Remove extern crate rustc_macros from numerous crates.

compiler/rustc_errors/src/diagnostic.rs

@@ -7,6 +7,7 @@ use rustc_data_structures::fx::FxIndexMap;
 use rustc_error_messages::fluent_value_from_str_list_sep_by_and;
 use rustc_error_messages::FluentValue;
 use rustc_lint_defs::{Applicability, LintExpectationId};
+use rustc_macros::{Decodable, Encodable};
 use rustc_span::source_map::Spanned;
 use rustc_span::symbol::Symbol;
 use rustc_span::{Span, DUMMY_SP};

compiler/rustc_errors/src/diagnostic_impls.rs

@@ -7,6 +7,7 @@ use crate::{
 use rustc_ast as ast;
 use rustc_ast_pretty::pprust;
 use rustc_hir as hir;
+use rustc_macros::Subdiagnostic;
 use rustc_span::edition::Edition;
 use rustc_span::symbol::{Ident, MacroRulesNormalizedIdent, Symbol};
 use rustc_span::Span;

compiler/rustc_errors/src/lib.rs

@@ -25,9 +25,6 @@
 #![feature(yeet_expr)]
 // tidy-alphabetical-end
 
-#[macro_use]
-extern crate rustc_macros;
-
 #[macro_use]
 extern crate tracing;
 
@@ -64,6 +61,7 @@ use rustc_data_structures::stable_hasher::{Hash128, StableHasher};
 use rustc_data_structures::sync::{Lock, Lrc};
 use rustc_data_structures::AtomicRef;
 use rustc_lint_defs::LintExpectationId;
+use rustc_macros::{Decodable, Encodable};
 use rustc_span::source_map::SourceMap;
 use rustc_span::{Loc, Span, DUMMY_SP};
 use std::backtrace::{Backtrace, BacktraceStatus};

compiler/rustc_errors/src/snippet.rs

@@ -1,6 +1,7 @@
 // Code for annotating snippets.
 
 use crate::{Level, Loc};
+use rustc_macros::{Decodable, Encodable};
 
 #[derive(Clone, Debug, PartialOrd, Ord, PartialEq, Eq)]
 pub struct Line {