Auto merge of #53258 - nikomatsakis:issue-53189-optimize-reassignment-immutable-state, r=pnkfelix

optimize reassignment immutable state

This is the "simple fix" when it comes to checking for reassignment. We just shoot for compatibility with the AST-based checker. Makes no attempt to solve #21232.

I opted for this simpler fix because I didn't want to think about complications [like the ones described here](https://github.com/rust-lang/rust/issues/21232#issuecomment-412219247).

Let's do some profiling measurements.

Fixes #53189

r? @pnkfelix

src/test/ui/borrowck/assign_mutable_fields.nll.stderr

@@ -0,0 +1,9 @@
+error[E0381]: use of possibly uninitialized variable: `x`
+  --> $DIR/assign_mutable_fields.rs:29:10
+   |
+LL |     drop(x); //~ ERROR
+   |          ^ use of possibly uninitialized `x`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0381`.

src/test/ui/borrowck/assign_mutable_fields.rs

@@ -0,0 +1,32 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Currently, we permit you to assign to individual fields of a mut
+// var, but we do not permit you to use the complete var afterwards.
+// We hope to fix this at some point.
+//
+// FIXME(#21232)
+
+fn assign_both_fields_and_use() {
+    let mut x: (u32, u32);
+    x.0 = 1;
+    x.1 = 22;
+    drop(x.0); //~ ERROR
+    drop(x.1); //~ ERROR
+}
+
+fn assign_both_fields_the_use_var() {
+    let mut x: (u32, u32);
+    x.0 = 1;
+    x.1 = 22;
+    drop(x); //~ ERROR
+}
+
+fn main() { }

src/test/ui/borrowck/assign_mutable_fields.stderr

@@ -0,0 +1,21 @@
+error[E0381]: use of possibly uninitialized variable: `x.0`
+  --> $DIR/assign_mutable_fields.rs:21:10
+   |
+LL |     drop(x.0); //~ ERROR
+   |          ^^^ use of possibly uninitialized `x.0`
+
+error[E0381]: use of possibly uninitialized variable: `x.1`
+  --> $DIR/assign_mutable_fields.rs:22:10
+   |
+LL |     drop(x.1); //~ ERROR
+   |          ^^^ use of possibly uninitialized `x.1`
+
+error[E0381]: use of possibly uninitialized variable: `x`
+  --> $DIR/assign_mutable_fields.rs:29:10
+   |
+LL |     drop(x); //~ ERROR
+   |          ^ use of possibly uninitialized `x`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0381`.

src/test/ui/borrowck/reassignment_immutable_fields.nll.stderr

@@ -0,0 +1,44 @@
+error[E0594]: cannot assign to `x.0`, as `x` is not declared as mutable
+  --> $DIR/reassignment_immutable_fields.rs:17:5
+   |
+LL |     let x: (u32, u32);
+   |         - help: consider changing this to be mutable: `mut x`
+LL |     x.0 = 1; //~ ERROR
+   |     ^^^^^^^ cannot assign
+
+error[E0594]: cannot assign to `x.1`, as `x` is not declared as mutable
+  --> $DIR/reassignment_immutable_fields.rs:18:5
+   |
+LL |     let x: (u32, u32);
+   |         - help: consider changing this to be mutable: `mut x`
+LL |     x.0 = 1; //~ ERROR
+LL |     x.1 = 22; //~ ERROR
+   |     ^^^^^^^^ cannot assign
+
+error[E0594]: cannot assign to `x.0`, as `x` is not declared as mutable
+  --> $DIR/reassignment_immutable_fields.rs:25:5
+   |
+LL |     let x: (u32, u32);
+   |         - help: consider changing this to be mutable: `mut x`
+LL |     x.0 = 1; //~ ERROR
+   |     ^^^^^^^ cannot assign
+
+error[E0594]: cannot assign to `x.1`, as `x` is not declared as mutable
+  --> $DIR/reassignment_immutable_fields.rs:26:5
+   |
+LL |     let x: (u32, u32);
+   |         - help: consider changing this to be mutable: `mut x`
+LL |     x.0 = 1; //~ ERROR
+LL |     x.1 = 22; //~ ERROR
+   |     ^^^^^^^^ cannot assign
+
+error[E0381]: use of possibly uninitialized variable: `x`
+  --> $DIR/reassignment_immutable_fields.rs:27:10
+   |
+LL |     drop(x); //~ ERROR
+   |          ^ use of possibly uninitialized `x`
+
+error: aborting due to 5 previous errors
+
+Some errors occurred: E0381, E0594.
+For more information about an error, try `rustc --explain E0381`.

src/test/ui/borrowck/reassignment_immutable_fields.rs

@@ -0,0 +1,30 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// This test is currently disallowed, but we hope someday to support it.
+//
+// FIXME(#21232)
+
+fn assign_both_fields_and_use() {
+    let x: (u32, u32);
+    x.0 = 1; //~ ERROR
+    x.1 = 22; //~ ERROR
+    drop(x.0); //~ ERROR
+    drop(x.1); //~ ERROR
+}
+
+fn assign_both_fields_the_use_var() {
+    let x: (u32, u32);
+    x.0 = 1; //~ ERROR
+    x.1 = 22; //~ ERROR
+    drop(x); //~ ERROR
+}
+
+fn main() { }

src/test/ui/borrowck/reassignment_immutable_fields.stderr

@@ -0,0 +1,56 @@
+error[E0594]: cannot assign to field `x.0` of immutable binding
+  --> $DIR/reassignment_immutable_fields.rs:17:5
+   |
+LL |     let x: (u32, u32);
+   |         - consider changing this to `mut x`
+LL |     x.0 = 1; //~ ERROR
+   |     ^^^^^^^ cannot mutably borrow field of immutable binding
+
+error[E0594]: cannot assign to field `x.1` of immutable binding
+  --> $DIR/reassignment_immutable_fields.rs:18:5
+   |
+LL |     let x: (u32, u32);
+   |         - consider changing this to `mut x`
+LL |     x.0 = 1; //~ ERROR
+LL |     x.1 = 22; //~ ERROR
+   |     ^^^^^^^^ cannot mutably borrow field of immutable binding
+
+error[E0381]: use of possibly uninitialized variable: `x.0`
+  --> $DIR/reassignment_immutable_fields.rs:19:10
+   |
+LL |     drop(x.0); //~ ERROR
+   |          ^^^ use of possibly uninitialized `x.0`
+
+error[E0381]: use of possibly uninitialized variable: `x.1`
+  --> $DIR/reassignment_immutable_fields.rs:20:10
+   |
+LL |     drop(x.1); //~ ERROR
+   |          ^^^ use of possibly uninitialized `x.1`
+
+error[E0594]: cannot assign to field `x.0` of immutable binding
+  --> $DIR/reassignment_immutable_fields.rs:25:5
+   |
+LL |     let x: (u32, u32);
+   |         - consider changing this to `mut x`
+LL |     x.0 = 1; //~ ERROR
+   |     ^^^^^^^ cannot mutably borrow field of immutable binding
+
+error[E0594]: cannot assign to field `x.1` of immutable binding
+  --> $DIR/reassignment_immutable_fields.rs:26:5
+   |
+LL |     let x: (u32, u32);
+   |         - consider changing this to `mut x`
+LL |     x.0 = 1; //~ ERROR
+LL |     x.1 = 22; //~ ERROR
+   |     ^^^^^^^^ cannot mutably borrow field of immutable binding
+
+error[E0381]: use of possibly uninitialized variable: `x`
+  --> $DIR/reassignment_immutable_fields.rs:27:10
+   |
+LL |     drop(x); //~ ERROR
+   |          ^ use of possibly uninitialized `x`
+
+error: aborting due to 7 previous errors
+
+Some errors occurred: E0381, E0594.
+For more information about an error, try `rustc --explain E0381`.

src/test/ui/borrowck/reassignment_immutable_fields_overlapping.nll.stderr

@@ -0,0 +1,20 @@
+error[E0594]: cannot assign to `x.a`, as `x` is not declared as mutable
+  --> $DIR/reassignment_immutable_fields_overlapping.rs:22:5
+   |
+LL |     let x: Foo;
+   |         - help: consider changing this to be mutable: `mut x`
+LL |     x.a = 1;  //~ ERROR
+   |     ^^^^^^^ cannot assign
+
+error[E0594]: cannot assign to `x.b`, as `x` is not declared as mutable
+  --> $DIR/reassignment_immutable_fields_overlapping.rs:23:5
+   |
+LL |     let x: Foo;
+   |         - help: consider changing this to be mutable: `mut x`
+LL |     x.a = 1;  //~ ERROR
+LL |     x.b = 22; //~ ERROR
+   |     ^^^^^^^^ cannot assign
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0594`.

src/test/ui/borrowck/reassignment_immutable_fields_overlapping.rs

@@ -0,0 +1,26 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// This should never be allowed -- `foo.a` and `foo.b` are
+// overlapping, so since `x` is not `mut` we should not permit
+// reassignment.
+
+union Foo {
+    a: u32,
+    b: u32,
+}
+
+unsafe fn overlapping_fields() {
+    let x: Foo;
+    x.a = 1;  //~ ERROR
+    x.b = 22; //~ ERROR
+}
+
+fn main() { }

src/test/ui/borrowck/reassignment_immutable_fields_overlapping.stderr

@@ -0,0 +1,20 @@
+error[E0594]: cannot assign to field `x.a` of immutable binding
+  --> $DIR/reassignment_immutable_fields_overlapping.rs:22:5
+   |
+LL |     let x: Foo;
+   |         - consider changing this to `mut x`
+LL |     x.a = 1;  //~ ERROR
+   |     ^^^^^^^ cannot mutably borrow field of immutable binding
+
+error[E0594]: cannot assign to field `x.b` of immutable binding
+  --> $DIR/reassignment_immutable_fields_overlapping.rs:23:5
+   |
+LL |     let x: Foo;
+   |         - consider changing this to `mut x`
+LL |     x.a = 1;  //~ ERROR
+LL |     x.b = 22; //~ ERROR
+   |     ^^^^^^^^ cannot mutably borrow field of immutable binding
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0594`.

src/test/ui/borrowck/reassignment_immutable_fields_twice.nll.stderr

@@ -0,0 +1,38 @@
+error[E0594]: cannot assign to `x.0`, as `x` is not declared as mutable
+  --> $DIR/reassignment_immutable_fields_twice.rs:17:5
+   |
+LL |     let x: (u32, u32);
+   |         - help: consider changing this to be mutable: `mut x`
+LL |     x = (22, 44);
+LL |     x.0 = 1; //~ ERROR
+   |     ^^^^^^^ cannot assign
+
+error[E0594]: cannot assign to `x.0`, as `x` is not declared as mutable
+  --> $DIR/reassignment_immutable_fields_twice.rs:22:5
+   |
+LL |     let x: (u32, u32);
+   |         - help: consider changing this to be mutable: `mut x`
+LL |     x.0 = 1; //~ ERROR
+   |     ^^^^^^^ cannot assign
+
+error[E0594]: cannot assign to `x.0`, as `x` is not declared as mutable
+  --> $DIR/reassignment_immutable_fields_twice.rs:23:5
+   |
+LL |     let x: (u32, u32);
+   |         - help: consider changing this to be mutable: `mut x`
+LL |     x.0 = 1; //~ ERROR
+LL |     x.0 = 22; //~ ERROR
+   |     ^^^^^^^^ cannot assign
+
+error[E0594]: cannot assign to `x.1`, as `x` is not declared as mutable
+  --> $DIR/reassignment_immutable_fields_twice.rs:24:5
+   |
+LL |     let x: (u32, u32);
+   |         - help: consider changing this to be mutable: `mut x`
+...
+LL |     x.1 = 44; //~ ERROR
+   |     ^^^^^^^^ cannot assign
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0594`.

src/test/ui/borrowck/reassignment_immutable_fields_twice.rs

@@ -0,0 +1,27 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// This should never be allowed -- since `x` is not `mut`, so `x.0`
+// cannot be assigned twice.
+
+fn var_then_field() {
+    let x: (u32, u32);
+    x = (22, 44);
+    x.0 = 1; //~ ERROR
+}
+
+fn same_field_twice() {
+    let x: (u32, u32);
+    x.0 = 1; //~ ERROR
+    x.0 = 22; //~ ERROR
+    x.1 = 44; //~ ERROR
+}
+
+fn main() { }

src/test/ui/borrowck/reassignment_immutable_fields_twice.stderr

@@ -0,0 +1,38 @@
+error[E0594]: cannot assign to field `x.0` of immutable binding
+  --> $DIR/reassignment_immutable_fields_twice.rs:17:5
+   |
+LL |     let x: (u32, u32);
+   |         - consider changing this to `mut x`
+LL |     x = (22, 44);
+LL |     x.0 = 1; //~ ERROR
+   |     ^^^^^^^ cannot mutably borrow field of immutable binding
+
+error[E0594]: cannot assign to field `x.0` of immutable binding
+  --> $DIR/reassignment_immutable_fields_twice.rs:22:5
+   |
+LL |     let x: (u32, u32);
+   |         - consider changing this to `mut x`
+LL |     x.0 = 1; //~ ERROR
+   |     ^^^^^^^ cannot mutably borrow field of immutable binding
+
+error[E0594]: cannot assign to field `x.0` of immutable binding
+  --> $DIR/reassignment_immutable_fields_twice.rs:23:5
+   |
+LL |     let x: (u32, u32);
+   |         - consider changing this to `mut x`
+LL |     x.0 = 1; //~ ERROR
+LL |     x.0 = 22; //~ ERROR
+   |     ^^^^^^^^ cannot mutably borrow field of immutable binding
+
+error[E0594]: cannot assign to field `x.1` of immutable binding
+  --> $DIR/reassignment_immutable_fields_twice.rs:24:5
+   |
+LL |     let x: (u32, u32);
+   |         - consider changing this to `mut x`
+...
+LL |     x.1 = 44; //~ ERROR
+   |     ^^^^^^^^ cannot mutably borrow field of immutable binding
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0594`.

src/test/ui/did_you_mean/issue-35937.nll.stderr

@@ -6,26 +6,23 @@ LL |     let f = Foo { v: Vec::new() };
 LL |     f.v.push("cat".to_string()); //~ ERROR cannot borrow
    |     ^^^ cannot borrow as mutable
 
-error[E0384]: cannot assign twice to immutable variable `s`
+error[E0594]: cannot assign to `s.x`, as `s` is not declared as mutable
   --> $DIR/issue-35937.rs:26:5
    |
 LL |     let s = S { x: 42 };
-   |         -
-   |         |
-   |         first assignment to `s`
-   |         consider changing this to `mut s`
+   |         - help: consider changing this to be mutable: `mut s`
 LL |     s.x += 1; //~ ERROR cannot assign
-   |     ^^^^^^^^ cannot assign twice to immutable variable
+   |     ^^^^^^^^ cannot assign
 
-error[E0384]: cannot assign to immutable argument `s`
+error[E0594]: cannot assign to `s.x`, as `s` is not declared as mutable
   --> $DIR/issue-35937.rs:30:5
    |
 LL | fn bar(s: S) {
-   |        - consider changing this to `mut s`
+   |        - help: consider changing this to be mutable: `mut s`
 LL |     s.x += 1; //~ ERROR cannot assign
-   |     ^^^^^^^^ cannot assign to immutable argument
+   |     ^^^^^^^^ cannot assign
 
 error: aborting due to 3 previous errors
 
-Some errors occurred: E0384, E0596.
-For more information about an error, try `rustc --explain E0384`.
+Some errors occurred: E0594, E0596.
+For more information about an error, try `rustc --explain E0594`.

src/test/ui/immut-function-arguments.ast.nll.stderr

@@ -1,19 +1,19 @@
-error[E0384]: cannot assign to immutable argument `y`
+error[E0594]: cannot assign to `*y`, as `y` is not declared as mutable
   --> $DIR/immut-function-arguments.rs:15:5
    |
 LL | fn f(y: Box<isize>) {
-   |      - consider changing this to `mut y`
+   |      - help: consider changing this to be mutable: `mut y`
 LL |     *y = 5; //[ast]~ ERROR cannot assign
-   |     ^^^^^^ cannot assign to immutable argument
+   |     ^^^^^^ cannot assign
 
-error[E0384]: cannot assign to immutable argument `q`
+error[E0594]: cannot assign to `*q`, as `q` is not declared as mutable
   --> $DIR/immut-function-arguments.rs:20:35
    |
 LL |     let _frob = |q: Box<isize>| { *q = 2; }; //[ast]~ ERROR cannot assign
-   |                  -                ^^^^^^ cannot assign to immutable argument
+   |                  -                ^^^^^^ cannot assign
    |                  |
-   |                  consider changing this to `mut q`
+   |                  help: consider changing this to be mutable: `mut q`
 
 error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0384`.
+For more information about this error, try `rustc --explain E0594`.

src/test/ui/immut-function-arguments.mir.stderr

@@ -1,19 +1,19 @@
-error[E0384]: cannot assign to immutable argument `y`
+error[E0594]: cannot assign to `*y`, as `y` is not declared as mutable
   --> $DIR/immut-function-arguments.rs:15:5
    |
 LL | fn f(y: Box<isize>) {
-   |      - consider changing this to `mut y`
+   |      - help: consider changing this to be mutable: `mut y`
 LL |     *y = 5; //[ast]~ ERROR cannot assign
-   |     ^^^^^^ cannot assign to immutable argument
+   |     ^^^^^^ cannot assign
 
-error[E0384]: cannot assign to immutable argument `q`
+error[E0594]: cannot assign to `*q`, as `q` is not declared as mutable
   --> $DIR/immut-function-arguments.rs:20:35
    |
 LL |     let _frob = |q: Box<isize>| { *q = 2; }; //[ast]~ ERROR cannot assign
-   |                  -                ^^^^^^ cannot assign to immutable argument
+   |                  -                ^^^^^^ cannot assign
    |                  |
-   |                  consider changing this to `mut q`
+   |                  help: consider changing this to be mutable: `mut q`
 
 error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0384`.
+For more information about this error, try `rustc --explain E0594`.

src/test/ui/mut/mutable-class-fields.ast.nll.stderr

@@ -1,14 +1,11 @@
-error[E0384]: cannot assign twice to immutable variable `nyan`
+error[E0594]: cannot assign to `nyan.how_hungry`, as `nyan` is not declared as mutable
   --> $DIR/mutable-class-fields.rs:28:3
    |
 LL |   let nyan : cat = cat(52, 99);
-   |       ----
-   |       |
-   |       first assignment to `nyan`
-   |       consider changing this to `mut nyan`
+   |       ---- help: consider changing this to be mutable: `mut nyan`
 LL |   nyan.how_hungry = 0; //[ast]~ ERROR cannot assign
-   |   ^^^^^^^^^^^^^^^^^^^ cannot assign twice to immutable variable
+   |   ^^^^^^^^^^^^^^^^^^^ cannot assign
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0384`.
+For more information about this error, try `rustc --explain E0594`.

src/test/ui/mut/mutable-class-fields.mir.stderr

@@ -1,14 +1,11 @@
-error[E0384]: cannot assign twice to immutable variable `nyan`
+error[E0594]: cannot assign to `nyan.how_hungry`, as `nyan` is not declared as mutable
   --> $DIR/mutable-class-fields.rs:28:3
    |
 LL |   let nyan : cat = cat(52, 99);
-   |       ----
-   |       |
-   |       first assignment to `nyan`
-   |       consider changing this to `mut nyan`
+   |       ---- help: consider changing this to be mutable: `mut nyan`
 LL |   nyan.how_hungry = 0; //[ast]~ ERROR cannot assign
-   |   ^^^^^^^^^^^^^^^^^^^ cannot assign twice to immutable variable
+   |   ^^^^^^^^^^^^^^^^^^^ cannot assign
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0384`.
+For more information about this error, try `rustc --explain E0594`.