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Fix proc macro panicked with Invalid type in const_random! in R…
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…ust 1.77.

With Rust nightly 1.77, any invocation of `const_random!` with a `u8`
array causes a compile-time panic. This can be seen when running `cargo
test`:
```
error: proc macro panicked
  --> tests/tests.rs:52:28
   |
52 |     const VALUE1: &[u8] = &const_random!([u8; 30]);
   |                            ^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: message: Invalid type
```
This is because the proc macro starts by calling `to_string` on the
input token stream, and then uses substring matching to "parse" it. In
Rust 1.77 the `Display` impl for `TokenStream` has changed, and what
used to be converted to the string `"[u8 ; 30]"` is now `"[u8; 30]"`. As
a result, the `byte_array.starts_with("[u8 ; ")` call fails.

Note that substring matching is inherently flawed because the whitespace
in the output of `to_string` is not guaranteed.

This commit rewrites the proc macro to be robust in the face of
`to_string` whitespace changes, by iterating over the individual
`TokenTrees`s.

The commit also adds a comment explaining why `usize` and `isize` are
handled differently, because it's subtle.

Note: I ran `cargo fmt` within `macro/` to format the changes to
`macro/src/lib.rs` and it made some minor changes to `macro/src/span.rs`
as well.
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@nnethercote
nnethercote committed Jan 26, 2024
1 parent e9f560b commit 025c9c5
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Showing 2 changed files with 86 additions and 44 deletions.
123 changes: 83 additions & 40 deletions macro/src/lib.rs
View file
Original file line number Diff line number Diff line change
@@ -1,11 +1,9 @@
#[allow(unused_extern_crates)]
extern crate proc_macro;

use proc_macro::*;
use std::iter::once;
mod span;
use crate::span::{gen_random_bytes, gen_random};

use crate::span::{gen_random, gen_random_bytes};

/// Create a TokenStream of an identifier out of a string
fn ident(ident: &str) -> TokenStream {
Expand All @@ -14,44 +12,89 @@ fn ident(ident: &str) -> TokenStream {

#[proc_macro]
pub fn const_random(input: TokenStream) -> TokenStream {
match &input.to_string()[..] {
"u8" => TokenTree::from(Literal::u8_suffixed(gen_random())).into(),
"u16" => TokenTree::from(Literal::u16_suffixed(gen_random())).into(),
"u32" => TokenTree::from(Literal::u32_suffixed(gen_random())).into(),
"u64" => TokenTree::from(Literal::u64_suffixed(gen_random())).into(),
"u128" => TokenTree::from(Literal::u128_suffixed(gen_random())).into(),
"i8" => TokenTree::from(Literal::i8_suffixed(gen_random())).into(),
"i16" => TokenTree::from(Literal::i16_suffixed(gen_random())).into(),
"i32" => TokenTree::from(Literal::i32_suffixed(gen_random())).into(),
"i64" => TokenTree::from(Literal::i64_suffixed(gen_random())).into(),
"i128" => TokenTree::from(Literal::i128_suffixed(gen_random())).into(),
"usize" => {
let value: TokenStream = TokenTree::from(Literal::u128_suffixed(gen_random())).into();
let type_cast: TokenStream = [value, ident("as"), ident("usize")]
.iter()
.cloned()
.collect();
TokenTree::from(Group::new(Delimiter::Parenthesis, type_cast)).into()
let mut iter = input.into_iter();
let Some(tt) = iter.next() else {
panic!("missing type arg");
};

let result = match &tt {
TokenTree::Ident(id) => {
let s = id.to_string();
match s.as_str() {
"u8" => TokenTree::from(Literal::u8_suffixed(gen_random())).into(),
"u16" => TokenTree::from(Literal::u16_suffixed(gen_random())).into(),
"u32" => TokenTree::from(Literal::u32_suffixed(gen_random())).into(),
"u64" => TokenTree::from(Literal::u64_suffixed(gen_random())).into(),
"u128" => TokenTree::from(Literal::u128_suffixed(gen_random())).into(),
"i8" => TokenTree::from(Literal::i8_suffixed(gen_random())).into(),
"i16" => TokenTree::from(Literal::i16_suffixed(gen_random())).into(),
"i32" => TokenTree::from(Literal::i32_suffixed(gen_random())).into(),
"i64" => TokenTree::from(Literal::i64_suffixed(gen_random())).into(),
"i128" => TokenTree::from(Literal::i128_suffixed(gen_random())).into(),
"usize" => {
// Note: usize does not implement `Random` and follow the pattern above. If it
// did, when cross-compiling from a 32-bit host to a 64-bit target,
// `usize::random()` would produce a 32-bit random usize which would then be
// turned into a suffixed literal (e.g. `0x1234_5678usize`). On the 64-bit
// target that literal would always have the upper 32 bits as zero, which would
// be bad. Instead we produce code that will generate a 128-bit integer literal
// (on the target) and then truncate it to usize (on the host).
let value: TokenStream =
TokenTree::from(Literal::u128_suffixed(gen_random())).into();
let type_cast: TokenStream = [value, ident("as"), ident("usize")]
.iter()
.cloned()
.collect();
TokenTree::from(Group::new(Delimiter::Parenthesis, type_cast)).into()
}
"isize" => {
// The same reasoning as `usize` applies for `isize`.
let value: TokenStream =
TokenTree::from(Literal::i128_suffixed(gen_random())).into();
let type_cast: TokenStream = [value, ident("as"), ident("isize")]
.iter()
.cloned()
.collect();
TokenTree::from(Group::new(Delimiter::Parenthesis, type_cast)).into()
}
_ => panic!("invalid integer type arg: `{}`", s),
}
}
"isize" => {
let value: TokenStream = TokenTree::from(Literal::i128_suffixed(gen_random())).into();
let type_cast: TokenStream = [value, ident("as"), ident("isize")]
.iter()
.cloned()
.collect();
TokenTree::from(Group::new(Delimiter::Parenthesis, type_cast)).into()
TokenTree::Group(group) if group.delimiter() == Delimiter::Bracket => {
let mut iter = group.stream().into_iter();
match (&iter.next(), &iter.next(), &iter.next(), &iter.next()) {
(
Some(TokenTree::Ident(ident)),
Some(TokenTree::Punct(punct)),
Some(TokenTree::Literal(literal)),
None,
) if ident.to_string().as_str() == "u8" && punct.as_char() == ';' => {
let Ok(len) = literal.to_string().parse() else {
panic!("invalid array length: `{}`", literal);
};
let mut random_bytes = vec![0; len];
gen_random_bytes(&mut random_bytes);
let array_parts: TokenStream = random_bytes
.into_iter()
.flat_map(|byte| {
let val = TokenTree::from(Literal::u8_suffixed(byte));
let comma = TokenTree::from(Punct::new(',', Spacing::Alone));
once(val).chain(once(comma))
})
.collect();
TokenTree::from(Group::new(Delimiter::Bracket, array_parts)).into()
}
_ => panic!("invalid array type arg: `{}`", tt),
}
}
byte_array if byte_array.starts_with("[u8 ; ") && byte_array.ends_with(']')=> {
let len = byte_array[6..byte_array.len()-1].parse().unwrap();
let mut random_bytes = vec![0; len];
gen_random_bytes(&mut random_bytes);
let array_parts: TokenStream = random_bytes.into_iter().flat_map(|byte| {
let val = TokenTree::from(Literal::u8_suffixed(byte));
let comma = TokenTree::from(Punct::new(',', Spacing::Alone));
once(val).chain(once(comma))
}).collect();
TokenTree::from(Group::new(Delimiter::Bracket, array_parts)).into()
_ => {
panic!("invalid type arg: `{}`", tt);
}
_ => panic!("Invalid type"),
}
};

if let Some(tt) = iter.next() {
panic!("invalid trailing token tree: `{}`", tt);
};

result
}
7 changes: 3 additions & 4 deletions macro/src/span.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -2,16 +2,15 @@ use proc_macro::Span;
use std::option_env;

use once_cell::race::OnceBox;
use tiny_keccak::{Xof, Hasher, Shake};

use tiny_keccak::{Hasher, Shake, Xof};

static SEED: OnceBox<Vec<u8>> = OnceBox::new();

fn get_seed() -> &'static [u8] {
&SEED.get_or_init(|| {
if let Some(value) = option_env!("CONST_RANDOM_SEED") {
Box::new(value.as_bytes().to_vec())
} else {
Box::new(value.as_bytes().to_vec())
} else {
let mut value = [0u8; 32];
getrandom::getrandom(&mut value).unwrap();
Box::new(value.to_vec())
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