Auto merge of #58981 - estebank:elseless-if, r=davidtwco

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

```
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 `()`
```

Fix #25228.

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 | |
 LL | |
 LL | |
 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