Implement necessary checks for tail calls

Author: WaffleLapkin

compiler/rustc_middle/src/query/mod.rs

@@ -864,6 +864,12 @@ rustc_queries! {
         cache_on_disk_if { true }
     }
 
+    /// Checks well-formedness of tail calls (`become f()`).
+    query thir_check_tail_calls(key: LocalDefId) -> Result<(), rustc_errors::ErrorGuaranteed> {
+        desc { |tcx| "tail-call-checking `{}`", tcx.def_path_str(key) }
+        cache_on_disk_if { true }
+    }
+
     /// Returns the types assumed to be well formed while "inside" of the given item.
     ///
     /// Note that we've liberated the late bound regions of function signatures, so

compiler/rustc_mir_build/src/build/expr/stmt.rs

@@ -101,9 +101,11 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             ),
             ExprKind::Become { value } => {
                 let v = &this.thir[value];
-                let ExprKind::Scope { region_scope, lint_level, value, .. } = v.kind else { span_bug!(v.span, "what {v:?}") };
+                let ExprKind::Scope { region_scope, lint_level, value, .. } = v.kind
+                    else { span_bug!(v.span, "`thir_check_tail_calls` should have disallowed this {v:?}") };
                 let v = &this.thir[value];
-                let ExprKind::Call { ref args, fun, fn_span, .. } = v.kind else { span_bug!(v.span, "what {v:?}") };
+                let ExprKind::Call { ref args, fun, fn_span, .. } = v.kind
+                    else { span_bug!(v.span, "`thir_check_tail_calls` should have disallowed this {v:?}") };
 
                 this.in_scope((region_scope, source_info), lint_level, |this| {
                     let fun = unpack!(block = this.as_local_operand(block, &this.thir[fun]));

compiler/rustc_mir_build/src/build/mod.rs

@@ -45,6 +45,10 @@ fn mir_build(tcx: TyCtxt<'_>, def: LocalDefId) -> Body<'_> {
         return construct_error(tcx, def, e);
     }
 
+    if let Err(err) = tcx.thir_check_tail_calls(def) {
+        return construct_error(tcx, def, err);
+    }
+
     let body = match tcx.thir_body(def) {
         Err(error_reported) => construct_error(tcx, def, error_reported),
         Ok((thir, expr)) => {