Box `DiagnosticBuilder`.

It's a large type -- 176 bytes on 64-bit. And it's passed around and
returned from a lot of functions, including within PResult.

This commit boxes it, which reduces memory traffic. In particular,
`PResult` shrinks to 16 bytes in the best case; this reduces instruction
counts by up to 2% on various workloads.

1 files changed, 7 insertions, 0 deletions

diff --git a/src/libsyntax/parse/mod.rs b/src/libsyntax/parse/mod.rs
index bc1bc00ac84..2441a027f99 100644
--- a/src/libsyntax/parse/mod.rs
+++ b/src/libsyntax/parse/mod.rs
@@ -13,6 +13,8 @@ use crate::symbol::Symbol;
 
 use errors::{Applicability, FatalError, Level, Handler, ColorConfig, Diagnostic, DiagnosticBuilder};
 use rustc_data_structures::fx::{FxHashSet, FxHashMap};
+#[cfg(target_arch = "x86_64")]
+use rustc_data_structures::static_assert_size;
 use rustc_data_structures::sync::{Lrc, Lock, Once};
 use syntax_pos::{Span, SourceFile, FileName, MultiSpan};
 use syntax_pos::edition::Edition;
@@ -38,6 +40,11 @@ crate mod unescape_error_reporting;
 
 pub type PResult<'a, T> = Result<T, DiagnosticBuilder<'a>>;
 
+// `PResult` is used a lot. Make sure it doesn't unintentionally get bigger.
+// (See also the comment on `DiagnosticBuilderInner`.)
+#[cfg(target_arch = "x86_64")]
+static_assert_size!(PResult<'_, bool>, 16);
+
 /// Collected spans during parsing for places where a certain feature was
 /// used and should be feature gated accordingly in `check_crate`.
 #[derive(Default)]