consts based on trivial consts

compiler/rustc_middle/src/mir/interpret/queries.rs

@@ -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 {

compiler/rustc_mir_transform/src/trivial_const.rs

@@ -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,

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

@@ -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`...
-  --> $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 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 caching mir of `<impl at $DIR/issue-24949-assoc-const-static-recursion-impl.rs:11:1: 11:19>::BAR` for CTFE...
+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 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
+   = 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 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

tests/ui/issues/issue-17252.stderr

@@ -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 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: ...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[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 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: ...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: 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

tests/ui/parallel-rustc/cycle_crash-issue-135870.rs

@@ -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() {}

tests/ui/parallel-rustc/cycle_crash-issue-135870.stderr

@@ -1,18 +1,12 @@
-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
 
-For more information about this error, try `rustc --explain E0391`.
+For more information about this error, try `rustc --explain E0391`.

tests/ui/recursion/issue-23302-3.stderr

@@ -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