Provide more context when mutably borrowing an imutably borrowed value #148508

estebank · 2025-11-05T00:12:25Z

Point at statics and consts being mutable borrowed or written to:

error[E0594]: cannot assign to immutable static item `NUM`
  --> $DIR/E0594.rs:4:5
   |
LL | static NUM: i32 = 18;
   | --------------- this `static` cannot be written to
...
LL |     NUM = 20;
   |     ^^^^^^^^ cannot assign

Point at the expression that couldn't be mutably borrowed from a pattern:

error[E0596]: cannot borrow data in a `&` reference as mutable
  --> $DIR/mut-pattern-of-immutable-borrow.rs:19:14
   |
LL |     match &arg.field {
   |           ---------- this cannot be borrowed as mutable
LL |         Some(ref mut s) => s.push('a'),
   |              ^^^^^^^^^ cannot borrow as mutable

Partially address #74617.

rustbot · 2025-11-05T00:12:28Z

This PR modifies tests/ui/issues/. If this PR is adding new tests to tests/ui/issues/,
please refrain from doing so, and instead add it to more descriptive subdirectories.

rustbot · 2025-11-05T00:12:31Z

r? @nnethercote

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They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

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hkBst · 2025-11-05T15:30:22Z

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

+                            {
+                                err.span_label(
+                                    local.source_info.span,
+                                    format!("this cannot be {acted_on}"),


This does not seem to have a testcase.

It's covered by a few for Boring like in tests/ui/nll/dont-print-desugared.stderr, and tests/ui/pattern/mut-pattern-of-immutable-borrow.rs covers the BlockTailTemp case.

I checked for the output "this cannot be written to" and did not find it. Since you added the "to", the output should be in the diff, but I did not find it...

I see. It is a combination that can't happen. It would trigger with something like

struct S; impl S { fn foo(&self) { *self = S; } }

but the explicit check for no overlapping spans means that the output is unchanged:

error[E0594]: cannot assign to `*self`, which is behind a `&` reference --> f16.rs:4:9 | 4 | *self = S; | ^^^^^^^^^ `self` is a `&` reference, so it cannot be written to | help: consider changing this to be a mutable reference | 3 | fn foo(&mut self) { | +++

tests/ui/borrowck/argument_number_mismatch_ice.stderr

nnethercote · 2025-11-09T21:32:49Z

r=me with the two commits squashed. Thanks.

Point at statics and consts being mutable borrowed or written to: ``` error[E0594]: cannot assign to immutable static item `NUM` --> $DIR/E0594.rs:4:5 | LL | static NUM: i32 = 18; | --------------- this `static` cannot be written to ... LL | NUM = 20; | ^^^^^^^^ cannot assign ``` Point at the expression that couldn't be mutably borrowed from a pattern: ``` error[E0596]: cannot borrow data in a `&` reference as mutable --> $DIR/mut-pattern-of-immutable-borrow.rs:19:14 | LL | match &arg.field { | ---------- this cannot be borrowed as mutable LL | Some(ref mut s) => s.push('a'), | ^^^^^^^^^ cannot borrow as mutable ```

estebank · 2025-11-10T18:46:46Z

@bors r=nnethercote

bors · 2025-11-10T18:46:48Z

📌 Commit 71b0755 has been approved by nnethercote

It is now in the queue for this repository.

bors · 2025-11-10T18:46:49Z

🌲 The tree is currently closed for pull requests below priority 1000. This pull request will be tested once the tree is reopened.

Provide more context when mutably borrowing an imutably borrowed value Point at statics and consts being mutable borrowed or written to: ``` error[E0594]: cannot assign to immutable static item `NUM` --> $DIR/E0594.rs:4:5 | LL | static NUM: i32 = 18; | --------------- this `static` cannot be written to ... LL | NUM = 20; | ^^^^^^^^ cannot assign ``` Point at the expression that couldn't be mutably borrowed from a pattern: ``` error[E0596]: cannot borrow data in a `&` reference as mutable --> $DIR/mut-pattern-of-immutable-borrow.rs:19:14 | LL | match &arg.field { | ---------- this cannot be borrowed as mutable LL | Some(ref mut s) => s.push('a'), | ^^^^^^^^^ cannot borrow as mutable ``` Partially address rust-lang#74617.

Rollup of 16 pull requests Successful merges: - #141470 (Add new `function_casts_as_integer` lint) - #143619 (`c_variadic`: Add future-incompatibility warning for `...` arguments without a pattern outside of `extern` blocks) - #146495 (rustdoc: Erase `#![doc(document_private_items)]`) - #147771 (Rename `*exact_{div,shr,shl}` to `*{div,shr,shl}_exact`) - #147833 (rustdoc-json: move `target` to `json::conversions`) - #147952 (Add a timeout to the `remote-test-client` connection) - #147955 (compiletest: Migrate `TestProps` directive handling to a system of named handlers) - #148480 (Add `Steal::risky_hack_borrow_mut`) - #148506 (Special case detecting `'static` lifetime requirement coming from `-> Box<dyn Trait>`) - #148508 (Provide more context when mutably borrowing an imutably borrowed value) - #148530 (update the bootstrap readme) - #148608 (Add test for --test-builder success path) - #148636 (bootstrap: respect `build.python` on macOS) - #148639 (test(rustdoc): move tests into jump-to-def) - #148647 (Check unsafety for non-macro attributes in `validate_attr`) - #148667 (a few small clippy fixes) r? `@ghost` `@rustbot` modify labels: rollup

Rollup of 15 pull requests Successful merges: - #141470 (Add new `function_casts_as_integer` lint) - #143619 (`c_variadic`: Add future-incompatibility warning for `...` arguments without a pattern outside of `extern` blocks) - #146495 (rustdoc: Erase `#![doc(document_private_items)]`) - #147771 (Rename `*exact_{div,shr,shl}` to `*{div,shr,shl}_exact`) - #147833 (rustdoc-json: move `target` to `json::conversions`) - #147955 (compiletest: Migrate `TestProps` directive handling to a system of named handlers) - #148480 (Add `Steal::risky_hack_borrow_mut`) - #148506 (Special case detecting `'static` lifetime requirement coming from `-> Box<dyn Trait>`) - #148508 (Provide more context when mutably borrowing an imutably borrowed value) - #148530 (update the bootstrap readme) - #148608 (Add test for --test-builder success path) - #148636 (bootstrap: respect `build.python` on macOS) - #148639 (test(rustdoc): move tests into jump-to-def) - #148647 (Check unsafety for non-macro attributes in `validate_attr`) - #148667 (a few small clippy fixes) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #148508 - estebank:issue-74617, r=nnethercote Provide more context when mutably borrowing an imutably borrowed value Point at statics and consts being mutable borrowed or written to: ``` error[E0594]: cannot assign to immutable static item `NUM` --> $DIR/E0594.rs:4:5 | LL | static NUM: i32 = 18; | --------------- this `static` cannot be written to ... LL | NUM = 20; | ^^^^^^^^ cannot assign ``` Point at the expression that couldn't be mutably borrowed from a pattern: ``` error[E0596]: cannot borrow data in a `&` reference as mutable --> $DIR/mut-pattern-of-immutable-borrow.rs:19:14 | LL | match &arg.field { | ---------- this cannot be borrowed as mutable LL | Some(ref mut s) => s.push('a'), | ^^^^^^^^^ cannot borrow as mutable ``` Partially address #74617.

Rollup of 15 pull requests Successful merges: - rust-lang/rust#141470 (Add new `function_casts_as_integer` lint) - rust-lang/rust#143619 (`c_variadic`: Add future-incompatibility warning for `...` arguments without a pattern outside of `extern` blocks) - rust-lang/rust#146495 (rustdoc: Erase `#![doc(document_private_items)]`) - rust-lang/rust#147771 (Rename `*exact_{div,shr,shl}` to `*{div,shr,shl}_exact`) - rust-lang/rust#147833 (rustdoc-json: move `target` to `json::conversions`) - rust-lang/rust#147955 (compiletest: Migrate `TestProps` directive handling to a system of named handlers) - rust-lang/rust#148480 (Add `Steal::risky_hack_borrow_mut`) - rust-lang/rust#148506 (Special case detecting `'static` lifetime requirement coming from `-> Box<dyn Trait>`) - rust-lang/rust#148508 (Provide more context when mutably borrowing an imutably borrowed value) - rust-lang/rust#148530 (update the bootstrap readme) - rust-lang/rust#148608 (Add test for --test-builder success path) - rust-lang/rust#148636 (bootstrap: respect `build.python` on macOS) - rust-lang/rust#148639 (test(rustdoc): move tests into jump-to-def) - rust-lang/rust#148647 (Check unsafety for non-macro attributes in `validate_attr`) - rust-lang/rust#148667 (a few small clippy fixes) r? `@ghost` `@rustbot` modify labels: rollup

Relevant upstream PR: - rust-lang/rust#148508 (Provide more context when mutably borrowing an imutably borrowed value) Resolves: model-checking#4468

Relevant upstream PR: - rust-lang/rust#148508 (Provide more context when mutably borrowing an imutably borrowed value) Resolves: #4468 By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Nov 5, 2025

rustbot assigned nnethercote Nov 5, 2025

hkBst suggested changes Nov 5, 2025

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nnethercote approved these changes Nov 9, 2025

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estebank force-pushed the issue-74617 branch from 417542f to 71b0755 Compare November 9, 2025 22:14

nnethercote added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Nov 10, 2025

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Nov 10, 2025

Zalathar mentioned this pull request Nov 11, 2025

Rollup of 17 pull requests #148810

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Zalathar mentioned this pull request Nov 11, 2025

Rollup of 16 pull requests #148813

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Zalathar mentioned this pull request Nov 11, 2025

Rollup of 15 pull requests #148817

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bors merged commit 417bea3 into rust-lang:main Nov 11, 2025
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rustbot added this to the 1.93.0 milestone Nov 11, 2025

tautschnig added a commit to tautschnig/kani that referenced this pull request Nov 12, 2025

Upgrade Rust toolchain to 2025-11-12

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Relevant upstream PR: - rust-lang/rust#148508 (Provide more context when mutably borrowing an imutably borrowed value) Resolves: model-checking#4468

tautschnig mentioned this pull request Nov 12, 2025

Upgrade Rust toolchain to 2025-11-12 model-checking/kani#4469

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Provide more context when mutably borrowing an imutably borrowed value #148508

Provide more context when mutably borrowing an imutably borrowed value #148508

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Provide more context when mutably borrowing an imutably borrowed value #148508

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