GCI: Imply outlives-bounds on free (generic) const items

Author: fmease

compiler/rustc_hir_analysis/src/outlives/implicit_infer.rs

@@ -8,18 +8,13 @@ use tracing::debug;
 use super::explicit::ExplicitPredicatesMap;
 use super::utils::*;
 
-/// Infer predicates for the items in the crate.
-///
-/// `global_inferred_outlives`: this is initially the empty map that
-///     was generated by walking the items in the crate. This will
-///     now be filled with inferred predicates.
+/// Infer outlives-predicates for the items in the local crate.
 pub(super) fn infer_predicates(
     tcx: TyCtxt<'_>,
 ) -> FxIndexMap<DefId, ty::EarlyBinder<'_, RequiredPredicates<'_>>> {
     debug!("infer_predicates");
 
     let mut explicit_map = ExplicitPredicatesMap::new();
-
     let mut global_inferred_outlives = FxIndexMap::default();
 
     // If new predicates were added then we need to re-calculate
@@ -70,6 +65,17 @@ pub(super) fn infer_predicates(
                     );
                 }
 
+                DefKind::Const => {
+                    insert_required_predicates_to_be_wf(
+                        tcx,
+                        tcx.type_of(item_did).instantiate_identity(),
+                        tcx.def_span(item_did),
+                        &global_inferred_outlives,
+                        &mut item_required_predicates,
+                        &mut explicit_map,
+                    );
+                }
+
                 _ => {}
             };
 
@@ -312,7 +318,7 @@ fn check_explicit_predicates<'tcx>(
     }
 }
 
-/// Check the inferred predicates declared on the type.
+/// Check the inferred predicates of the type.
 ///
 /// ### Example
 ///

compiler/rustc_hir_analysis/src/outlives/mod.rs

@@ -15,7 +15,7 @@ pub(crate) fn provide(providers: &mut Providers) {
 
 fn inferred_outlives_of(tcx: TyCtxt<'_>, item_def_id: LocalDefId) -> &[(ty::Clause<'_>, Span)] {
     match tcx.def_kind(item_def_id) {
-        DefKind::Struct | DefKind::Enum | DefKind::Union => {
+        DefKind::Struct | DefKind::Enum | DefKind::Union | DefKind::Const => {
             let crate_map = tcx.inferred_outlives_crate(());
             crate_map.predicates.get(&item_def_id.to_def_id()).copied().unwrap_or(&[])
         }
@@ -48,15 +48,8 @@ fn inferred_outlives_of(tcx: TyCtxt<'_>, item_def_id: LocalDefId) -> &[(ty::Clau
     }
 }
 
+/// Compute a map from each applicable item in the local crate to its inferred/implied outlives-predicates.
 fn inferred_outlives_crate(tcx: TyCtxt<'_>, (): ()) -> CratePredicatesMap<'_> {
-    // Compute a map from each ADT (struct/enum/union) and lazy type alias to
-    // the **explicit** outlives predicates (`T: 'a`, `'a: 'b`) that the user wrote.
-    // Typically there won't be many of these, except in older code where
-    // they were mandatory. Nonetheless, we have to ensure that every such
-    // predicate is satisfied, so they form a kind of base set of requirements
-    // for the type.
-
-    // Compute the inferred predicates
     let global_inferred_outlives = implicit_infer::infer_predicates(tcx);
 
     // Convert the inferred predicates into the "collected" form the

src/doc/rustc-dev-guide/src/traits/implied-bounds.md

@@ -9,9 +9,9 @@ handled... well... implicitly.
 
 ## explicit implied bounds
 
-The explicit implied bounds are computed in [`fn inferred_outlives_of`]. Only ADTs and
-lazy type aliases have explicit implied bounds which are computed via a fixpoint algorithm
-in the [`fn inferred_outlives_crate`] query.
+The explicit implied bounds are computed in [`fn inferred_outlives_of`]. Only ADTs,
+[lazy type aliases][lta] and [free (generic) const items][gci] have explicit implied bounds
+which are computed via a fixpoint algorithm in the [`fn inferred_outlives_crate`] query.
 
 We use [`fn insert_required_predicates_to_be_wf`] on all fields of all ADTs in the crate.
 This function computes the outlives bounds for each component of the field using a
@@ -31,6 +31,8 @@ if the outlived region is a region parameter. [It does not add `'static` require
  [`fn check_explicit_predicates`]: https://github.com/rust-lang/rust/blob/5b8bc568d28b2e922290c9a966b3231d0ce9398b/compiler/rustc_hir_analysis/src/outlives/implicit_infer.rs#L238
  [`fn insert_outlives_predicate`]: https://github.com/rust-lang/rust/blob/5b8bc568d28b2e922290c9a966b3231d0ce9398b/compiler/rustc_hir_analysis/src/outlives/utils.rs#L15
  [nostatic]: https://github.com/rust-lang/rust/blob/5b8bc568d28b2e922290c9a966b3231d0ce9398b/compiler/rustc_hir_analysis/src/outlives/utils.rs#L159-L165
+ [lta]: https://github.com/rust-lang/rust/issues/112792
+ [gci]: https://github.com/rust-lang/rust/issues/113521
 
 ## implicit implied bounds
 

tests/ui/generic-const-items/implied-outlives-bounds.neg.stderr

@@ -0,0 +1,18 @@
+error: lifetime may not live long enough
+  --> $DIR/implied-outlives-bounds.rs:20:9
+   |
+LL | fn env0<'any>() {
+   |         ---- lifetime `'any` defined here
+LL |     _ = TYPE_OUTLIVES_0::<'static, &'any ()>;
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ requires that `'any` must outlive `'static`
+
+error: lifetime may not live long enough
+  --> $DIR/implied-outlives-bounds.rs:25:9
+   |
+LL | fn env1<'any>() {
+   |         ---- lifetime `'any` defined here
+LL |     _ = REGION_OUTLIVES_0::<'static, 'any>;
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ requires that `'any` must outlive `'static`
+
+error: aborting due to 2 previous errors
+

tests/ui/generic-const-items/implied-outlives-bounds.rs

@@ -0,0 +1,28 @@
+// Check that we imply outlives-bounds on free const items.
+//@ revisions: pos neg
+//@[pos] check-pass
+#![feature(generic_const_items)]
+#![feature(freeze)] // only used in the test case `TYPE_OUTLIVES_1`
+#![expect(incomplete_features)]
+
+const REGION_OUTLIVES_0<'a, 'b>: Option<&'a &'b ()> = None; // we imply `'a: 'b`
+const REGION_OUTLIVES_1<'a, 'b>: &'a &'b () = &&(); // we imply `'a: 'b`
+
+const TYPE_OUTLIVES_0<'a, T>: Option<&'a T> = None; // we imply `T: 'a`
+
+const TYPE_OUTLIVES_1<'a, T: Def>: &'a T = &T::DEF; // we imply `T: 'a`
+trait Def: std::marker::Freeze { const DEF: Self; }
+
+// Ensure that we actually enforce these implied bounds at usage sites:
+
+#[cfg(neg)]
+fn env0<'any>() {
+    _ = TYPE_OUTLIVES_0::<'static, &'any ()>; //[neg]~ ERROR lifetime may not live long enough
+}
+
+#[cfg(neg)]
+fn env1<'any>() {
+    _ = REGION_OUTLIVES_0::<'static, 'any>; //[neg]~ ERROR lifetime may not live long enough
+}
+
+fn main() {}

tests/ui/generic-const-items/reference-outlives-referent.rs

@@ -2,8 +2,16 @@
 // successfully emit a diagnostic. Regression test for issue #114714.
 
 #![feature(generic_const_items)]
-#![allow(incomplete_features)]
+#![expect(incomplete_features)]
 
-const Q<'a, 'b>: &'a &'b () = &&(); //~ ERROR reference has a longer lifetime than the data it references
+struct S<'a>(&'a ());
+
+impl<'a> S<'a> {
+    const K<'b>: &'a &'b () = &&(); //~ ERROR reference has a longer lifetime than the data it references
+}
+
+const Q<'a, 'b>: () = {
+    let _: &'a &'b () = &&(); //~ ERROR lifetime may not live long enough
+};
 
 fn main() {}

tests/ui/generic-const-items/reference-outlives-referent.stderr

@@ -1,20 +1,32 @@
 error[E0491]: in type `&'a &'b ()`, reference has a longer lifetime than the data it references
-  --> $DIR/reference-outlives-referent.rs:7:18
+  --> $DIR/reference-outlives-referent.rs:10:5
    |
-LL | const Q<'a, 'b>: &'a &'b () = &&();
-   |                  ^^^^^^^^^^
+LL |     const K<'b>: &'a &'b () = &&();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
 note: the pointer is valid for the lifetime `'a` as defined here
-  --> $DIR/reference-outlives-referent.rs:7:9
+  --> $DIR/reference-outlives-referent.rs:9:6
    |
-LL | const Q<'a, 'b>: &'a &'b () = &&();
-   |         ^^
+LL | impl<'a> S<'a> {
+   |      ^^
 note: but the referenced data is only valid for the lifetime `'b` as defined here
-  --> $DIR/reference-outlives-referent.rs:7:13
+  --> $DIR/reference-outlives-referent.rs:10:13
    |
-LL | const Q<'a, 'b>: &'a &'b () = &&();
+LL |     const K<'b>: &'a &'b () = &&();
    |             ^^
 
-error: aborting due to 1 previous error
+error: lifetime may not live long enough
+  --> $DIR/reference-outlives-referent.rs:14:12
+   |
+LL | const Q<'a, 'b>: () = {
+   |         --  -- lifetime `'b` defined here
+   |         |
+   |         lifetime `'a` defined here
+LL |     let _: &'a &'b () = &&();
+   |            ^^^^^^^^^^ requires that `'b` must outlive `'a`
+   |
+   = help: consider adding the following bound: `'b: 'a`
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0491`.