6 files changed, 54 insertions, 20 deletions
diff --git a/compiler/rustc_hir_typeck/src/cast.rs b/compiler/rustc_hir_typeck/src/cast.rs
index c948b6343b7..07d64918e8b 100644
--- a/compiler/rustc_hir_typeck/src/cast.rs
+++ b/compiler/rustc_hir_typeck/src/cast.rs
@@ -40,17 +40,19 @@ use rustc_middle::mir::Mutability;
 use rustc_middle::ty::adjustment::AllowTwoPhase;
 use rustc_middle::ty::cast::{CastKind, CastTy};
 use rustc_middle::ty::error::TypeError;
+use rustc_middle::ty::TyCtxt;
 use rustc_middle::ty::{self, Ty, TypeAndMut, TypeVisitableExt, VariantDef};
 use rustc_session::lint;
 use rustc_span::def_id::{DefId, LOCAL_CRATE};
 use rustc_span::symbol::sym;
 use rustc_span::Span;
+use rustc_span::DUMMY_SP;
 use rustc_trait_selection::infer::InferCtxtExt;
 
 /// Reifies a cast check to be checked once we have full type information for
 /// a function context.
 #[derive(Debug)]
-pub struct CastCheck<'tcx> {
+pub(crate) struct CastCheck<'tcx> {
     /// The expression whose value is being casted
     expr: &'tcx hir::Expr<'tcx>,
     /// The source type for the cast expression
@@ -60,8 +62,6 @@ pub struct CastCheck<'tcx> {
     cast_ty: Ty<'tcx>,
     cast_span: Span,
     span: Span,
-    /// whether the cast is made in a const context or not.
-    pub constness: hir::Constness,
 }
 
 /// The kind of pointer and associated metadata (thin, length or vtable) - we
@@ -194,18 +194,45 @@ fn make_invalid_casting_error<'a, 'tcx>(
     )
 }
 
+/// If a cast from `from_ty` to `to_ty` is valid, returns a `Some` containing the kind
+/// of the cast.
+///
+/// This is a helper used from clippy.
+pub fn check_cast<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+    e: &'tcx hir::Expr<'tcx>,
+    from_ty: Ty<'tcx>,
+    to_ty: Ty<'tcx>,
+) -> Option<CastKind> {
+    let hir_id = e.hir_id;
+    let local_def_id = hir_id.owner.def_id;
+
+    let root_ctxt = crate::TypeckRootCtxt::new(tcx, local_def_id);
+    let fn_ctxt = FnCtxt::new(&root_ctxt, param_env, local_def_id);
+
+    if let Ok(check) = CastCheck::new(
+        &fn_ctxt, e, from_ty, to_ty,
+        // We won't show any errors to the user, so the span is irrelevant here.
+        DUMMY_SP, DUMMY_SP,
+    ) {
+        check.do_check(&fn_ctxt).ok()
+    } else {
+        None
+    }
+}
+
 impl<'a, 'tcx> CastCheck<'tcx> {
-    pub fn new(
+    pub(crate) fn new(
         fcx: &FnCtxt<'a, 'tcx>,
         expr: &'tcx hir::Expr<'tcx>,
         expr_ty: Ty<'tcx>,
         cast_ty: Ty<'tcx>,
         cast_span: Span,
         span: Span,
-        constness: hir::Constness,
     ) -> Result<CastCheck<'tcx>, ErrorGuaranteed> {
         let expr_span = expr.span.find_ancestor_inside(span).unwrap_or(expr.span);
-        let check = CastCheck { expr, expr_ty, expr_span, cast_ty, cast_span, span, constness };
+        let check = CastCheck { expr, expr_ty, expr_span, cast_ty, cast_span, span };
 
         // For better error messages, check for some obviously unsized
         // cases now. We do a more thorough check at the end, once
@@ -644,7 +671,7 @@ impl<'a, 'tcx> CastCheck<'tcx> {
     /// Checks a cast, and report an error if one exists. In some cases, this
     /// can return Ok and create type errors in the fcx rather than returning
     /// directly. coercion-cast is handled in check instead of here.
-    pub fn do_check(&self, fcx: &FnCtxt<'a, 'tcx>) -> Result<CastKind, CastError> {
+    fn do_check(&self, fcx: &FnCtxt<'a, 'tcx>) -> Result<CastKind, CastError> {
         use rustc_middle::ty::cast::CastTy::*;
         use rustc_middle::ty::cast::IntTy::*;
 
diff --git a/compiler/rustc_hir_typeck/src/coercion.rs b/compiler/rustc_hir_typeck/src/coercion.rs
index 44b19318d5d..51d01afc4eb 100644
--- a/compiler/rustc_hir_typeck/src/coercion.rs
+++ b/compiler/rustc_hir_typeck/src/coercion.rs
@@ -1318,6 +1318,20 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     }
 }
 
+/// Check whether `ty` can be coerced to `output_ty`.
+/// Used from clippy.
+pub fn can_coerce<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+    body_id: LocalDefId,
+    ty: Ty<'tcx>,
+    output_ty: Ty<'tcx>,
+) -> bool {
+    let root_ctxt = crate::typeck_root_ctxt::TypeckRootCtxt::new(tcx, body_id);
+    let fn_ctxt = FnCtxt::new(&root_ctxt, param_env, body_id);
+    fn_ctxt.can_coerce(ty, output_ty)
+}
+
 /// CoerceMany encapsulates the pattern you should use when you have
 /// many expressions that are all getting coerced to a common
 /// type. This arises, for example, when you have a match (the result
diff --git a/compiler/rustc_hir_typeck/src/expr.rs b/compiler/rustc_hir_typeck/src/expr.rs
index 4da45303d12..d3df3dd3885 100644
--- a/compiler/rustc_hir_typeck/src/expr.rs
+++ b/compiler/rustc_hir_typeck/src/expr.rs
@@ -1390,15 +1390,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         } else {
             // Defer other checks until we're done type checking.
             let mut deferred_cast_checks = self.deferred_cast_checks.borrow_mut();
-            match cast::CastCheck::new(
-                self,
-                e,
-                t_expr,
-                t_cast,
-                t.span,
-                expr.span,
-                hir::Constness::NotConst,
-            ) {
+            match cast::CastCheck::new(self, e, t_expr, t_cast, t.span, expr.span) {
                 Ok(cast_check) => {
                     debug!(
                         "check_expr_cast: deferring cast from {:?} to {:?}: {:?}",
diff --git a/compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs b/compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs
index 05e7c5b2b41..0b69c7a2431 100644
--- a/compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs
+++ b/compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs
@@ -38,7 +38,7 @@ use std::ops::Deref;
 ///
 /// [`ItemCtxt`]: rustc_hir_analysis::collect::ItemCtxt
 /// [`InferCtxt`]: infer::InferCtxt
-pub struct FnCtxt<'a, 'tcx> {
+pub(crate) struct FnCtxt<'a, 'tcx> {
     pub(super) body_id: LocalDefId,
 
     /// The parameter environment used for proving trait obligations
diff --git a/compiler/rustc_hir_typeck/src/lib.rs b/compiler/rustc_hir_typeck/src/lib.rs
index 80fd4be53e1..700dde184f2 100644
--- a/compiler/rustc_hir_typeck/src/lib.rs
+++ b/compiler/rustc_hir_typeck/src/lib.rs
@@ -42,8 +42,9 @@ mod typeck_root_ctxt;
 mod upvar;
 mod writeback;
 
-pub use fn_ctxt::FnCtxt;
-pub use typeck_root_ctxt::TypeckRootCtxt;
+pub use coercion::can_coerce;
+use fn_ctxt::FnCtxt;
+use typeck_root_ctxt::TypeckRootCtxt;
 
 use crate::check::check_fn;
 use crate::coercion::DynamicCoerceMany;
diff --git a/compiler/rustc_hir_typeck/src/typeck_root_ctxt.rs b/compiler/rustc_hir_typeck/src/typeck_root_ctxt.rs
index e493e6a0a7e..88777bf5b02 100644
--- a/compiler/rustc_hir_typeck/src/typeck_root_ctxt.rs
+++ b/compiler/rustc_hir_typeck/src/typeck_root_ctxt.rs
@@ -27,7 +27,7 @@ use std::ops::Deref;
 /// `bar()` will each have their own `FnCtxt`, but they will
 /// share the inference context, will process obligations together,
 /// can access each other's local types (scoping permitted), etc.
-pub struct TypeckRootCtxt<'tcx> {
+pub(crate) struct TypeckRootCtxt<'tcx> {
     pub(super) infcx: InferCtxt<'tcx>,
 
     pub(super) typeck_results: RefCell<ty::TypeckResults<'tcx>>,