v[v.len()-1] not accepted by borrow checker

[Danvil]

I tried this code:

let mut v = vec![1,2,3];
v[v.len()-1] = 5;

and expected it to compile, but the borrow checker does not accept it.

This looks like a bug, because the following similar code works thanks to two-phase borrow:

use std::ops::IndexMut;
let mut v = vec![1,2,3];
*v.index_mut(v.len() - 1) = 5;

Another case which works fine:

let mut v = vec![1,2,3];
v.push(v.len());

[guchengxi1994]

let mut v = vec![1,2,3];
let v_len = v.len() -1;
v[v_len] = 5;
println!("{:?}",v)

this works fine.

[pnkfelix]

This is a duplicate of #74319 #58419.

We deliberately made a (conservative) choice to not apply two-phase borrow in the v[...] = ... scenario today:

rust/compiler/rustc_typeck/src/check/place_op.rs

Lines 376 to 380 in 96859db

	// Deref/indexing can be desugared to a method call,
	// so maybe we could use two-phase here.
	// See the documentation of AllowTwoPhase for why that's
	// not the case today.
	allow_two_phase_borrow: AllowTwoPhase::No,

as documented here:

rust/compiler/rustc_middle/src/ty/adjustment.rs

Lines 135 to 151 in 96859db

	/// At least for initial deployment, we want to limit two-phase borrows to
	/// only a few specific cases. Right now, those are mostly "things that desugar"
	/// into method calls:
	/// - using `x.some_method()` syntax, where some_method takes `&mut self`,
	/// - using `Foo::some_method(&mut x, ...)` syntax,
	/// - binary assignment operators (`+=`, `-=`, `*=`, etc.).
	/// Anything else should be rejected until generalized two-phase borrow support
	/// is implemented. Right now, dataflow can't handle the general case where there
	/// is more than one use of a mutable borrow, and we don't want to accept too much
	/// new code via two-phase borrows, so we try to limit where we create two-phase
	/// capable mutable borrows.
	/// See #49434 for tracking.
	#[derive(Copy, Clone, PartialEq, Debug, TyEncodable, TyDecodable, HashStable)]
	pub enum AllowTwoPhase {
	Yes,
	No,
	}

Closing as duplicate of #74319 #58419 (I'll post some more thoughts there though while this is on my mind).