Replace AstBuilder with inherent methods

Author: Mark-Simulacrum

src/doc/unstable-book/src/language-features/plugin.md

@@ -59,7 +59,6 @@ extern crate rustc_plugin;
 use syntax::parse::token::{self, Token};
 use syntax::tokenstream::TokenTree;
 use syntax::ext::base::{ExtCtxt, MacResult, DummyResult, MacEager};
-use syntax::ext::build::AstBuilder;  // A trait for expr_usize.
 use syntax_pos::Span;
 use rustc_plugin::Registry;
 
@@ -164,13 +163,6 @@ can continue and find further errors.
 To print syntax fragments for debugging, you can use `span_note` together with
 `syntax::print::pprust::*_to_string`.
 
-The example above produced an integer literal using `AstBuilder::expr_usize`.
-As an alternative to the `AstBuilder` trait, `libsyntax` provides a set of
-quasiquote macros. They are undocumented and very rough around the edges.
-However, the implementation may be a good starting point for an improved
-quasiquote as an ordinary plugin library.
-
-
 # Lint plugins
 
 Plugins can extend [Rust's lint

src/libsyntax/attr/builtin.rs

@@ -3,7 +3,6 @@
 use crate::ast::{self, Attribute, MetaItem, NestedMetaItem};
 use crate::early_buffered_lints::BufferedEarlyLintId;
 use crate::ext::base::ExtCtxt;
-use crate::ext::build::AstBuilder;
 use crate::feature_gate::{Features, GatedCfg};
 use crate::parse::ParseSess;
 

src/libsyntax/diagnostics/plugin.rs

@@ -4,7 +4,6 @@ use std::env;
 use crate::ast::{self, Ident, Name};
 use crate::source_map;
 use crate::ext::base::{ExtCtxt, MacEager, MacResult};
-use crate::ext::build::AstBuilder;
 use crate::parse::token::{self, Token};
 use crate::ptr::P;
 use crate::symbol::kw;