Auto merge of #123737 - compiler-errors:alias-wf, r=lcnr

bors · bors · commit d163e5e515d4 · 2024-07-03T03:48:06.000Z
Check alias args for WF even if they have escaping bound vars #### What This PR stops skipping arguments of aliases if they have escaping bound vars, instead recursing into them and only discarding the resulting obligations referencing bounds vars. #### An example: From the test: ``` trait Trait { type Gat<U: ?Sized>; } fn test<T>(f: for<'a> fn(<&'a T as Trait>::Gat<&'a [str]>)) where for<'a> &'a T: Trait {} //~^ ERROR the size for values of type `[()]` cannot be known at compilation time fn main() {} ``` We now prove that `str: Sized` in order for `&'a [str]` to be well-formed. We were previously unconditionally skipping over `&'a [str]` as it referenced a buond variable. We now recurse into it and instead only discard the `[str]: 'a` obligation because of the escaping bound vars. #### Why? This is a change that improves consistency about proving well-formedness earlier in the pipeline, which is necessary for future work on where-bounds in binders and correctly handling higher-ranked implied bounds. I don't expect this to fix any unsoundness. #### What doesn't it fix? Specifically, this doesn't check projection predicates' components are well-formed, because there are too many regressions: #123737 (comment)
diff --git a/compiler/rustc_trait_selection/src/traits/wf.rs b/compiler/rustc_trait_selection/src/traits/wf.rs
@@ -435,12 +435,6 @@ impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
         }
     }
 
-    /// Pushes the obligations required for an alias (except inherent) to be WF
-    /// into `self.out`.
-    fn compute_alias_ty(&mut self, data: ty::AliasTy<'tcx>) {
-        self.compute_alias_term(data.into());
-    }
-
     /// Pushes the obligations required for an alias (except inherent) to be WF
     /// into `self.out`.
     fn compute_alias_term(&mut self, data: ty::AliasTerm<'tcx>) {
@@ -498,7 +492,7 @@ impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
             self.out.extend(obligations);
         }
 
-        self.compute_projection_args(data.args);
+        data.args.visit_with(self);
     }
 
     fn compute_projection_args(&mut self, args: GenericArgsRef<'tcx>) {
@@ -702,8 +696,8 @@ impl<'a, 'tcx> TypeVisitor<TyCtxt<'tcx>> for WfPredicates<'a, 'tcx> {
             }
 
             ty::Alias(ty::Projection | ty::Opaque | ty::Weak, data) => {
-                self.compute_alias_ty(data);
-                return; // Subtree handled by compute_projection.
+                let obligations = self.nominal_obligations(data.def_id, data.args);
+                self.out.extend(obligations);
             }
             ty::Alias(ty::Inherent, data) => {
                 self.compute_inherent_projection(data);
diff --git a/tests/ui/higher-ranked/well-formed-aliases.rs b/tests/ui/higher-ranked/well-formed-aliases.rs
@@ -0,0 +1,8 @@
+trait Trait {
+    type Gat<U: ?Sized>;
+}
+
+fn test<T>(f: for<'a> fn(<&'a T as Trait>::Gat<&'a [str]>)) where for<'a> &'a T: Trait {}
+//~^ ERROR the size for values of type `str` cannot be known at compilation time
+
+fn main() {}
diff --git a/tests/ui/higher-ranked/well-formed-aliases.stderr b/tests/ui/higher-ranked/well-formed-aliases.stderr
@@ -0,0 +1,12 @@
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/well-formed-aliases.rs:5:52
+   |
+LL | fn test<T>(f: for<'a> fn(<&'a T as Trait>::Gat<&'a [str]>)) where for<'a> &'a T: Trait {}
+   |                                                    ^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `str`
+   = note: slice and array elements must have `Sized` type
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.

Original file line number	Diff line number	Diff line change
`@@ -435,12 +435,6 @@ impl<'a, 'tcx> WfPredicates<'a, 'tcx> {`
`435`	`435`	`}`
`436`	`436`	`}`
`437`	`437`
`438`		`- /// Pushes the obligations required for an alias (except inherent) to be WF`
`439`		- /// into `self.out`.
`440`		`- fn compute_alias_ty(&mut self, data: ty::AliasTy<'tcx>) {`
`441`		`- self.compute_alias_term(data.into());`
`442`		`- }`
`443`		`-`
`444`	`438`	`/// Pushes the obligations required for an alias (except inherent) to be WF`
`445`	`439`	/// into `self.out`.
`446`	`440`	`fn compute_alias_term(&mut self, data: ty::AliasTerm<'tcx>) {`
`@@ -498,7 +492,7 @@ impl<'a, 'tcx> WfPredicates<'a, 'tcx> {`
`498`	`492`	`self.out.extend(obligations);`
`499`	`493`	`}`
`500`	`494`
`501`		`- self.compute_projection_args(data.args);`
	`495`	`+ data.args.visit_with(self);`
`502`	`496`	`}`
`503`	`497`
`504`	`498`	`fn compute_projection_args(&mut self, args: GenericArgsRef<'tcx>) {`
`@@ -702,8 +696,8 @@ impl<'a, 'tcx> TypeVisitor<TyCtxt<'tcx>> for WfPredicates<'a, 'tcx> {`
`702`	`696`	`}`
`703`	`697`
`704`	`698`	`ty::Alias(ty::Projection \| ty::Opaque \| ty::Weak, data) => {`
`705`		`- self.compute_alias_ty(data);`
`706`		`- return; // Subtree handled by compute_projection.`
	`699`	`+ let obligations = self.nominal_obligations(data.def_id, data.args);`
	`700`	`+ self.out.extend(obligations);`
`707`	`701`	`}`
`708`	`702`	`ty::Alias(ty::Inherent, data) => {`
`709`	`703`	`self.compute_inherent_projection(data);`