Rollup merge of rust-lang#58204 - estebank:impl-trait-semi, r=zackmdavis

On return type `impl Trait` for block with no expr point at last semi

Partial solution, doesn't actually validate that the last statement in the function body can satisfy the trait bound, but it's a good incremental improvement over the status quo.

```
error[E0277]: the trait bound `(): Bar` is not satisfied
  --> $DIR/impl-trait-return-trailing-semicolon.rs:3:13
   |
LL | fn foo() -> impl Bar {
   |             ^^^^^^^^ the trait `Bar` is not implemented for `()`
LL |     5;
   |      - consider removing this semicolon
   |
   = note: the return type of a function must have a statically known size
```

Partially addresses rust-lang#54771.

src/librustc/traits/error_reporting.rs

@@ -599,11 +599,12 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         }
     }
 
-    pub fn report_selection_error(&self,
-                                  obligation: &PredicateObligation<'tcx>,
-                                  error: &SelectionError<'tcx>,
-                                  fallback_has_occurred: bool)
-    {
+    pub fn report_selection_error(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        error: &SelectionError<'tcx>,
+        fallback_has_occurred: bool,
+    ) {
         let span = obligation.cause.span;
 
         let mut err = match *error {
@@ -673,6 +674,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
 
                         self.suggest_borrow_on_unsized_slice(&obligation.cause.code, &mut err);
                         self.suggest_remove_reference(&obligation, &mut err, &trait_ref);
+                        self.suggest_semicolon_removal(&obligation, &mut err, span, &trait_ref);
 
                         // Try to report a help message
                         if !trait_ref.has_infer_types() &&
@@ -901,9 +903,11 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
 
     /// When encountering an assignment of an unsized trait, like `let x = ""[..];`, provide a
     /// suggestion to borrow the initializer in order to use have a slice instead.
-    fn suggest_borrow_on_unsized_slice(&self,
-                                       code: &ObligationCauseCode<'tcx>,
-                                       err: &mut DiagnosticBuilder<'tcx>) {
+    fn suggest_borrow_on_unsized_slice(
+        &self,
+        code: &ObligationCauseCode<'tcx>,
+        err: &mut DiagnosticBuilder<'tcx>,
+    ) {
         if let &ObligationCauseCode::VariableType(node_id) = code {
             let parent_node = self.tcx.hir().get_parent_node(node_id);
             if let Some(Node::Local(ref local)) = self.tcx.hir().find(parent_node) {
@@ -925,10 +929,12 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
 
     /// Whenever references are used by mistake, like `for (i, e) in &vec.iter().enumerate()`,
     /// suggest removing these references until we reach a type that implements the trait.
-    fn suggest_remove_reference(&self,
-                                obligation: &PredicateObligation<'tcx>,
-                                err: &mut DiagnosticBuilder<'tcx>,
-                                trait_ref: &ty::Binder<ty::TraitRef<'tcx>>) {
+    fn suggest_remove_reference(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        err: &mut DiagnosticBuilder<'tcx>,
+        trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
+    ) {
         let trait_ref = trait_ref.skip_binder();
         let span = obligation.cause.span;
 
@@ -970,6 +976,40 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         }
     }
 
+    fn suggest_semicolon_removal(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        err: &mut DiagnosticBuilder<'tcx>,
+        span: Span,
+        trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
+    ) {
+        let hir = self.tcx.hir();
+        let parent_node = hir.get_parent_node(
+            hir.hir_to_node_id(obligation.cause.body_id),
+        );
+        let node = hir.find(parent_node);
+        if let Some(hir::Node::Item(hir::Item {
+            node: hir::ItemKind::Fn(decl, _, _, body_id),
+            ..
+        })) = node {
+            let body = hir.body(*body_id);
+            if let hir::ExprKind::Block(blk, _) = &body.value.node {
+                if decl.output.span().overlaps(span) && blk.expr.is_none() &&
+                    "()" == &trait_ref.self_ty().to_string()
+                {
+                    // FIXME(estebank): When encountering a method with a trait
+                    // bound not satisfied in the return type with a body that has
+                    // no return, suggest removal of semicolon on last statement.
+                    // Once that is added, close #54771.
+                    if let Some(ref stmt) = blk.stmts.last() {
+                        let sp = self.tcx.sess.source_map().end_point(stmt.span);
+                        err.span_label(sp, "consider removing this semicolon");
+                    }
+                }
+            }
+        }
+    }
+
     /// Given some node representing a fn-like thing in the HIR map,
     /// returns a span and `ArgKind` information that describes the
     /// arguments it expects. This can be supplied to

src/test/ui/suggestions/impl-trait-return-trailing-semicolon.rs

@@ -0,0 +1,7 @@
+trait Bar {}
+impl Bar for u8 {}
+fn foo() -> impl Bar {
+    5; //~^ ERROR the trait bound `(): Bar` is not satisfied
+}
+
+fn main() {}

src/test/ui/suggestions/impl-trait-return-trailing-semicolon.stderr

@@ -0,0 +1,13 @@
+error[E0277]: the trait bound `(): Bar` is not satisfied
+  --> $DIR/impl-trait-return-trailing-semicolon.rs:3:13
+   |
+LL | fn foo() -> impl Bar {
+   |             ^^^^^^^^ the trait `Bar` is not implemented for `()`
+LL |     5; //~^ ERROR the trait bound `(): Bar` is not satisfied
+   |      - consider removing this semicolon
+   |
+   = note: the return type of a function must have a statically known size
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.