add error for unknown jump target

Author: folkertdev

compiler/rustc_mir_build/messages.ftl

@@ -84,7 +84,10 @@ mir_build_call_to_unsafe_fn_requires_unsafe_unsafe_op_in_unsafe_fn_allowed =
 
 mir_build_confused = missing patterns are not covered because `{$variable}` is interpreted as a constant pattern, not a new variable
 
-mir_build_const_continue_missing_value = a `const_continue` must break to a label with a value
+mir_build_const_continue_missing_value = a `#[const_continue]` must break to a label with a value
+
+mir_build_const_continue_unknown_jump_target = the target of this `#[const_continue]` is not statically known
+    .note = this value must be an integer or enum literal
 
 mir_build_const_defined_here = constant defined here
 

compiler/rustc_mir_build/src/builder/scope.rs

@@ -99,6 +99,7 @@ use tracing::{debug, instrument};
 
 use super::matches::BuiltMatchTree;
 use crate::builder::{BlockAnd, BlockAndExtension, BlockFrame, Builder, CFG};
+use crate::errors::ConstContinueUnknownJumpTarget;
 
 #[derive(Debug)]
 pub(crate) struct Scopes<'tcx> {
@@ -813,8 +814,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 let Some(real_target) =
                     self.static_pattern_match(&cx, value, &*scope.arms, &scope.built_match_tree)
                 else {
-                    // self.tcx.dcx().emit_fatal()
-                    todo!()
+                    self.tcx.dcx().emit_fatal(ConstContinueUnknownJumpTarget { span })
                 };
 
                 self.block_context.push(BlockFrame::SubExpr);

compiler/rustc_mir_build/src/errors.rs

@@ -1194,3 +1194,11 @@ pub(crate) struct ConstContinueMissingValue {
     #[primary_span]
     pub span: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(mir_build_const_continue_unknown_jump_target)]
+#[note]
+pub(crate) struct ConstContinueUnknownJumpTarget {
+    #[primary_span]
+    pub span: Span,
+}

tests/ui/loop-match/invalid.rs

@@ -136,7 +136,7 @@ fn break_without_value_unit() {
                 () => {
                     #[const_continue]
                     break 'blk;
-                    //~^ ERROR a `const_continue` must break to a label with a value
+                    //~^ ERROR a `#[const_continue]` must break to a label with a value
                 }
             }
         }

tests/ui/loop-match/invalid.stderr

@@ -74,7 +74,7 @@ LL | |
 LL | |     }
    | |_____^
 
-error: a `const_continue` must break to a label with a value
+error: a `#[const_continue]` must break to a label with a value
   --> $DIR/invalid.rs:138:21
    |
 LL |                     break 'blk;