Rollup merge of #104504 - compiler-errors:fru-syntax-note, r=estebank

Add a detailed note for missing comma typo w/ FRU syntax

Thanks to `@pierwill` for working on this with me!

Fixes #104373, perhaps `@alice-i-cecile` can comment on the new error for the example provided on that issue -- feedback is welcome.

```
error[E0063]: missing field `defaulted` in initializer of `Outer`
  --> $DIR/multi-line-fru-suggestion.rs:14:5
   |
LL |     Outer {
   |     ^^^^^ missing `defaulted`
   |
note: this expression may have been misinterpreted as a `..` range expression
  --> $DIR/multi-line-fru-suggestion.rs:16:16
   |
LL |           inner: Inner {
   |  ________________^
LL | |             a: 1,
LL | |             b: 2,
LL | |         }
   | |_________^ this expression does not end in a comma...
LL |           ..Default::default()
   |           ^^^^^^^^^^^^^^^^^^^^ ... so this is interpreted as a `..` range expression, instead of functional record update syntax
help: to set the remaining fields from `Default::default()`, separate the last named field with a comma
   |
LL |         },
   |          +

error: aborting due to previous error

For more information about this error, try `rustc --explain E0063`.
```

src/test/ui/structs/multi-line-fru-suggestion.rs

@@ -0,0 +1,22 @@
+#[derive(Default)]
+struct Inner {
+    a: u8,
+    b: u8,
+}
+
+#[derive(Default)]
+struct Outer {
+    inner: Inner,
+    defaulted: u8,
+}
+
+fn main(){
+    Outer {
+        //~^ ERROR missing field `defaulted` in initializer of `Outer`
+        inner: Inner {
+            a: 1,
+            b: 2,
+        }
+        ..Default::default()
+    };
+}

src/test/ui/structs/multi-line-fru-suggestion.stderr

@@ -0,0 +1,25 @@
+error[E0063]: missing field `defaulted` in initializer of `Outer`
+  --> $DIR/multi-line-fru-suggestion.rs:14:5
+   |
+LL |     Outer {
+   |     ^^^^^ missing `defaulted`
+   |
+note: this expression may have been misinterpreted as a `..` range expression
+  --> $DIR/multi-line-fru-suggestion.rs:16:16
+   |
+LL |           inner: Inner {
+   |  ________________^
+LL | |             a: 1,
+LL | |             b: 2,
+LL | |         }
+   | |_________^ this expression does not end in a comma...
+LL |           ..Default::default()
+   |           ^^^^^^^^^^^^^^^^^^^^ ... so this is interpreted as a `..` range expression, instead of functional record update syntax
+help: to set the remaining fields from `Default::default()`, separate the last named field with a comma
+   |
+LL |         }, 
+   |          +
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0063`.

src/test/ui/structs/struct-record-suggestion.fixed

@@ -7,9 +7,8 @@ struct A {
 }
 
 fn a() {
-    let q = A { c: 5,..Default::default() };
-    //~^ ERROR mismatched types
-    //~| ERROR missing fields
+    let q = A { c: 5, ..Default::default() };
+    //~^ ERROR missing fields
     //~| HELP separate the last named field with a comma
     let r = A { c: 5, ..Default::default() };
     assert_eq!(q, r);
@@ -21,7 +20,7 @@ struct B {
 }
 
 fn b() {
-    let q = B { b: 1,..Default::default() };
+    let q = B { b: 1, ..Default::default() };
     //~^ ERROR mismatched types
     //~| HELP separate the last named field with a comma
     let r = B { b: 1 };

src/test/ui/structs/struct-record-suggestion.rs

@@ -8,8 +8,7 @@ struct A {
 
 fn a() {
     let q = A { c: 5..Default::default() };
-    //~^ ERROR mismatched types
-    //~| ERROR missing fields
+    //~^ ERROR missing fields
     //~| HELP separate the last named field with a comma
     let r = A { c: 5, ..Default::default() };
     assert_eq!(q, r);

src/test/ui/structs/struct-record-suggestion.stderr

@@ -1,37 +1,38 @@
-error[E0308]: mismatched types
-  --> $DIR/struct-record-suggestion.rs:10:20
-   |
-LL |     let q = A { c: 5..Default::default() };
-   |                    ^^^^^^^^^^^^^^^^^^^^^ expected `u64`, found struct `std::ops::Range`
-   |
-   = note: expected type `u64`
-            found struct `std::ops::Range<{integer}>`
-
 error[E0063]: missing fields `b` and `d` in initializer of `A`
   --> $DIR/struct-record-suggestion.rs:10:13
    |
 LL |     let q = A { c: 5..Default::default() };
    |             ^ missing `b` and `d`
    |
+note: this expression may have been misinterpreted as a `..` range expression
+  --> $DIR/struct-record-suggestion.rs:10:20
+   |
+LL |     let q = A { c: 5..Default::default() };
+   |                    ^^^^^^^^^^^^^^^^^^^^^
 help: to set the remaining fields from `Default::default()`, separate the last named field with a comma
    |
-LL |     let q = A { c: 5,..Default::default() };
+LL |     let q = A { c: 5, ..Default::default() };
    |                     +
 
 error[E0308]: mismatched types
-  --> $DIR/struct-record-suggestion.rs:24:20
+  --> $DIR/struct-record-suggestion.rs:23:20
    |
 LL |     let q = B { b: 1..Default::default() };
    |                    ^^^^^^^^^^^^^^^^^^^^^ expected `u32`, found struct `std::ops::Range`
    |
    = note: expected type `u32`
             found struct `std::ops::Range<{integer}>`
+note: this expression may have been misinterpreted as a `..` range expression
+  --> $DIR/struct-record-suggestion.rs:23:20
+   |
+LL |     let q = B { b: 1..Default::default() };
+   |                    ^^^^^^^^^^^^^^^^^^^^^
 help: to set the remaining fields from `Default::default()`, separate the last named field with a comma
    |
-LL |     let q = B { b: 1,..Default::default() };
+LL |     let q = B { b: 1, ..Default::default() };
    |                     +
 
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 
 Some errors have detailed explanations: E0063, E0308.
 For more information about an error, try `rustc --explain E0063`.