Stop using DiagnosticBuilder::buffer in the parser.

One consequence is that errors returned by
`maybe_new_parser_from_source_str` now must be consumed, so a bunch of
places that previously ignored those errors now cancel them. (Most of
them explicitly dropped the errors before. I guess that was to indicate
"we are explicitly ignoring these", though I'm not 100% sure.)

src/tools/clippy/clippy_lints/src/doc/needless_doctest_main.rs

@@ -53,7 +53,7 @@ pub fn check(
                 let mut parser = match maybe_new_parser_from_source_str(&sess, filename, code) {
                     Ok(p) => p,
                     Err(errs) => {
-                        drop(errs);
+                        errs.into_iter().for_each(|err| err.cancel());
                         return (false, test_attr_spans);
                     },
                 };

src/tools/rustfmt/src/parse/parser.rs

@@ -3,7 +3,7 @@ use std::path::{Path, PathBuf};
 
 use rustc_ast::token::TokenKind;
 use rustc_ast::{ast, attr, ptr};
-use rustc_errors::Diagnostic;
+use rustc_errors::DiagnosticBuilder;
 use rustc_parse::{new_parser_from_file, parser::Parser as RawParser};
 use rustc_span::{sym, Span};
 use thin_vec::ThinVec;
@@ -65,7 +65,7 @@ impl<'a> ParserBuilder<'a> {
     fn parser(
         sess: &'a rustc_session::parse::ParseSess,
         input: Input,
-    ) -> Result<rustc_parse::parser::Parser<'a>, Option<Vec<Diagnostic>>> {
+    ) -> Result<rustc_parse::parser::Parser<'a>, Option<Vec<DiagnosticBuilder<'a>>>> {
         match input {
             Input::File(ref file) => catch_unwind(AssertUnwindSafe(move || {
                 new_parser_from_file(sess, file, None)

src/tools/rustfmt/src/parse/session.rs

@@ -4,7 +4,9 @@ use std::sync::atomic::{AtomicBool, Ordering};
 use rustc_data_structures::sync::{IntoDynSyncSend, Lrc};
 use rustc_errors::emitter::{DynEmitter, Emitter, HumanEmitter};
 use rustc_errors::translation::Translate;
-use rustc_errors::{ColorConfig, DiagCtxt, Diagnostic, Level as DiagnosticLevel};
+use rustc_errors::{
+    ColorConfig, DiagCtxt, Diagnostic, DiagnosticBuilder, Level as DiagnosticLevel,
+};
 use rustc_session::parse::ParseSess as RawParseSess;
 use rustc_span::{
     source_map::{FilePathMapping, SourceMap},
@@ -283,9 +285,9 @@ impl ParseSess {
 
 // Methods that should be restricted within the parse module.
 impl ParseSess {
-    pub(super) fn emit_diagnostics(&self, diagnostics: Vec<Diagnostic>) {
+    pub(super) fn emit_diagnostics(&self, diagnostics: Vec<DiagnosticBuilder<'_>>) {
         for diagnostic in diagnostics {
-            self.parse_sess.dcx.emit_diagnostic(diagnostic);
+            diagnostic.emit();
         }
     }