Deduce readonly in generic parameters

Author: tmiasko

compiler/rustc_middle/src/ty/context.rs

@@ -3300,6 +3300,7 @@ pub struct DeducedParamAttrs {
     /// The parameter is marked immutable in the function and contains no `UnsafeCell` (i.e. its
     /// type is freeze).
     pub read_only: bool,
+    pub read_only_when_freeze: bool,
 }
 
 pub fn provide(providers: &mut Providers) {

compiler/rustc_mir_transform/src/deduce_param_attrs.rs

@@ -169,26 +169,11 @@ pub(super) fn deduced_param_attrs<'tcx>(
 
     // Set the `readonly` attribute for every argument that we concluded is immutable and that
     // contains no UnsafeCells.
-    //
-    // FIXME: This is overly conservative around generic parameters: `is_freeze()` will always
-    // return false for them. For a description of alternatives that could do a better job here,
-    // see [1].
-    //
-    // [1]: https://github.com/rust-lang/rust/pull/103172#discussion_r999139997
-    let typing_env = body.typing_env(tcx);
-    let mut deduced_param_attrs = tcx.arena.alloc_from_iter(
-        body.local_decls.iter().skip(1).take(body.arg_count).enumerate().map(
-            |(arg_index, local_decl)| DeducedParamAttrs {
-                read_only: deduce.read_only.contains(arg_index) || (
-                            deduce.read_only_when_freeze.contains(arg_index)
-                            // We must normalize here to reveal opaques and normalize
-                            // their generic parameters, otherwise we'll see exponential
-                            // blow-up in compile times: #113372
-                            && tcx.normalize_erasing_regions(typing_env, local_decl.ty).is_freeze(tcx, typing_env))
-
-            }
-        ),
-    );
+    let mut deduced_param_attrs =
+        tcx.arena.alloc_from_iter((0..body.arg_count).map(|arg_index| DeducedParamAttrs {
+            read_only: deduce.read_only.contains(arg_index),
+            read_only_when_freeze: deduce.read_only_when_freeze.contains(arg_index),
+        }));
 
     // Trailing parameters past the size of the `deduced_param_attrs` array are assumed to have the
     // default set of attributes, so we don't have to store them explicitly. Pop them off to save a

compiler/rustc_ty_utils/src/abi.rs

@@ -712,7 +712,10 @@ fn fn_abi_adjust_for_abi<'tcx>(
                 // we can't deduce any parameters for, so make sure the argument index is in
                 // bounds.
                 if let Some(deduced_param_attrs) = deduced_param_attrs.get(arg_idx) {
-                    if deduced_param_attrs.read_only {
+                    if deduced_param_attrs.read_only
+                        || (deduced_param_attrs.read_only_when_freeze
+                            && arg.layout.ty.is_freeze(tcx, cx.typing_env))
+                    {
                         attrs.regular.insert(ArgAttribute::ReadOnly);
                         debug!("added deduced read-only attribute");
                     }