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extern crate proc_macro;
use self::proc_macro::TokenStream;
use quote::{quote, ToTokens};
use std::collections::HashSet as Set;
use syn::fold::{self, Fold};
use syn::parse::{Parse, ParseStream, Result};
use syn::punctuated::Punctuated;
use syn::{parse_macro_input, parse_quote, Expr, Ident, ItemFn, Local, Pat, Stmt, Token};
/// Parses a list of variable names separated by commas.
///
/// a, b, c
///
/// This is how the compiler passes in arguments to our attribute -- it is
/// everything inside the delimiters after the attribute name.
///
/// #[trace_var(a, b, c)]
/// ^^^^^^^
struct Args {
vars: Set<Ident>,
}
impl Parse for Args {
fn parse(input: ParseStream) -> Result<Self> {
let vars = Punctuated::<Ident, Token![,]>::parse_terminated(input)?;
Ok(Args {
vars: vars.into_iter().collect(),
})
}
}
impl Args {
/// Determines whether the given `Expr` is a path referring to one of the
/// variables we intend to print. Expressions are used as the left-hand side
/// of the assignment operator.
fn should_print_expr(&self, e: &Expr) -> bool {
match *e {
Expr::Path(ref e) => {
if e.path.leading_colon.is_some() {
false
} else if e.path.segments.len() != 1 {
false
} else {
let first = e.path.segments.first().unwrap();
self.vars.contains(&first.ident) && first.arguments.is_empty()
}
}
_ => false,
}
}
/// Determines whether the given `Pat` is an identifier equal to one of the
/// variables we intend to print. Patterns are used as the left-hand side of
/// a `let` binding.
fn should_print_pat(&self, p: &Pat) -> bool {
match p {
Pat::Ident(ref p) => self.vars.contains(&p.ident),
_ => false,
}
}
/// Produces an expression that assigns the right-hand side to the left-hand
/// side and then prints the value.
///
/// // Before
/// VAR = INIT
///
/// // After
/// { VAR = INIT; println!("VAR = {:?}", VAR); }
fn assign_and_print(&mut self, left: Expr, op: &dyn ToTokens, right: Expr) -> Expr {
let right = fold::fold_expr(self, right);
parse_quote!({
#left #op #right;
println!(concat!(stringify!(#left), " = {:?}"), #left);
})
}
/// Produces a let-binding that assigns the right-hand side to the left-hand
/// side and then prints the value.
///
/// // Before
/// let VAR = INIT;
///
/// // After
/// let VAR = { let VAR = INIT; println!("VAR = {:?}", VAR); VAR };
fn let_and_print(&mut self, local: Local) -> Stmt {
let Local { pat, init, .. } = local;
let init = self.fold_expr(*init.unwrap().1);
let ident = match pat {
Pat::Ident(ref p) => &p.ident,
_ => unreachable!(),
};
parse_quote! {
let #pat = {
#[allow(unused_mut)]
let #pat = #init;
println!(concat!(stringify!(#ident), " = {:?}"), #ident);
#ident
};
}
}
}
/// The `Fold` trait is a way to traverse an owned syntax tree and replace some
/// of its nodes.
///
/// Syn provides two other syntax tree traversal traits: `Visit` which walks a
/// shared borrow of a syntax tree, and `VisitMut` which walks an exclusive
/// borrow of a syntax tree and can mutate it in place.
///
/// All three traits have a method corresponding to each type of node in Syn's
/// syntax tree. All of these methods have default no-op implementations that
/// simply recurse on any child nodes. We can override only those methods for
/// which we want non-default behavior. In this case the traversal needs to
/// transform `Expr` and `Stmt` nodes.
impl Fold for Args {
fn fold_expr(&mut self, e: Expr) -> Expr {
match e {
Expr::Assign(e) => {
if self.should_print_expr(&e.left) {
self.assign_and_print(*e.left, &e.eq_token, *e.right)
} else {
Expr::Assign(fold::fold_expr_assign(self, e))
}
}
Expr::AssignOp(e) => {
if self.should_print_expr(&e.left) {
self.assign_and_print(*e.left, &e.op, *e.right)
} else {
Expr::AssignOp(fold::fold_expr_assign_op(self, e))
}
}
_ => fold::fold_expr(self, e),
}
}
fn fold_stmt(&mut self, s: Stmt) -> Stmt {
match s {
Stmt::Local(s) => {
if s.init.is_some() && self.should_print_pat(&s.pat) {
self.let_and_print(s)
} else {
Stmt::Local(fold::fold_local(self, s))
}
}
_ => fold::fold_stmt(self, s),
}
}
}
/// Attribute to print the value of the given variables each time they are
/// reassigned.
///
/// # Example
///
/// ```
/// #[trace_var(p, n)]
/// fn factorial(mut n: u64) -> u64 {
/// let mut p = 1;
/// while n > 1 {
/// p *= n;
/// n -= 1;
/// }
/// p
/// }
/// ```
#[proc_macro_attribute]
pub fn trace_var(args: TokenStream, input: TokenStream) -> TokenStream {
let input = parse_macro_input!(input as ItemFn);
// Parse the list of variables the user wanted to print.
let mut args = parse_macro_input!(args as Args);
// Use a syntax tree traversal to transform the function body.
let output = args.fold_item_fn(input);
// Hand the resulting function body back to the compiler.
TokenStream::from(quote!(#output))
}
|
