Point at coercion reason for if expressions without else clause if caused by return type

src/librustc_typeck/check/mod.rs

@@ -3472,8 +3472,22 @@ 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 ret_reason = self.maybe_get_coercion_reason(then_expr.hir_id, sp);
+
             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());
+                } else if let ExprKind::Block(block, _) = &then_expr.node {
+                    if let Some(expr) = &block.expr {
+                        err.span_label(expr.span, "found here".to_string());
+                    }
+                }
+                err.note("`if` expressions without `else` evaluate to `()`");
+                err.help("consider adding an `else` block that evaluates to the expected type");
+            }, ret_reason.is_none());
 
             // If the condition is false we can't diverge.
             self.diverges.set(cond_diverges);
@@ -3487,6 +3501,37 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
         }
     }
 
+    fn maybe_get_coercion_reason(&self, hir_id: hir::HirId, sp: Span) -> Option<(Span, String)> {
+        let node = 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(hir_id),
+        ));
+        if let Node::Block(block) = node {
+            // 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 {
+                    return self.get_fn_decl(hir_id).map(|(fn_decl, _)| (
+                        fn_decl.output.span(),
+                        format!("expected `{}` because of this return type", fn_decl.output),
+                    ));
+                }
+            }
+        }
+        if let Node::Local(hir::Local {
+            ty: Some(_), pat, ..
+        }) = node {
+            return Some((pat.span, "expected because of this assignment".to_string()));
+        }
+        None
+    }
+
     // Check field access expressions
     fn check_field(&self,
                    expr: &'gcx hir::Expr,

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

@@ -0,0 +1,25 @@
+fn foo(bar: usize) -> usize {
+    if bar % 5 == 0 {
+        return 3;
+    }
+    //~^^^ ERROR if may be missing an else clause
+}
+
+fn foo2(bar: usize) -> usize {
+    let x: usize = if bar % 5 == 0 {
+        return 3;
+    };
+    //~^^^ ERROR if may be missing an else clause
+    x
+}
+
+fn foo3(bar: usize) -> usize {
+    if bar % 5 == 0 {
+        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 +1,49 @@
+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 {
+   |                         ----- expected `usize` because of this return type
+LL | /     if bar % 5 == 0 {
+LL | |         return 3;
+LL | |     }
+   | |_____^ expected usize, found ()
+   |
+   = note: expected type `usize`
+              found type `()`
+   = note: `if` expressions without `else` evaluate to `()`
+   = help: consider adding an `else` block that evaluates to the expected type
+
+error[E0317]: if may be missing an else clause
+  --> $DIR/if-without-else-as-fn-expr.rs:9:20
+   |
+LL |       let x: usize = if bar % 5 == 0 {
+   |  _________-__________^
+   | |         |
+   | |         expected because of this assignment
+LL | |         return 3;
+LL | |     };
+   | |_____^ expected usize, found ()
+   |
+   = note: expected type `usize`
+              found type `()`
+   = note: `if` expressions without `else` evaluate to `()`
+   = help: consider adding an `else` block that evaluates to the expected type
+
+error[E0317]: if may be missing an else clause
+  --> $DIR/if-without-else-as-fn-expr.rs:17:5
+   |
+LL |   fn foo3(bar: usize) -> usize {
+   |                          ----- expected `usize` because of this return type
+LL | /     if bar % 5 == 0 {
+LL | |         3
+LL | |     }
+   | |_____^ expected usize, found ()
+   |
+   = note: expected type `usize`
+              found type `()`
+   = note: `if` expressions without `else` evaluate to `()`
+   = help: consider adding an `else` block that evaluates to the expected type
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0317`.

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

@@ -2,10 +2,15 @@ error[E0317]: if may be missing an else clause
   --> $DIR/if-without-else-result.rs:2:13
    |
 LL |     let a = if true { true };
-   |             ^^^^^^^^^^^^^^^^ expected (), found bool
+   |             ^^^^^^^^^^----^^
+   |             |         |
+   |             |         found here
+   |             expected (), found bool
    |
    = note: expected type `()`
               found type `bool`
+   = note: `if` expressions without `else` evaluate to `()`
+   = help: consider adding an `else` block that evaluates to the expected type
 
 error: aborting due to previous error
 

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

@@ -8,11 +8,14 @@ LL | | //~| expected type `()`
 LL | | //~| found type `{integer}`
 LL | | //~| expected (), found integer
 LL | |         1
+   | |         - found here
 LL | |     };
    | |_____^ expected (), found integer
    |
    = note: expected type `()`
               found type `{integer}`
+   = note: `if` expressions without `else` evaluate to `()`
+   = help: consider adding an `else` block that evaluates to the expected type
 
 error: aborting due to previous error
 

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

@@ -2,10 +2,15 @@ error[E0317]: if may be missing an else clause
   --> $DIR/issue-50577.rs:3:16
    |
 LL |         Drop = assert_eq!(1, 1)
-   |                ^^^^^^^^^^^^^^^^ expected (), found isize
+   |                ^^^^^^^^^^^^^^^^
+   |                |
+   |                expected (), found isize
+   |                found here
    |
    = note: expected type `()`
               found type `isize`
+   = note: `if` expressions without `else` evaluate to `()`
+   = help: consider adding an `else` block that evaluates to the expected type
    = 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