Rollup merge of #82789 - csmoe:issue-82772, r=estebank

Get with field index from pattern slice instead of directly indexing

Closes #82772
r? ``@estebank``

https://github.com/rust-lang/rust/pull/82789#issuecomment-796921977
> ``@estebank`` So the real cause is we only generate single pattern for Box here
615b03aeaa/compiler/rustc_mir_build/src/thir/pattern/deconstruct_pat.rs (L1130-L1132)
But in the replacing function, it tries to index on the 1-length pattern slice with field 1, thus out of bounds.
615b03aeaa/compiler/rustc_mir_build/src/thir/pattern/deconstruct_pat.rs (L1346)

compiler/rustc_mir_build/src/thir/pattern/deconstruct_pat.rs

@@ -1343,7 +1343,9 @@ impl<'p, 'tcx> Fields<'p, 'tcx> {
         match &mut fields {
             Fields::Vec(pats) => {
                 for (i, pat) in new_pats {
-                    pats[i] = pat
+                    if let Some(p) = pats.get_mut(i) {
+                        *p = pat;
+                    }
                 }
             }
             Fields::Filtered { fields, .. } => {

src/test/ui/structs/struct-variant-privacy-xc.rs

@@ -1,7 +1,8 @@
 // aux-build:struct_variant_privacy.rs
 extern crate struct_variant_privacy;
 
-fn f(b: struct_variant_privacy::Bar) { //~ ERROR enum `Bar` is private
+fn f(b: struct_variant_privacy::Bar) {
+    //~^ ERROR enum `Bar` is private
     match b {
         struct_variant_privacy::Bar::Baz { a: _a } => {} //~ ERROR enum `Bar` is private
     }

src/test/ui/structs/struct-variant-privacy-xc.stderr

@@ -11,7 +11,7 @@ LL | enum Bar {
    | ^^^^^^^^
 
 error[E0603]: enum `Bar` is private
-  --> $DIR/struct-variant-privacy-xc.rs:6:33
+  --> $DIR/struct-variant-privacy-xc.rs:7:33
    |
 LL |         struct_variant_privacy::Bar::Baz { a: _a } => {}
    |                                 ^^^ private enum

src/test/ui/structs/struct-variant-privacy.rs

@@ -1,10 +1,11 @@
 mod foo {
     enum Bar {
-        Baz { a: isize }
+        Baz { a: isize },
     }
 }
 
-fn f(b: foo::Bar) { //~ ERROR enum `Bar` is private
+fn f(b: foo::Bar) {
+    //~^ ERROR enum `Bar` is private
     match b {
         foo::Bar::Baz { a: _a } => {} //~ ERROR enum `Bar` is private
     }

src/test/ui/structs/struct-variant-privacy.stderr

@@ -11,7 +11,7 @@ LL |     enum Bar {
    |     ^^^^^^^^
 
 error[E0603]: enum `Bar` is private
-  --> $DIR/struct-variant-privacy.rs:9:14
+  --> $DIR/struct-variant-privacy.rs:10:14
    |
 LL |         foo::Bar::Baz { a: _a } => {}
    |              ^^^ private enum

src/test/ui/typeck/issue-82772.rs

@@ -0,0 +1,13 @@
+// edition:2018
+
+fn main() {
+    use a::ModPrivateStruct;
+    let Box { 0: _, .. }: Box<()>; //~ ERROR field `0` of
+    let Box { 1: _, .. }: Box<()>; //~ ERROR field `1` of
+    let ModPrivateStruct { 1: _, .. } = ModPrivateStruct::default(); //~ ERROR field `1` of
+}
+
+mod a {
+    #[derive(Default)]
+    pub struct ModPrivateStruct(u8, u8);
+}

src/test/ui/typeck/issue-82772.stderr

@@ -0,0 +1,21 @@
+error[E0451]: field `0` of struct `Box` is private
+  --> $DIR/issue-82772.rs:5:15
+   |
+LL |     let Box { 0: _, .. }: Box<()>;
+   |               ^^^^ private field
+
+error[E0451]: field `1` of struct `Box` is private
+  --> $DIR/issue-82772.rs:6:15
+   |
+LL |     let Box { 1: _, .. }: Box<()>;
+   |               ^^^^ private field
+
+error[E0451]: field `1` of struct `ModPrivateStruct` is private
+  --> $DIR/issue-82772.rs:7:28
+   |
+LL |     let ModPrivateStruct { 1: _, .. } = ModPrivateStruct::default();
+   |                            ^^^^ private field
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0451`.