Point at coercion reason for if exprs without else clause

```
error[E0317]: if may be missing an else clause
  --> $DIR/if-without-else-as-fn-expr.rs:2:5
   |
LL |   fn foo(bar: usize) -> usize {
   |                         ----- found `usize` because of this return type
LL | /     if bar % 5 == 0 {
LL | |         return 3;
LL | |     }
   | |_____^ expected (), found usize
   |
   = note: expected type `()`
              found type `usize`
   = note: `if` expressions without `else` must evaluate to `()`
```

Author: estebank

src/librustc_typeck/check/mod.rs

@@ -3472,8 +3472,39 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
             // We won't diverge unless both branches do (or the condition does).
             self.diverges.set(cond_diverges | then_diverges & else_diverges);
         } else {
+            // If this `if` expr is the parent's function return expr, the cause of the type
+            // coercion is the return type, point at it. (#25228)
+            let mut ret_reason = None;
+            if let Node::Block(block) = self.tcx.hir().get_by_hir_id(
+                self.tcx.hir().get_parent_node_by_hir_id(
+                    self.tcx.hir().get_parent_node_by_hir_id(then_expr.hir_id),
+                ),
+            ) {
+                // check that the body's parent is an fn
+                let parent = self.tcx.hir().get_by_hir_id(
+                    self.tcx.hir().get_parent_node_by_hir_id(
+                        self.tcx.hir().get_parent_node_by_hir_id(block.hir_id),
+                    ),
+                );
+                if let (Some(expr), Node::Item(hir::Item {
+                    node: hir::ItemKind::Fn(..), ..
+                })) = (&block.expr, parent) {
+                    // check that the `if` expr without `else` is the fn body's expr
+                    if expr.span == sp {
+                        ret_reason = self.get_fn_decl(then_expr.hir_id).map(|(fn_decl, _)| (
+                            fn_decl.output.span(),
+                            format!("found `{}` because of this return type", fn_decl.output),
+                        ));
+                    }
+                }
+            }
             let else_cause = self.cause(sp, ObligationCauseCode::IfExpressionWithNoElse);
-            coerce.coerce_forced_unit(self, &else_cause, &mut |_| (), true);
+            coerce.coerce_forced_unit(self, &else_cause, &mut |err| {
+                if let Some((sp, msg)) = &ret_reason {
+                    err.span_label(*sp, msg.as_str());
+                }
+                err.note("`if` expressions without `else` must evaluate to `()`");
+            }, true);
 
             // If the condition is false we can't diverge.
             self.diverges.set(cond_diverges);

src/test/ui/if/if-without-else-as-fn-expr.rs

@@ -0,0 +1,10 @@
+fn foo(bar: usize) -> usize {
+    if bar % 5 == 0 {
+        return 3;
+    }
+    //~^^^ ERROR if may be missing an else clause
+}
+
+fn main() {
+    let _ = foo(1);
+}

src/test/ui/if/if-without-else-as-fn-expr.stderr

@@ -0,0 +1,17 @@
+error[E0317]: if may be missing an else clause
+  --> $DIR/if-without-else-as-fn-expr.rs:2:5
+   |
+LL |   fn foo(bar: usize) -> usize {
+   |                         ----- found `usize` because of this return type
+LL | /     if bar % 5 == 0 {
+LL | |         return 3;
+LL | |     }
+   | |_____^ expected (), found usize
+   |
+   = note: expected type `()`
+              found type `usize`
+   = note: `if` expressions without `else` must evaluate to `()`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0317`.

src/test/ui/if/if-without-else-result.stderr

@@ -6,6 +6,7 @@ LL |     let a = if true { true };
    |
    = note: expected type `()`
               found type `bool`
+   = note: `if` expressions without `else` must evaluate to `()`
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-4201.stderr

@@ -13,6 +13,7 @@ LL | |     };
    |
    = note: expected type `()`
               found type `{integer}`
+   = note: `if` expressions without `else` must evaluate to `()`
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-50577.stderr

@@ -6,6 +6,7 @@ LL |         Drop = assert_eq!(1, 1)
    |
    = note: expected type `()`
               found type `isize`
+   = note: `if` expressions without `else` must evaluate to `()`
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
 
 error: aborting due to previous error