[wip] library: mark more functions that return String as #[must_use] #50462

matthiaskrgr · 2018-05-05T10:36:59Z

   library: mark more functions that return String as #[must_use]
    If the return value (String) of the function is not used, the function call is technically in vain
    (unless there are side effects) and the entire function call could be ommitted.
    Warn about unused return values of -> String functions.
    Example code:
       let mut x = Sting::from("hello");
       x.push_str(" world!");
       x.to_uppercase();
       println!("{}", x);
    will print "hello world!" instead of "HELLO WORLD!") because the result of .to_uppercase() is not caught
    in a variable.
    std::str::to_lowercase()
    std::str::to_uppercase()
    std::str::escape_debug()
    std::str::escape_default()
    std::str::escape_unicode()
    std::str::into_string()
    std::str::repeat()
    std::str::to_ascii_uppercase()
    std::str::to_ascii_lowercase()
    std::String::with_capacity()
    std::String::from_str()
    std::String::from_utf16_lossy()
    std::String::from_raw_parts()
    std::String::from_utf8_unchecked()
    std::String::split_off()

TODO:

if String::new() is #[must_use], Vec::new() shoud have it too?
mark traits as #[must_use]
add messages [wip] library: mark more functions that return String as #[must_use] #50462 (comment)
[wip] library: mark more functions that return String as #[must_use] #50462 (review)
[wip] library: mark more functions that return String as #[must_use] #50462 (review)
[wip] library: mark more functions that return String as #[must_use] #50462 (review)

rust-highfive · 2018-05-05T10:37:03Z

r? @shepmaster

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rust-highfive · 2018-05-05T11:48:58Z

The job x86_64-gnu-llvm-3.9 of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

Testing alloc stage1 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
[01:10:15]    Compiling libc v0.2.40
[01:10:16]    Compiling rand v0.4.2
[01:10:19]    Compiling alloc v0.0.0 (file:///checkout/src/liballoc)
[01:10:30] error: unused return value of `std::string::String::split_off` which must be used
[01:10:30]     |
[01:10:30]     |
[01:10:30] 247 |     split.split_off(orig.len() + 1);
[01:10:30]     |
[01:10:30]     = note: `-D unused-must-use` implied by `-D warnings`
[01:10:30] 
[01:10:30] 
[01:10:30] error: unused return value of `std::string::String::split_off` which must be used
[01:10:30]     |
[01:10:30]     |
[01:10:30] 254 |     orig.split_off(1);
[01:10:30] 
[01:10:30] error: aborting due to 2 previous errors
[01:10:30] 
[01:10:30] error: Could not compile `alloc`.
[01:10:30] error: Could not compile `alloc`.
[01:10:30] 
[01:10:30] To learn more, run the command again with --verbose.
[01:10:30] 
[01:10:30] 
[01:10:30] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "test" "--target" "x86_64-unknown-linux-gnu" "--release" "--locked" "--color" "always" "--features" "panic-unwind jemalloc backtrace" "--manifest-path" "/checkout/src/libstd/Cargo.toml" "-p" "alloc" "--" "--quiet"
[01:10:30] 
[01:10:30] 
[01:10:30] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test
[01:10:30] Build completed unsuccessfully in 0:26:40
[01:10:30] Build completed unsuccessfully in 0:26:40
[01:10:30] Makefile:58: recipe for target 'check' failed
[01:10:30] make: *** [check] Error 1

The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:2a7ba538
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
---
60840 ./src/llvm-emscripten/lib
56092 ./obj/build/x86_64-unknown-linux-gnu/stage0/lib/rustlib/x86_64-unknown-linux-gnu/bin
55380 ./obj/build/x86_64-unknown-linux-gnu/stage0-rustc/release
53660 ./obj/build/x86_64-unknown-linux-gnu/stage1-rustc/x86_64-unknown-linux-gnu/release/incremental/syntax-33ta18b3panbi
53656 ./obj/build/x86_64-unknown-linux-gnu/stage1-rustc/x86_64-unknown-linux-gnu/release/incremental/syntax-33ta18b3panbi/s-f0r1w561vd-1iks737-9sj2cmggjcjg
48604 ./obj/build/x86_64-unknown-linux-gnu/stage0/bin
47892 ./obj/build/x86_64-unknown-linux-gnu/stage0-std
47052 ./src/test
46720 ./obj/build/x86_64-unknown-linux-gnu/stage1-std/release

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

shepmaster · 2018-05-05T14:51:49Z

Could you provide some rationale for this change? I'm too lazy to look for it now, but I seem to recall there being discussions around why we shouldn't add #[must_use] to every method in the standard library.

matthiaskrgr · 2018-05-05T15:46:09Z

Ah, I didn't not know about this discussion.
Anyway, this is similar to #50177

The idea is, if a user uses code like

    let x = String::from("Hello world");
    x.to_uppercase();
    println!("{}", x);

,the x.to_uppercase(); is essentially dead code (unless there are side effects and since the String is not modified inplace) however there is no warning about it.

These functions that return a new String should have their return value captured, otherwise the operation has no effect and might lead to confusion.

ollie27 · 2018-05-05T22:25:28Z

src/liballoc/string.rs

@@ -2142,6 +2150,7 @@ pub trait ToString {
 /// since `fmt::Write for String` never returns an error itself.
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: fmt::Display + ?Sized> ToString for T {
+    #[must_use]


The #[must_use] attribute needs to be in the trait definition. It doesn't work when added to impls: #48486.

zackmdavis · 2018-05-06T06:01:26Z

(Discussion issue for #[must_use] functions in the standard library is #48926.)

frol · 2018-05-06T10:46:18Z

Could you provide some rationale for this change?

@shepmaster I have just recently seen an answer on Reddit which made this kind of mistake using .to_uppercase() (the comment is already removed), so it seems worthwhile to me to be warned by a compiler given it is already possible to do that.

shepmaster · 2018-05-06T11:51:28Z

I'd also prefer that the rationale go into the commit message. Commit messages that duplicate the diff are mostly useless because we can always look at the diff again; we can't look back at the author's rationale.

I'd also recommend squashing the commits, once the review feedback is finished.

matthiaskrgr · 2018-05-06T12:06:14Z

sure, will do

rust-highfive · 2018-05-06T14:59:37Z

The job x86_64-gnu-llvm-3.9 of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

[00:03:13]    Compiling rustc_tsan v0.0.0 (file:///checkout/src/librustc_tsan)
[00:03:29]    Compiling libc v0.0.0 (file:///checkout/src/rustc/libc_shim)
[00:03:29]    Compiling alloc v0.0.0 (file:///checkout/src/liballoc)
[00:03:29]    Compiling std_unicode v0.0.0 (file:///checkout/src/libstd_unicode)
[00:03:29] error[E0518]: attribute should be applied to function
[00:03:29]      |
[00:03:29] 2184 |   #[inline]
[00:03:29]      |   ^^^^^^^^^
[00:03:29]      |   ^^^^^^^^^
[00:03:29] 2185 |   #[stable(feature = "string_to_string_specialization", since = "1.17.0")]
[00:03:29] 2186 | / impl ToString for String {
[00:03:29] 2187 | |     #[must_use]
[00:03:29] 2188 | |     fn to_string(&self) -> String {
[00:03:29] 2190 | |     }
[00:03:29] 2191 | | }
[00:03:29] 2191 | | }
[00:03:29]      | |_- not a function
[00:03:30]    Compiling alloc_system v0.0.0 (file:///checkout/src/liballoc_system)
[00:03:30]    Compiling panic_abort v0.0.0 (file:///checkout/src/libpanic_abort)
[00:03:32] error: aborting due to previous error
[00:03:32] 
[00:03:32] 
[00:03:32] For more information about this error, try `rustc --explain E0518`.
[00:03:32] error: Could not compile `alloc`.
[00:03:32] 
[00:03:32] Caused by:
[00:03:32]   process didn't exit successfully: `/checkout/obj/build/bootstrap/debug/rustc --crate-name alloc liballoc/lib.rs --color always --error-format json --crate-type lib --emit=dep-info,link -C opt-level=3 -C metadata=5c6ca57f52fc716b -C extra-filename=-5c6ca57f52fc716b --out-dir /checkout/obj/build/x86_64-unknown-linux-gnu/stage0-std/x86_64-unknown-linux-gnu/release/deps --target x86_64-unknown-linux-gnu -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-std/x86_64-unknown-linux-gnu/release/deps -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-std/release/deps --extern core=/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-std/x86_64-unknown-linux-gnu/release/deps/libcore-fb1e36473ec4786e.rlib --extern compiler_builtins=/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-std/x86_64-unknown-linux-gnu/release/deps/libcompiler_builtins-bad063b3019d016c.rlib -L native=/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-std/x86_64-unknown-linux-gnu/release/build/compiler_builtins-af41331a61619951/out` (exit code: 101)
[00:03:34] error: build failed
[00:03:34] error: build failed
[00:03:34] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "build" "--target" "x86_64-unknown-linux-gnu" "--release" "--locked" "--color" "always" "--features" "panic-unwind jemalloc backtrace" "--manifest-path" "/checkout/src/libstd/Cargo.toml" "--message-format" "json"
[00:03:34] expected success, got: exit code: 101
[00:03:34] thread 'main' panicked at 'cargo must succeed', bootstrap/compile.rs:1091:9
[00:03:34] travis_fold:end:stage0-std

[00:03:34] travis_time:end:stage0-std:start=1525618708642499454,finish=1525618743514838858,duration=34872339404


[00:03:34] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
[00:03:34] Build completed unsuccessfully in 0:00:36
[00:03:34] make: *** [tidy] Error 1
[00:03:34] Makefile:79: recipe for target 'tidy' failed

The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:21378f00
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

shepmaster · 2018-05-06T18:11:15Z

src/liballoc/string.rs

@@ -11,7 +11,7 @@
 //! A UTF-8 encoded, growable string.
 //!
 //! This module contains the [`String`] type, a trait for converting
-//! [`ToString`]s, and several error types that may result from working with
+//! [`ToString`]s, and several error types that may result from working withF


Looks like a typo snuck in.

shepmaster · 2018-05-06T18:12:31Z

src/liballoc/string.rs

@@ -689,6 +693,7 @@ impl String {
    ///     assert_eq!(String::from("hello"), s);
    /// }
    /// ```
+    #[must_use]


I think I'd disagree with this one — this is used for FFI when passing back a decomposed String to deallocate it.

Wouldn't the code be more readable if one is forced to use drop() explicitly if the only intention is to deallocate?

Not in my opinion, no. Idiomatic Rust doesn't tend to use explicit calls to drop frequently.

I am just a stranger here, but I have some experience with Python, which mantra is "explicit is better than implicit".

Idiomatic Rust doesn't tend to use unsafe blocks frequently, either. I would even say that explicitness is even more valuable in unsafe code.

shepmaster · 2018-05-06T18:13:15Z

src/liballoc/string.rs

 #[stable(feature = "string_to_string_specialization", since = "1.17.0")]
 impl ToString for String {
-    #[inline]


This should not be changed.

shepmaster · 2018-05-06T18:16:50Z

src/liballoc/string.rs

@@ -1419,6 +1425,7 @@ impl String {
    /// assert_eq!(world, "World!");
    /// # }
    /// ```
+    #[must_use]


This function does have side effects (&mut self) — why should the user need to care about the return value here? What should they use instead of ignoring the result?

If I ignore the result of split_off, it seems that I should better use truncate, shouldn't I?

Is there a way to customize the error message provided by #[must_use] on a case-by-case basis to suggest that truncate is more appropriate? Otherwise, should that go into the documentation?

The attribute can take a string literal with an additional message: usage, output. (The output format may change soon.)

This one I like provided it has the message. It sets a nice "if it allows redirecting to a cheaper method" precedent (which, in retrospect, collect is also following).

(Should probably be on Vec::split_off too, though.)

scottmcm · 2018-05-07T00:52:24Z

src/liballoc/str.rs

@@ -418,6 +423,7 @@ impl str {
    ///
    /// assert_eq!(boxed_str.into_string(), string);
    /// ```
+    #[must_use]
    #[stable(feature = "box_str", since = "1.4.0")]
    #[inline]
    pub fn into_string(self: Box<str>) -> String {


This one feels insufficiently justified. It's not allocating. It consumes the input so if you didn't need it you'll probably know. It does almost nothing, so will probably disappear on its own if unused.

I agree that it could be must_use, but as I understand it the goal isn't to put it everywhere possible.

I don't understand why we should apply so hardly-defined rules to where must_use should be used and where it shouldn't. I suggest the following reasoning: if the function call doesn't make sense without consuming the result, it should be marked with must_use.

It does almost nothing, so will probably disappear on its own if unused.

Why does Rust warn about unused uses then? They do nothing and just "disappear" completely if not used.

scottmcm · 2018-05-07T00:54:33Z

src/liballoc/string.rs

@@ -378,6 +378,7 @@ impl String {
    /// ```
    /// let s = String::new();
    /// ```
+    #[must_use]
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn new() -> String {


This one's another case of a trivial method where, sure, not using it isn't ideal stylistically, but it's just setting 3 pointers to constants, so overall I feel "meh".

Edit: Also, it's inconsistent for String::new to have it but not Vec::new.

Ok, for now I removed #[must_use] from std::String::new()

Manishearth · 2018-05-07T16:12:57Z

#[must_use] takes a string argument (#[must_use = "reasons reasons"]). Please add some explanatory text to each one of these to help users know why the result should be used.

Manishearth · 2018-05-07T16:34:20Z

I did this for a bunch of existing ones in #50511

If the return value (String) of the function is not used, the function call is technically in vain (unless there are side effects) and the entire function call could be ommitted. Warn about unused return values of -> String functions. Example code: let mut x = Sting::from("hello"); x.push_str(" world!"); x.to_uppercase(); println!("{}", x); will print "hello world!" instead of "HELLO WORLD!") because the result of .to_uppercase() is not caught in a variable. std::str::to_lowercase() std::str::to_uppercase() std::str::escape_debug() std::str::escape_default() std::str::escape_unicode() std::str::into_string() std::str::repeat() std::str::to_ascii_uppercase() std::str::to_ascii_lowercase() std::String::with_capacity() std::String::from_str() std::String::from_utf16_lossy() std::String::from_raw_parts() std::String::from_utf8_unchecked() std::String::split_off()

pietroalbini · 2018-05-14T10:27:09Z

Ping from triage @shepmaster! The author pushed some new commits.

matthiaskrgr · 2018-05-14T11:28:48Z

This still WIP, I haven't had s lot of time in the recent days unfortunately.

matthiaskrgr · 2018-05-15T08:37:54Z

I made a separate pull request for Vec::new() and String::new() here: #50766

pietroalbini · 2018-05-28T08:42:25Z

Marking as blocked on #48926.

TimNN · 2018-07-10T17:26:41Z

Ping from triage! Thanks for your PR, @matthiaskrgr. It looks like blocking issue / RFC will need some time before it is resolved, so we're closing this PR for now. Feel free to reopen it in the future.

rust-highfive assigned shepmaster May 5, 2018

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label May 5, 2018

matthiaskrgr changed the title ~~[wip] library: mark more functions that return String as #[must_use]~~ library: mark more functions that return String as #[must_use] May 5, 2018

ollie27 reviewed May 5, 2018

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frol mentioned this pull request May 6, 2018

mark most of std::str methods as #[must_use] #50479

Closed

matthiaskrgr force-pushed the must_use_strings branch from 03b98d5 to efe47ba Compare May 6, 2018 14:54

shepmaster reviewed May 6, 2018

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shepmaster mentioned this pull request May 7, 2018

discussion/tracking issue for #[must_use] functions in the standard library #48926

Closed

scottmcm reviewed May 7, 2018

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matthiaskrgr force-pushed the must_use_strings branch from efe47ba to 42c4faf Compare May 10, 2018 12:29

matthiaskrgr force-pushed the must_use_strings branch from 42c4faf to aa127d8 Compare May 10, 2018 12:31

matthiaskrgr changed the title ~~library: mark more functions that return String as #[must_use]~~ [wip] library: mark more functions that return String as #[must_use] May 10, 2018

pietroalbini 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 May 21, 2018

pietroalbini added S-blocked Status: Marked as blocked ❌ on something else such as an RFC or other implementation work. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels May 28, 2018

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pietroalbini added S-blocked Status: Marked as blocked ❌ on something else such as an RFC or other implementation work. and removed S-blocked Status: Marked as blocked ❌ on something else such as an RFC or other implementation work. labels Jun 25, 2018

TimNN added A-allocators Area: Custom and system allocators and removed A-allocators Area: Custom and system allocators labels Jul 3, 2018

kennytm mentioned this pull request Jul 10, 2018

Add #[must_use] to compare_and_swap #52201

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matthiaskrgr deleted the must_use_strings branch August 28, 2018 17:35

m-ou-se mentioned this pull request Oct 22, 2020

Warn using last and count to exhaust an iterator #78225

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[wip] library: mark more functions that return String as #[must_use] #50462

[wip] library: mark more functions that return String as #[must_use] #50462

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[wip] library: mark more functions that return String as #[must_use] #50462

[wip] library: mark more functions that return String as #[must_use] #50462

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