loop_match: suggest extracting to a const item

if the expression cannot be evaluated in a straightforward way

compiler/rustc_mir_build/messages.ftl

@@ -86,10 +86,16 @@ mir_build_confused = missing patterns are not covered because `{$variable}` is i
 
 mir_build_const_continue_bad_const = could not determine the target branch for this `#[const_continue]`
     .label = this value is too generic
-    .note = the value must be a literal or a monomorphic const
 
 mir_build_const_continue_missing_value = a `#[const_continue]` must break to a label with a value
 
+mir_build_const_continue_not_const = could not determine the target branch for this `#[const_continue]`
+    .help = try extracting the expression into a `const` item
+
+mir_build_const_continue_not_const_const_block = `const` blocks may use generics, and are not evaluated early enough
+mir_build_const_continue_not_const_const_other = this value must be a literal or a monomorphic const
+mir_build_const_continue_not_const_constant_parameter = constant parameters may use generics, and are not evaluated early enough
+
 mir_build_const_continue_unknown_jump_target = the target of this `#[const_continue]` is not statically known
     .label = this value must be a literal or a monomorphic const
 

compiler/rustc_mir_build/src/builder/scope.rs

@@ -100,7 +100,9 @@ use tracing::{debug, instrument};
 
 use super::matches::BuiltMatchTree;
 use crate::builder::{BlockAnd, BlockAndExtension, BlockFrame, Builder, CFG};
-use crate::errors::{ConstContinueBadConst, ConstContinueUnknownJumpTarget};
+use crate::errors::{
+    ConstContinueBadConst, ConstContinueNotMonomorphicConst, ConstContinueUnknownJumpTarget,
+};
 
 #[derive(Debug)]
 pub(crate) struct Scopes<'tcx> {
@@ -867,7 +869,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             span_bug!(span, "break value must be a scope")
         };
 
-        let constant = match &self.thir[value].kind {
+        let expr = &self.thir[value];
+        let constant = match &expr.kind {
             ExprKind::Adt(box AdtExpr { variant_index, fields, base, .. }) => {
                 assert!(matches!(base, AdtExprBase::None));
                 assert!(fields.is_empty());
@@ -887,7 +890,27 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     ),
                 }
             }
-            _ => self.as_constant(&self.thir[value]),
+
+            ExprKind::Literal { .. }
+            | ExprKind::NonHirLiteral { .. }
+            | ExprKind::ZstLiteral { .. }
+            | ExprKind::NamedConst { .. } => self.as_constant(&self.thir[value]),
+
+            other => {
+                use crate::errors::ConstContinueNotMonomorphicConstReason as Reason;
+
+                let span = expr.span;
+                let reason = match other {
+                    ExprKind::ConstParam { .. } => Reason::ConstantParameter { span },
+                    ExprKind::ConstBlock { .. } => Reason::ConstBlock { span },
+                    _ => Reason::Other { span },
+                };
+
+                self.tcx
+                    .dcx()
+                    .emit_err(ConstContinueNotMonomorphicConst { span: expr.span, reason });
+                return block.unit();
+            }
         };
 
         let break_index = self

compiler/rustc_mir_build/src/errors.rs

@@ -1213,6 +1213,38 @@ pub(crate) struct LoopMatchArmWithGuard {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(mir_build_const_continue_not_const)]
+#[help]
+pub(crate) struct ConstContinueNotMonomorphicConst {
+    #[primary_span]
+    pub span: Span,
+
+    #[subdiagnostic]
+    pub reason: ConstContinueNotMonomorphicConstReason,
+}
+
+#[derive(Subdiagnostic)]
+pub(crate) enum ConstContinueNotMonomorphicConstReason {
+    #[label(mir_build_const_continue_not_const_constant_parameter)]
+    ConstantParameter {
+        #[primary_span]
+        span: Span,
+    },
+
+    #[label(mir_build_const_continue_not_const_const_block)]
+    ConstBlock {
+        #[primary_span]
+        span: Span,
+    },
+
+    #[label(mir_build_const_continue_not_const_const_other)]
+    Other {
+        #[primary_span]
+        span: Span,
+    },
+}
+
 #[derive(Diagnostic)]
 #[diag(mir_build_const_continue_bad_const)]
 pub(crate) struct ConstContinueBadConst {