Implement borrowck for &pin mut|const $place

[frank-king]

Part of Pin Ergonomics.

This forbids places to be moved or mutably borrowed ever since they are pinned until they are reassigned (even after the pinned borrows themselves expire).

#![feature(pin_ergnonmics)]
#![allow(incomplete_features)]

#[pin_v2]
struct Foo;

fn foo(mut x: Foo) {
    {
        let _x = &pin mut x; // x is pinned
    } // now the pinned reference expires
    let _x = &mut x; //~ ERROR cannot borrow `x` as mutable because it is pinned
    let _x = x; //~ ERROR cannot move out of `x` because it is pinned
    
    x = Foo; // x is reassigned
    let _x = &mut x; // ok because x is reassigned and not pinned yet
}

Tasks:

• Add Pins dataflow analysis
• Improve the diagnostics of pinning errors
• Test pin pattern matching
• Forbid &pin borrows for ADTs without #[pin_v2] (for soundness concerns)

[rust-log-analyzer]

The job aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

Executing "/scripts/stage_2_test_set1.sh"
+ /scripts/stage_2_test_set1.sh
PR_CI_JOB set; skipping tidy
+ '[' 1 == 1 ']'
+ echo 'PR_CI_JOB set; skipping tidy'
+ SKIP_TIDY='--skip tidy'
+ ../x.py --stage 2 test --skip tidy --skip compiler --skip src
##[group]Building bootstrap
    Finished `dev` profile [unoptimized] target(s) in 0.04s
##[endgroup]
downloading https://static.rust-lang.org/dist/2026-03-05/rustfmt-nightly-aarch64-unknown-linux-gnu.tar.xz
---
---- [ui] tests/ui/pin-ergonomics/pin-coercion.rs stdout ----

error: test compilation failed although it shouldn't!
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/pin-ergonomics/pin-coercion.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "-O" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "-C" "prefer-dynamic" "-o" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/pin-ergonomics/pin-coercion/a" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers" "--edition=2024"
stdout: none
--- stderr -------------------------------
error: cannot borrow `x` as mutable because it is pinned
##[error]  --> /checkout/tests/ui/pin-ergonomics/pin-coercion.rs:32:43
   |
LL |     x.get_mut().get_mut().push_str("&mut T -> &pin mut T\n");
   |     - pin of `*x.pointer` occurs here
LL |     let x_ref: &_ = coerce_pin_mut_to_ref(x.as_mut());
   |                                           ^ borrow of `x` as mutable occurs here

error: aborting due to 1 previous error
------------------------------------------

---- [ui] tests/ui/pin-ergonomics/pin-coercion.rs stdout end ----