Remove special-casing for argument patterns in MIR typeck

Author: dianne

Diff for: compiler/rustc_borrowck/src/type_check/mod.rs

@@ -892,19 +892,6 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                     Some(l) if !body.local_decls[l].is_user_variable() => {
                         ConstraintCategory::Boring
                     }
-                    Some(_)
-                        if let Some(body_id) = tcx
-                            .hir_node_by_def_id(body.source.def_id().expect_local())
-                            .body_id()
-                            && let params = tcx.hir().body(body_id).params
-                            && params
-                                .iter()
-                                .any(|param| param.span.contains(stmt.source_info.span)) =>
-                    {
-                        // Assignments generated from lowering argument patterns shouldn't be called
-                        // "assignments" in diagnostics and aren't interesting to blame for errors.
-                        ConstraintCategory::Boring
-                    }
                     _ => ConstraintCategory::Assignment,
                 };
                 debug!(

Diff for: tests/ui/lifetimes/lifetime-errors/ex3-both-anon-regions-2.rs

@@ -1,6 +1,6 @@
 fn foo(&mut (ref mut v, w): &mut (&u8, &u8), x: &u8) {
-    *v = x;
     //~^ ERROR lifetime may not live long enough
+    *v = x;
 }
 
 fn main() { }

Diff for: tests/ui/lifetimes/lifetime-errors/ex3-both-anon-regions-2.stderr

@@ -1,13 +1,15 @@
 error: lifetime may not live long enough
-  --> $DIR/ex3-both-anon-regions-2.rs:2:5
+  --> $DIR/ex3-both-anon-regions-2.rs:1:14
    |
 LL | fn foo(&mut (ref mut v, w): &mut (&u8, &u8), x: &u8) {
-   |                                   -             - let's call the lifetime of this reference `'1`
-   |                                   |
-   |                                   let's call the lifetime of this reference `'2`
-LL |     *v = x;
-   |     ^^^^^^ assignment requires that `'1` must outlive `'2`
+   |              ^^^^^^^^^            -             - let's call the lifetime of this reference `'1`
+   |              |                    |
+   |              |                    let's call the lifetime of this reference `'2`
+   |              assignment requires that `'1` must outlive `'2`
    |
+   = note: requirement occurs because of a mutable reference to `&u8`
+   = note: mutable references are invariant over their type parameter
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 help: consider introducing a named lifetime parameter
    |
 LL | fn foo<'a>(&mut (ref mut v, w): &mut (&'a u8, &u8), x: &'a u8) {