move `check_opaque_type_parameter_valid`

3 files changed, 178 insertions, 1 deletions

diff --git a/compiler/rustc_trait_selection/src/errors.rs b/compiler/rustc_trait_selection/src/errors.rs
index b30390a9330..9f7bfe5101a 100644
--- a/compiler/rustc_trait_selection/src/errors.rs
+++ b/compiler/rustc_trait_selection/src/errors.rs
@@ -12,7 +12,7 @@ use rustc_hir::intravisit::{Visitor, VisitorExt, walk_ty};
 use rustc_hir::{self as hir, AmbigArg, FnRetTy, GenericParamKind, IsAnonInPath, Node};
 use rustc_macros::{Diagnostic, Subdiagnostic};
 use rustc_middle::ty::print::{PrintTraitRefExt as _, TraitRefPrintOnlyTraitPath};
-use rustc_middle::ty::{self, Binder, ClosureKind, FnSig, Region, Ty, TyCtxt};
+use rustc_middle::ty::{self, Binder, ClosureKind, FnSig, GenericArg, Region, Ty, TyCtxt};
 use rustc_span::{BytePos, Ident, Span, Symbol, kw};
 
 use crate::error_reporting::infer::ObligationCauseAsDiagArg;
@@ -1922,3 +1922,14 @@ impl Subdiagnostic for AddPreciseCapturingForOvercapture {
         }
     }
 }
+
+#[derive(Diagnostic)]
+#[diag(trait_selection_opaque_type_non_generic_param, code = E0792)]
+pub(crate) struct NonGenericOpaqueTypeParam<'a, 'tcx> {
+    pub ty: GenericArg<'tcx>,
+    pub kind: &'a str,
+    #[primary_span]
+    pub span: Span,
+    #[label]
+    pub param_span: Span,
+}
diff --git a/compiler/rustc_trait_selection/src/lib.rs b/compiler/rustc_trait_selection/src/lib.rs
index b18fb0fb8fd..93c11805304 100644
--- a/compiler/rustc_trait_selection/src/lib.rs
+++ b/compiler/rustc_trait_selection/src/lib.rs
@@ -36,6 +36,7 @@
 pub mod error_reporting;
 pub mod errors;
 pub mod infer;
+pub mod opaque_types;
 pub mod regions;
 pub mod solve;
 pub mod traits;
diff --git a/compiler/rustc_trait_selection/src/opaque_types.rs b/compiler/rustc_trait_selection/src/opaque_types.rs
new file mode 100644
index 00000000000..5af912fee54
--- /dev/null
+++ b/compiler/rustc_trait_selection/src/opaque_types.rs
@@ -0,0 +1,165 @@
+use rustc_data_structures::fx::FxIndexMap;
+use rustc_hir::OpaqueTyOrigin;
+use rustc_hir::def_id::LocalDefId;
+use rustc_infer::infer::outlives::env::OutlivesEnvironment;
+use rustc_infer::infer::{InferCtxt, TyCtxtInferExt};
+use rustc_middle::ty::{
+    self, GenericArgKind, GenericArgs, OpaqueTypeKey, TyCtxt, TypeVisitableExt,
+    TypingMode, fold_regions,
+};
+use rustc_span::{ErrorGuaranteed, Span};
+
+use crate::errors::NonGenericOpaqueTypeParam;
+use crate::regions::OutlivesEnvironmentBuildExt;
+use crate::traits::ObligationCtxt;
+
+/// Opaque type parameter validity check as documented in the [rustc-dev-guide chapter].
+///
+/// [rustc-dev-guide chapter]:
+/// https://rustc-dev-guide.rust-lang.org/opaque-types-region-infer-restrictions.html
+pub fn check_opaque_type_parameter_valid<'tcx>(
+    infcx: &InferCtxt<'tcx>,
+    opaque_type_key: OpaqueTypeKey<'tcx>,
+    span: Span,
+) -> Result<(), ErrorGuaranteed> {
+    let tcx = infcx.tcx;
+    let opaque_generics = tcx.generics_of(opaque_type_key.def_id);
+    let opaque_env = LazyOpaqueTyEnv::new(tcx, opaque_type_key.def_id);
+    let mut seen_params: FxIndexMap<_, Vec<_>> = FxIndexMap::default();
+
+    for (i, arg) in opaque_type_key.iter_captured_args(tcx) {
+        let arg_is_param = match arg.unpack() {
+            GenericArgKind::Type(ty) => matches!(ty.kind(), ty::Param(_)),
+            GenericArgKind::Lifetime(lt) => {
+                matches!(*lt, ty::ReEarlyParam(_) | ty::ReLateParam(_))
+                    || (lt.is_static() && opaque_env.param_equal_static(i))
+            }
+            GenericArgKind::Const(ct) => matches!(ct.kind(), ty::ConstKind::Param(_)),
+        };
+
+        if arg_is_param {
+            // Register if the same lifetime appears multiple times in the generic args.
+            // There is an exception when the opaque type *requires* the lifetimes to be equal.
+            // See [rustc-dev-guide chapter] § "An exception to uniqueness rule".
+            let seen_where = seen_params.entry(arg).or_default();
+            if !seen_where.first().is_some_and(|&prev_i| opaque_env.params_equal(i, prev_i)) {
+                seen_where.push(i);
+            }
+        } else {
+            // Prevent `fn foo() -> Foo<u32>` from being defining.
+            let opaque_param = opaque_generics.param_at(i, tcx);
+            let kind = opaque_param.kind.descr();
+
+            opaque_env.param_is_error(i)?;
+
+            return Err(infcx.dcx().emit_err(NonGenericOpaqueTypeParam {
+                ty: arg,
+                kind,
+                span,
+                param_span: tcx.def_span(opaque_param.def_id),
+            }));
+        }
+    }
+
+    for (_, indices) in seen_params {
+        if indices.len() > 1 {
+            let descr = opaque_generics.param_at(indices[0], tcx).kind.descr();
+            let spans: Vec<_> = indices
+                .into_iter()
+                .map(|i| tcx.def_span(opaque_generics.param_at(i, tcx).def_id))
+                .collect();
+            return Err(infcx
+                .dcx()
+                .struct_span_err(span, "non-defining opaque type use in defining scope")
+                .with_span_note(spans, format!("{descr} used multiple times"))
+                .emit());
+        }
+    }
+
+    Ok(())
+}
+
+/// Computes if an opaque type requires a lifetime parameter to be equal to
+/// another one or to the `'static` lifetime.
+/// These requirements are derived from the explicit and implied bounds.
+struct LazyOpaqueTyEnv<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    def_id: LocalDefId,
+
+    /// Equal parameters will have the same name. Computed Lazily.
+    /// Example:
+    ///     `type Opaque<'a: 'static, 'b: 'c, 'c: 'b> = impl Sized;`
+    ///     Identity args: `['a, 'b, 'c]`
+    ///     Canonical args: `['static, 'b, 'b]`
+    canonical_args: std::cell::OnceCell<ty::GenericArgsRef<'tcx>>,
+}
+
+impl<'tcx> LazyOpaqueTyEnv<'tcx> {
+    fn new(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Self {
+        Self { tcx, def_id, canonical_args: std::cell::OnceCell::new() }
+    }
+
+    fn param_equal_static(&self, param_index: usize) -> bool {
+        self.get_canonical_args()[param_index].expect_region().is_static()
+    }
+
+    fn params_equal(&self, param1: usize, param2: usize) -> bool {
+        let canonical_args = self.get_canonical_args();
+        canonical_args[param1] == canonical_args[param2]
+    }
+
+    fn param_is_error(&self, param_index: usize) -> Result<(), ErrorGuaranteed> {
+        self.get_canonical_args()[param_index].error_reported()
+    }
+
+    fn get_canonical_args(&self) -> ty::GenericArgsRef<'tcx> {
+        if let Some(&canonical_args) = self.canonical_args.get() {
+            return canonical_args;
+        }
+
+        let &Self { tcx, def_id, .. } = self;
+        let origin = tcx.local_opaque_ty_origin(def_id);
+        let parent = match origin {
+            OpaqueTyOrigin::FnReturn { parent, .. }
+            | OpaqueTyOrigin::AsyncFn { parent, .. }
+            | OpaqueTyOrigin::TyAlias { parent, .. } => parent,
+        };
+        let param_env = tcx.param_env(parent);
+        let args = GenericArgs::identity_for_item(tcx, parent).extend_to(
+            tcx,
+            def_id.to_def_id(),
+            |param, _| {
+                tcx.map_opaque_lifetime_to_parent_lifetime(param.def_id.expect_local()).into()
+            },
+        );
+
+        // FIXME(#132279): It feels wrong to use `non_body_analysis` here given that we're
+        // in a body here.
+        let infcx = tcx.infer_ctxt().build(TypingMode::non_body_analysis());
+        let ocx = ObligationCtxt::new(&infcx);
+
+        let wf_tys = ocx.assumed_wf_types(param_env, parent).unwrap_or_else(|_| {
+            tcx.dcx().span_delayed_bug(tcx.def_span(def_id), "error getting implied bounds");
+            Default::default()
+        });
+        let outlives_env = OutlivesEnvironment::new(&infcx, parent, param_env, wf_tys);
+
+        let mut seen = vec![tcx.lifetimes.re_static];
+        let canonical_args = fold_regions(tcx, args, |r1, _| {
+            if r1.is_error() {
+                r1
+            } else if let Some(&r2) = seen.iter().find(|&&r2| {
+                let free_regions = outlives_env.free_region_map();
+                free_regions.sub_free_regions(tcx, r1, r2)
+                    && free_regions.sub_free_regions(tcx, r2, r1)
+            }) {
+                r2
+            } else {
+                seen.push(r1);
+                r1
+            }
+        });
+        self.canonical_args.set(canonical_args).unwrap();
+        canonical_args
+    }
+}