Add more details to elseless if error

Author: estebank

Diff for: src/librustc_typeck/check/mod.rs

@@ -3480,6 +3480,10 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
             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");
@@ -3498,11 +3502,10 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
     }
 
     fn maybe_get_coercion_reason(&self, hir_id: hir::HirId, sp: Span) -> Option<(Span, String)> {
-        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(hir_id),
-            ),
-        ) {
+        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(
@@ -3521,6 +3524,11 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                 }
             }
         }
+        if let Node::Local(hir::Local {
+            ty: Some(_), pat, ..
+        }) = node {
+            return Some((pat.span, "expected because of this assignment".to_string()));
+        }
         None
     }
 

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

@@ -5,6 +5,21 @@ fn foo(bar: usize) -> usize {
     //~^^^ 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);
 }

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

@@ -13,6 +13,37 @@ LL | |     }
    = 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
+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`.

Diff for: src/test/ui/if/if-without-else-result.stderr

@@ -2,7 +2,10 @@ 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`

Diff for: src/test/ui/issues/issue-4201.stderr

@@ -8,6 +8,7 @@ LL | | //~| expected type `()`
 LL | | //~| found type `{integer}`
 LL | | //~| expected (), found integer
 LL | |         1
+   | |         - found here
 LL | |     };
    | |_____^ expected (), found integer
    |

Diff for: src/test/ui/issues/issue-50577.stderr

@@ -2,7 +2,10 @@ 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`