Fix some double indents on exprs containing blocks

The `print_expr` method already places an `ibox(INDENT_UNIT)` around
every expr that gets printed. Some exprs were then using `self.head`
inside of that, which does its own `cbox(INDENT_UNIT)`, resulting in two
levels of indentation:

    while true {
            stuff;
        }

This commit fixes those cases to produce the expected single level of
indentation within every expression containing a block.

    while true {
        stuff;
    }

compiler/rustc_ast_pretty/src/pprust/state/expr.rs

@@ -320,7 +320,9 @@ impl<'a> State<'a> {
                     self.print_ident(label.ident);
                     self.word_space(":");
                 }
-                self.head("while");
+                self.cbox(0);
+                self.ibox(0);
+                self.word_nbsp("while");
                 self.print_expr_as_cond(test);
                 self.space();
                 self.print_block_with_attrs(blk, attrs);
@@ -330,7 +332,9 @@ impl<'a> State<'a> {
                     self.print_ident(label.ident);
                     self.word_space(":");
                 }
-                self.head("for");
+                self.cbox(0);
+                self.ibox(0);
+                self.word_nbsp("for");
                 self.print_pat(pat);
                 self.space();
                 self.word_space("in");
@@ -343,12 +347,14 @@ impl<'a> State<'a> {
                     self.print_ident(label.ident);
                     self.word_space(":");
                 }
-                self.head("loop");
+                self.cbox(0);
+                self.ibox(0);
+                self.word_nbsp("loop");
                 self.print_block_with_attrs(blk, attrs);
             }
             ast::ExprKind::Match(ref expr, ref arms) => {
-                self.cbox(INDENT_UNIT);
-                self.ibox(INDENT_UNIT);
+                self.cbox(0);
+                self.ibox(0);
                 self.word_nbsp("match");
                 self.print_expr_as_cond(expr);
                 self.space();
@@ -388,7 +394,7 @@ impl<'a> State<'a> {
                     self.word_space(":");
                 }
                 // containing cbox, will be closed by print-block at }
-                self.cbox(INDENT_UNIT);
+                self.cbox(0);
                 // head-box, will be closed by print-block after {
                 self.ibox(0);
                 self.print_block_with_attrs(blk, attrs);
@@ -397,7 +403,7 @@ impl<'a> State<'a> {
                 self.word_nbsp("async");
                 self.print_capture_clause(capture_clause);
                 // cbox/ibox in analogy to the `ExprKind::Block` arm above
-                self.cbox(INDENT_UNIT);
+                self.cbox(0);
                 self.ibox(0);
                 self.print_block_with_attrs(blk, attrs);
             }
@@ -500,7 +506,9 @@ impl<'a> State<'a> {
                 self.word("?")
             }
             ast::ExprKind::TryBlock(ref blk) => {
-                self.head("try");
+                self.cbox(0);
+                self.ibox(0);
+                self.word_nbsp("try");
                 self.print_block_with_attrs(blk, attrs)
             }
             ast::ExprKind::Err => {

compiler/rustc_ast_pretty/src/pprust/state/item.rs

@@ -1,5 +1,5 @@
 use crate::pp::Breaks::Inconsistent;
-use crate::pprust::state::{AnnNode, PrintState, State, INDENT_UNIT};
+use crate::pprust::state::{AnnNode, PrintState, State};
 
 use rustc_ast as ast;
 use rustc_ast::GenericBound;
@@ -377,7 +377,7 @@ impl<'a> State<'a> {
             self.space_if_not_bol();
             self.maybe_print_comment(v.span.lo());
             self.print_outer_attributes(&v.attrs);
-            self.ibox(INDENT_UNIT);
+            self.ibox(0);
             self.print_variant(v);
             self.word(",");
             self.end();