Add more docs - mostly warnings - to std::mem::transmute #34609

strega-nil · 2016-07-02T06:34:53Z

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rust-highfive · 2016-07-02T06:34:59Z

r? @aturon

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strega-nil · 2016-07-02T06:35:18Z

r? @steveklabnik

strega-nil · 2016-07-02T06:36:48Z

Perhaps I should also add an example for casting to usize.

arielb1 · 2016-07-02T15:16:58Z

transmute can also be used to transform a Vec<Option<&T>> to a Vec<&T>.

strega-nil · 2016-07-02T15:31:15Z

@arielb1 Ah, good for the alternatives section!

ollie27 · 2016-07-03T08:46:54Z

src/libcore/intrinsics.rs

@@ -278,17 +278,157 @@ extern "rust-intrinsic" {
    /// Moves a value out of scope without running drop glue.
    pub fn forget<T>(_: T) -> ();

-    /// Unsafely transforms a value of one type into a value of another type.
+    /// Reinterprets the bits of a value of one type as another type. Both types


The first paragraph needs to be a single sentence as it's used as a short summary on the module page and search results.

ollie27 · 2016-07-03T09:03:24Z

A lot of these examples aren't compiling, check the travis output and try them locally. I suggest you put each example in it's own code block so they run individually and some examples like the libc one need to be marked ignore as that won't even compile.

strega-nil · 2016-07-03T09:06:59Z

@ollie27 Yeah, I've obviously never written documentation like this before xP

strega-nil · 2016-07-04T09:19:35Z

@ollie27 Okay, I can't actually try it locally due to LLDB failures. I'm... not willing to continue to try this. If someone else wants to pick this up, they can, but goddamnit, I've spent a fucking day trying to get rustc to compile. This is ridiculous.

Edit: Alright, it's working now. The rustc new-dev experience is... not great :P

ollie27 · 2016-07-05T09:26:29Z

You shouldn't need to compile rustc to try these. If you have a working rustdoc you can copy this doc block into it's own file and run rustdoc --test on that. You can also use https://play.rust-lang.org/.

steveklabnik · 2016-07-05T15:50:56Z

I suggest you put each example in it's own code block so they run individually and some examples like the libc one need to be marked ignore as that won't even compile.

Yes please to both of these!

steveklabnik · 2016-07-05T18:02:02Z

src/libcore/intrinsics.rs

+    /// are as follows:
+    ///
+    /// Turning a pointer into a `usize`:
+    /// ```


can you put a blank line between the triple backticks and the descriptions? This applies throughout the file.

steveklabnik · 2016-07-21T19:58:59Z

@bors: r+ rollup

bors · 2016-07-21T19:59:01Z

📌 Commit 24f8589 has been approved by steveklabnik

bors · 2016-07-21T19:59:04Z

⌛ Testing commit 24f8589 with merge 3850b22...

bors · 2016-07-21T20:04:31Z

💔 Test failed - auto-win-gnu-32-opt

alexcrichton · 2016-07-21T20:29:44Z

@bors: retry

On Thu, Jul 21, 2016 at 1:04 PM, bors [email protected] wrote:

💔 Test failed - auto-win-gnu-32-opt
https://buildbot.rust-lang.org/builders/auto-win-gnu-32-opt/builds/5024

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steveklabnik · 2016-07-25T18:28:15Z

@bors: retry

alexcrichton · 2016-07-25T18:39:05Z

@bors: retry (maybe third time's the charm)

Add more docs - mostly warnings - to std::mem::transmute

Rollup of 13 pull requests - Successful merges: #34461, #34609, #34732, #34850, #34894, #34935, #34974, #34990, #34995, #35001, #35009, #35010, #35028 - Failed merges:

bors · 2016-07-26T10:58:46Z

⌛ Testing commit 24f8589 with merge 2094bc3...

bors · 2016-07-26T11:03:11Z

💔 Test failed - auto-win-msvc-64-opt

alexcrichton · 2016-07-26T17:17:14Z

@bors: retry

On Tue, Jul 26, 2016 at 4:03 AM, bors [email protected] wrote:

💔 Test failed - auto-win-msvc-64-opt
https://buildbot.rust-lang.org/builders/auto-win-msvc-64-opt/builds/5083

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Add more docs - mostly warnings - to std::mem::transmute

Rollup of 15 pull requests - Successful merges: #34461, #34609, #34732, #34850, #34935, #34974, #34990, #34995, #35001, #35009, #35010, #35019, #35028, #35029, #35043 - Failed merges:

tamird

(leaving a comment here in case the author and reviewers have opinions)

Why is using pointer-to-pointer as casts most sensible (or safer) than transmute? In 2025, involving pointers might trigger questions about provenance. Is that better?

This came up in a discussion on the Rust-for-Linux mailing list, and I commented about it another issue as well.

There's also been some whiplash on this in std itself. transmute was replaced with this pattern in 2633b85, and then replaced with transmute again in https://github.com/rust-lang/rust/pull/75157/files.

I'd love to understand how this came to be the suggested pattern.

EDIT: @sfackler said it in #34609 (comment). Shame this wasn't addressed.

ChrisDenton · 2025-03-27T02:12:03Z

Nine years is a long time in software. It's very possible everyone involved in this PR has moved on.

You'd be better off opening a new issue or asking on the Zulip chat.

tamird · 2025-03-27T14:49:29Z

Nine years is a long time in software. It's very possible everyone involved in this PR has moved on.

You'd be better off opening a new issue or asking on the Zulip chat.

#general > casting &[T] to `#[repr(transparent)]` wrapper struct @ 💬 for a start.

Add more docs - mostly warnings - to std::mem::transmute

3d03f75

rust-highfive assigned aturon Jul 2, 2016

rust-highfive assigned steveklabnik and unassigned aturon Jul 2, 2016

ubsan added 3 commits July 1, 2016 23:57

Fix up some things which scott mentioned

233b45f

Fix some other small nits

6928bbb

More nits :P

2413b52

ubsan added 2 commits July 2, 2016 08:45

Add a new alternative

377bbfe

Make sure the documentation compiles

9e94ebf

ollie27 reviewed Jul 3, 2016
View reviewed changes

Fix tests

7ec44e6

steveklabnik reviewed Jul 5, 2016
View reviewed changes

Fix nits

24f8589

steveklabnik added a commit to steveklabnik/rust that referenced this pull request Jul 25, 2016

Rollup merge of rust-lang#34609 - ubsan:transmute_docs, r=steveklabnik

ee771bf

Add more docs - mostly warnings - to std::mem::transmute

steveklabnik mentioned this pull request Jul 25, 2016

Rollup of 13 pull requests #35036

Closed

steveklabnik added a commit to steveklabnik/rust that referenced this pull request Jul 26, 2016

Rollup merge of rust-lang#34609 - ubsan:transmute_docs, r=steveklabnik

4c95b66

Add more docs - mostly warnings - to std::mem::transmute

steveklabnik mentioned this pull request Jul 26, 2016

Rollup of 15 pull requests #35053

Merged

bors merged commit 24f8589 into rust-lang:master Jul 27, 2016

strega-nil deleted the transmute_docs branch September 11, 2018 05:38

wchargin mentioned this pull request Nov 24, 2020

transmute_ptr_to_ptr doesn’t justify its existence rust-lang/rust-clippy#6372

Open

tamird mentioned this pull request Mar 26, 2025

Lint against transmutes which could be as casts. #34249

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tamird reviewed Mar 26, 2025

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Add more docs - mostly warnings - to std::mem::transmute #34609

Add more docs - mostly warnings - to std::mem::transmute #34609

strega-nil commented Jul 2, 2016

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tamird commented Mar 27, 2025

Add more docs - mostly warnings - to std::mem::transmute #34609

Add more docs - mostly warnings - to std::mem::transmute #34609

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