warning: Quasi-quoting might make incremental compilation very inefficient: NtIdent(..)

@michaelwoerister

I have a macro like this:

#[macro_export]
macro_rules! define_proc_macros {
    (
        $(
            $( #[$attr:meta] )*
            pub fn $proc_macro_name: ident ($input: ident : &str) -> String
            $body: block
        )+
    ) => {
        $(
            $( #[$attr] )*
            #[proc_macro_derive($proc_macro_name)]
            pub fn $proc_macro_name(derive_input: ::proc_macro::TokenStream)
                                    -> ::proc_macro::TokenStream {
                let $input = derive_input.to_string();
                let $input = $crate::_extract_input(&$input);
                $body.parse().unwrap()
            }
        )+
    }
}

pub fn _extract_input(derive_input: &str) -> &str {
    // …
}

When I use it, I get the warning in the title twice for every generated functions. When I move things into one crate to avoid "note: this error originates in a macro outside of the current crate", the span of the warning is on $proc_macro_name in #[proc_macro_derive($proc_macro_name)].

Steps to reproduce: runCARGO_INCREMENTAL=1 cargo +nightly build -p cssparser-macros in https://github.com/servo/rust-cssparser/.

I don’t understand this warning and what I can do about it. What does quasi-quoting mean in this context? How bad is the inefficiency really? Is it possible to fix it while keeping this macro’s flexibility?

CC @michaelwoerister and @nikomatsakis for #37787 that added this warning.

@jseyfried

That's strange. @jseyfried, do you see anything in the macro above that in a Token::Interpolated?
This is where the warning is generated by the way:

rust/src/librustc_incremental/calculate_svh/svh_visitor.rs

Line 1068 in 5309a3e

token::Token::Interpolated(ref non_terminal) => {

@jseyfried

ping @jseyfried: Do you know what could be going on here? Where does the macro definition above introduce a Token::Interpolated?

The $proc_macro_name turns into a Nonterminal::Ident token. Normally, nonterminals are parsed into AST and don't cause warnings. However, now that #40346 has landed, attributes can have arbitrary tokens and the Nonterminal::Ident token ends up in the final AST/HIR.

I believe the solution here is to expand interpolated/nonterminal tokens into their underlying terminal tokens as we lower to HIR.

@jseyfried

@jseyfried If I understand correctly, something is inefficient but what needs to change to fix it is in rustc rather than in my code?

@SimonSapin

@SimonSapin That is correct. It's an inefficiency in rustc's handling of some parts of the HIR/AST.

@jseyfried

I believe the solution here is to expand interpolated/nonterminal tokens into their underlying terminal tokens as we lower to HIR.

@jseyfried, that sounds like a great solution. Would that be much work?

@michaelwoerister

@michaelwoerister
Once #40939 lands it should be simple.

@dtolnay

I'm getting this warning too when using proc-macro-hack (cc @dtolnay). Is there any way to suppress it?

warning: Quasi-quoting might make incremental compilation very inefficient: NtIdent(..)
  --> derive/src/lib.rs:9:1
   |
9  |   proc_macro_expr_impl! {
   |  _^ starting here...
10 | |     pub fn args_impl(input: &str) -> String {
11 | |         args::Args::new(input)
12 | |             .process().tokens
13 | |             .to_string()
14 | |     }
15 | | }
   | |_^ ...ending here
   |
   = note: this error originates in a macro outside of the current crate

If it will take a while to fix this, it would be nice to at least be able to suppress the warning.

@jseyfried

@jseyfried Is there an ETA for #40939?

@jseyfried

Now that #40939 has landed, will you look into a fix, @jseyfried?

@michaelwoerister

@michaelwoerister Sure, I still think #40946 (comment) is the best fix; I'll implement this week.

@jseyfried

Thanks a lot, @jseyfried ❤️

Fix "Quasi-quoting is inefficient" warning in incremental rustbuild.

Rollup merge of rust-lang#43292 - kennytm:fix-quasi-quoting-warning-i…

@jseyfried

@jseyfried any updates on status of this?

@jseyfried

I've made a PR for this at #44601, although @jseyfried if that's not at all what you were thinking, just let me know!

rustc: Forbid interpolated tokens in the HIR

Auto merge of #44601 - alexcrichton:lower-attributes-in-hir, r=nrc

Navigation Menu

Repository navigation

warning: Quasi-quoting might make incremental compilation very inefficient: NtIdent(..) #40946

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

warning: Quasi-quoting might make incremental compilation very inefficient: NtIdent(..) #40946

Description

Activity

michaelwoerister commented on Apr 3, 2017

michaelwoerister commented on Apr 10, 2017

jseyfried commented on Apr 24, 2017

SimonSapin commented on Apr 24, 2017

michaelwoerister commented on Apr 24, 2017

michaelwoerister commented on Apr 24, 2017

jseyfried commented on Apr 24, 2017

rushmorem commented on Apr 26, 2017

seanmonstar commented on Jun 7, 2017

michaelwoerister commented on Jun 8, 2017

michaelwoerister commented on Jul 7, 2017

jseyfried commented on Jul 10, 2017

michaelwoerister commented on Jul 11, 2017

nikomatsakis commented on Aug 16, 2017

alexcrichton commented on Sep 15, 2017

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions