typeck: workaround WF hole in to_const.

Author: eddyb

src/librustc_typeck/check/mod.rs

@@ -3311,8 +3311,31 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     }
 
     pub fn to_const(&self, ast_c: &hir::AnonConst) -> &'tcx ty::Const<'tcx> {
-        let c = self.tcx.hir().local_def_id(ast_c.hir_id).expect_local();
-        ty::Const::from_anon_const(self.tcx, c)
+        let const_def_id = self.tcx.hir().local_def_id(ast_c.hir_id).expect_local();
+        let c = ty::Const::from_anon_const(self.tcx, const_def_id);
+
+        // HACK(eddyb) emulate what a `WellFormedConst` obligation would do.
+        // This code should be replaced with the proper WF handling ASAP.
+        if let ty::ConstKind::Unevaluated(def_id, substs, promoted) = c.val {
+            assert!(promoted.is_none());
+
+            // HACK(eddyb) let's hope these are always empty.
+            // let obligations = self.nominal_obligations(def_id, substs);
+            // self.out.extend(obligations);
+
+            let cause = traits::ObligationCause::new(
+                self.tcx.def_span(const_def_id.to_def_id()),
+                self.body_id,
+                traits::MiscObligation,
+            );
+            self.register_predicate(traits::Obligation::new(
+                cause,
+                self.param_env,
+                ty::Predicate::ConstEvaluatable(def_id, substs),
+            ));
+        }
+
+        c
     }
 
     // If the type given by the user has free regions, save it for later, since

src/test/compile-fail/issue-52443.rs

@@ -8,4 +8,9 @@ fn main() {
     //~| WARN denote infinite loops with
     [(); { for _ in 0usize.. {}; 0}];
     //~^ ERROR `for` is not allowed in a `const`
+    //~| ERROR calls in constants are limited to constant functions
+    //~| ERROR references in constants may only refer to immutable values
+    //~| ERROR calls in constants are limited to constant functions
+    //~| ERROR constant contains unimplemented expression type
+    //~| ERROR evaluation of constant value failed
 }

src/test/ui/const-generics/issues/issue-62456.rs

@@ -1,10 +1,9 @@
 #![feature(const_generics)]
 //~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
 
-// build-pass
-
 fn foo<const N: usize>() {
     let _ = [0u64; N + 1];
+    //~^ ERROR constant expression depends on a generic parameter
 }
 
 fn main() {}

src/test/ui/const-generics/issues/issue-62456.stderr

@@ -6,5 +6,13 @@ LL | #![feature(const_generics)]
    |
    = note: `#[warn(incomplete_features)]` on by default
 
-warning: 1 warning emitted
+error: constant expression depends on a generic parameter
+  --> $DIR/issue-62456.rs:5:20
+   |
+LL |     let _ = [0u64; N + 1];
+   |                    ^^^^^
+   |
+   = note: this may fail depending on what value the parameter takes
+
+error: aborting due to previous error; 1 warning emitted
 

src/test/ui/const-generics/issues/issue-62504.rs

@@ -17,6 +17,7 @@ impl<const X: usize> ArrayHolder<X> {
     pub const fn new() -> Self {
         ArrayHolder([0; Self::SIZE])
         //~^ ERROR: mismatched types
+        //~| ERROR constant expression depends on a generic parameter
     }
 }
 

src/test/ui/const-generics/issues/issue-62504.stderr

@@ -7,6 +7,14 @@ LL |         ArrayHolder([0; Self::SIZE])
    = note: expected array `[u32; _]`
               found array `[u32; _]`
 
-error: aborting due to previous error
+error: constant expression depends on a generic parameter
+  --> $DIR/issue-62504.rs:18:25
+   |
+LL |         ArrayHolder([0; Self::SIZE])
+   |                         ^^^^^^^^^^
+   |
+   = note: this may fail depending on what value the parameter takes
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0308`.

src/test/ui/const-generics/issues/issue-66205.rs

@@ -1,10 +1,9 @@
-// check-pass
-
 #![allow(incomplete_features, dead_code, unconditional_recursion)]
 #![feature(const_generics)]
 
 fn fact<const N: usize>() {
     fact::<{ N - 1 }>();
+    //~^ ERROR constant expression depends on a generic parameter
 }
 
 fn main() {}

src/test/ui/const-generics/issues/issue-66205.stderr

@@ -0,0 +1,10 @@
+error: constant expression depends on a generic parameter
+  --> $DIR/issue-66205.rs:5:12
+   |
+LL |     fact::<{ N - 1 }>();
+   |            ^^^^^^^^^
+   |
+   = note: this may fail depending on what value the parameter takes
+
+error: aborting due to previous error
+

src/test/ui/const-generics/issues/issue-67739.rs

@@ -1,7 +1,5 @@
 // Regression test for #67739
 
-// check-pass
-
 #![allow(incomplete_features)]
 #![feature(const_generics)]
 
@@ -12,6 +10,7 @@ pub trait Trait {
 
     fn associated_size(&self) -> usize {
         [0u8; mem::size_of::<Self::Associated>()];
+        //~^ ERROR constant expression depends on a generic parameter
         0
     }
 }

src/test/ui/const-generics/issues/issue-67739.stderr

@@ -0,0 +1,10 @@
+error: constant expression depends on a generic parameter
+  --> $DIR/issue-67739.rs:12:15
+   |
+LL |         [0u8; mem::size_of::<Self::Associated>()];
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: this may fail depending on what value the parameter takes
+
+error: aborting due to previous error
+