Incremental compilation does not account for changes to -Zpolonius

[shepmaster]

I tried this code:

/// Insert a new data element at a given key.
pub fn insert<'a, K: Eq, V>(this: &'a mut Vec<(K, V)>, key: K, val: V) -> &'a mut V {
    for (key1, val1) in &mut *this {
        if key == *key1 {
            return val1;
        }
    }
    let this = &mut *this;
    this.push((key, val));
    &mut this.last_mut().unwrap().1
}

fn main() {
    let mut things = vec![(1, 2), (3, 4)];

    let v = insert(&mut things, 1, 2);
    println!("{} ({:p})", v, v);

    let v = insert(&mut things, 1, 2);
    println!("{} ({:p})", v, v);

    let v = insert(&mut things, 5, 6);
    println!("{} ({:p})", v, v);

    let v = insert(&mut things, 5, 6);
    println!("{} ({:p})", v, v);
}

I expected this to fail when not compiled with -Zpolonius, but it does compile if we previously built with -Zpolonius:

% RUSTFLAGS='' cargo +nightly build
   Compiling p v0.1.0 (/private/tmp/p)
error[E0499]: cannot borrow `*this` as mutable more than once at a time
 --> src/main.rs:8:16
  |
2 | pub fn insert<'a, K: Eq, V>(this: &'a mut Vec<(K, V)>, key: K, val: V) -> &'a mut V {
  |               -- lifetime `'a` defined here
3 |     for (key1, val1) in &mut *this {
  |                         ---------- first mutable borrow occurs here
4 |         if key == *key1 {
5 |             return val1;
  |                    ---- returning this value requires that `*this` is borrowed for `'a`
...
8 |     let this = &mut *this;
  |                ^^^^^^^^^^ second mutable borrow occurs here

% RUSTFLAGS='-Zpolonius' cargo +nightly build
   Compiling p v0.1.0 (/private/tmp/p)
    Finished dev [unoptimized + debuginfo] target(s) in 0.37s

% RUSTFLAGS='' cargo +nightly build
   Compiling p v0.1.0 (/private/tmp/p)
    Finished dev [unoptimized + debuginfo] target(s) in 0.25s

If I disable incremental compilation, this does not occur:

[profile.dev]
incremental = false

% RUSTFLAGS='' cargo +nightly build
   Compiling p v0.1.0 (/private/tmp/p)
error[E0499]: cannot borrow `*this` as mutable more than once at a time
 --> src/main.rs:8:16
  |
2 | pub fn insert<'a, K: Eq, V>(this: &'a mut Vec<(K, V)>, key: K, val: V) -> &'a mut V {
  |               -- lifetime `'a` defined here
3 |     for (key1, val1) in &mut *this {
  |                         ---------- first mutable borrow occurs here
4 |         if key == *key1 {
5 |             return val1;
  |                    ---- returning this value requires that `*this` is borrowed for `'a`
...
8 |     let this = &mut *this;
  |                ^^^^^^^^^^ second mutable borrow occurs here

% RUSTFLAGS='-Zpolonius' cargo +nightly build
   Compiling p v0.1.0 (/private/tmp/p)
    Finished dev [unoptimized + debuginfo] target(s) in 0.32s

% RUSTFLAGS='' cargo +nightly build
   Compiling p v0.1.0 (/private/tmp/p)
error[E0499]: cannot borrow `*this` as mutable more than once at a time
 --> src/main.rs:8:16
  |
2 | pub fn insert<'a, K: Eq, V>(this: &'a mut Vec<(K, V)>, key: K, val: V) -> &'a mut V {
  |               -- lifetime `'a` defined here
3 |     for (key1, val1) in &mut *this {
  |                         ---------- first mutable borrow occurs here
4 |         if key == *key1 {
5 |             return val1;
  |                    ---- returning this value requires that `*this` is borrowed for `'a`
...
8 |     let this = &mut *this;
  |                ^^^^^^^^^^ second mutable borrow occurs here

Meta

% rustc +nightly --version --verbose
rustc 1.50.0-nightly (da3846948 2020-11-21)
binary: rustc
commit-hash: da384694807172f0ca40eca2e49a11688aba6e93
commit-date: 2020-11-21
host: x86_64-apple-darwin
release: 1.50.0-nightly

[jonas-schievink]

This needs to be changed to TRACKED:

rust/compiler/rustc_session/src/options.rs

Lines 1011 to 1012 in 52e3cf1

	polonius: bool = (false, parse_bool, [UNTRACKED],
	"enable polonius-based borrow-checker (default: no)"),