implement checks for tail calls

this implements checks necessary to guarantee that we can actually
perform a tail call. while extremely restrictive, this is what is
documented in the RFC, and all these checks are needed for one reason or
another.

Author: WaffleLapkin

compiler/rustc_middle/src/query/mod.rs

@@ -916,6 +916,12 @@ rustc_queries! {
         cache_on_disk_if { true }
     }
 
+    /// Checks well-formedness of tail calls (`become f()`).
+    query 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/mod.rs

@@ -50,6 +50,10 @@ pub(crate) fn mir_build<'tcx>(tcx: TyCtxtAt<'tcx>, def: LocalDefId) -> Body<'tcx
         return construct_error(tcx, def, e);
     }
 
+    if let Err(err) = tcx.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)) => {