disallow non-static lifetimes in const generics

src/librustc_error_codes/error_codes.rs

@@ -453,6 +453,7 @@ E0767: include_str!("./error_codes/E0767.md"),
 E0768: include_str!("./error_codes/E0768.md"),
 E0769: include_str!("./error_codes/E0769.md"),
 E0770: include_str!("./error_codes/E0770.md"),
+E0771: include_str!("./error_codes/E0771.md"),
 ;
 //  E0006, // merged with E0005
 //  E0008, // cannot bind by-move into a pattern guard

src/librustc_error_codes/error_codes/E0771.md

@@ -0,0 +1,23 @@
+A non-`'static` lifetime was used in a const generic. This is currently not
+allowed.
+
+Erroneous code example:
+
+```compile_fail,E0771
+#![feature(const_generics)]
+
+fn function_with_str<'a, const STRING: &'a str>() {} // error!
+```
+
+To fix this issue, the lifetime in the const generic need to be changed to
+`'static`:
+
+```
+#![feature(const_generics)]
+
+fn function_with_str<const STRING: &'static str>() {} // ok!
+```
+
+For more information, see [GitHub issue #74052].
+
+[GitHub issue #74052]: https://github.com/rust-lang/rust/issues/74052

src/librustc_resolve/late/diagnostics.rs

@@ -1141,6 +1141,24 @@ impl<'tcx> LifetimeContext<'_, 'tcx> {
         err.emit();
     }
 
+    // FIXME(const_generics): This patches over a ICE caused by non-'static lifetimes in const
+    // generics. We are disallowing this until we can decide on how we want to handle non-'static
+    // lifetimes in const generics. See issue #74052 for discussion.
+    crate fn emit_non_static_lt_in_const_generic_error(&self, lifetime_ref: &hir::Lifetime) {
+        let mut err = struct_span_err!(
+            self.tcx.sess,
+            lifetime_ref.span,
+            E0771,
+            "use of non-static lifetime `{}` in const generic",
+            lifetime_ref
+        );
+        err.note(
+            "for more information, see issue #74052 \
+            <https://github.com/rust-lang/rust/issues/74052>",
+        );
+        err.emit();
+    }
+
     crate fn is_trait_ref_fn_scope(&mut self, trait_ref: &'tcx hir::PolyTraitRef<'tcx>) -> bool {
         if let def::Res::Def(_, did) = trait_ref.trait_ref.path.res {
             if [

src/librustc_resolve/late/lifetimes.rs

@@ -173,6 +173,8 @@ crate struct LifetimeContext<'a, 'tcx> {
     /// Used to disallow the use of in-band lifetimes in `fn` or `Fn` syntax.
     is_in_fn_syntax: bool,
 
+    is_in_const_generic: bool,
+
     /// List of labels in the function/method currently under analysis.
     labels_in_fn: Vec<Ident>,
 
@@ -333,6 +335,7 @@ fn krate(tcx: TyCtxt<'_>) -> NamedRegionMap {
             scope: ROOT_SCOPE,
             trait_ref_hack: false,
             is_in_fn_syntax: false,
+            is_in_const_generic: false,
             labels_in_fn: vec![],
             xcrate_object_lifetime_defaults: Default::default(),
             lifetime_uses: &mut Default::default(),
@@ -828,6 +831,10 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
             self.insert_lifetime(lifetime_ref, Region::Static);
             return;
         }
+        if self.is_in_const_generic && lifetime_ref.name != LifetimeName::Error {
+            self.emit_non_static_lt_in_const_generic_error(lifetime_ref);
+            return;
+        }
         self.resolve_lifetime_ref(lifetime_ref);
     }
 
@@ -860,8 +867,11 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
                     }
                 }
                 GenericParamKind::Const { ref ty, .. } => {
+                    let was_in_const_generic = self.is_in_const_generic;
+                    self.is_in_const_generic = true;
                     walk_list!(self, visit_param_bound, param.bounds);
                     self.visit_ty(&ty);
+                    self.is_in_const_generic = was_in_const_generic;
                 }
             }
         }
@@ -1317,6 +1327,7 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
             scope: &wrap_scope,
             trait_ref_hack: self.trait_ref_hack,
             is_in_fn_syntax: self.is_in_fn_syntax,
+            is_in_const_generic: self.is_in_const_generic,
             labels_in_fn,
             xcrate_object_lifetime_defaults,
             lifetime_uses,

src/test/ui/const-generics/const-argument-non-static-lifetime.rs

@@ -0,0 +1,17 @@
+// run-pass
+
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete
+#![allow(dead_code)]
+
+fn test<const N: usize>() {}
+
+fn wow<'a>() -> &'a () {
+    test::<{
+        let _: &'a ();
+        3
+    }>();
+    &()
+}
+
+fn main() {}

src/test/ui/const-generics/const-argument-non-static-lifetime.stderr

@@ -0,0 +1,11 @@
+warning: the feature `const_generics` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/const-argument-non-static-lifetime.rs:3:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #44580 <https://github.com/rust-lang/rust/issues/44580> for more information
+
+warning: 1 warning emitted
+

src/test/ui/error-codes/E0771.rs

@@ -0,0 +1,8 @@
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete
+
+fn function_with_str<'a, const STRING: &'a str>() {} //~ ERROR E0771
+
+fn main() {
+    function_with_str::<"Hello, world!">()
+}

src/test/ui/error-codes/E0771.stderr

@@ -0,0 +1,20 @@
+warning: the feature `const_generics` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/E0771.rs:1:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #44580 <https://github.com/rust-lang/rust/issues/44580> for more information
+
+error[E0771]: use of non-static lifetime `'a` in const generic
+  --> $DIR/E0771.rs:4:41
+   |
+LL | fn function_with_str<'a, const STRING: &'a str>() {}
+   |                                         ^^
+   |
+   = note: for more information, see issue #74052 <https://github.com/rust-lang/rust/issues/74052>
+
+error: aborting due to previous error; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0771`.