Add more logarithm constants

[clarfonthey]

Right now, we have ln(2) and ln(10), but only log2(e) and log10(e). This also adds log2(10) and log10(2) for consistency.

[rust-highfive]

r? @dtolnay

(rust_highfive has picked a reviewer for you, use r? to override)

[dtolnay]

Could you share some example use cases that require these constants? I think I've never seen log10(2) before.

[clarfonthey]

@dtolnay log2(e) and log10(e) are used to convert from binary/decimal logs to natural logs, and log2(10) and log10(2) are used to convert between binary/decimal logs. These are actually useful in string conversion, as you can use log2(10) to get an upper bound on the length of an integer string and log10(2) to get an upper bound on the number of bits required, given the length of a string.

Obviously these aren't super useful in every context, but I'd argue that log2(10) and log10(2) are more useful than log2(e) and log10(e), which are currently included.

[rust-highfive]

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.

[00:03:22]    Compiling std v0.0.0 (file:///checkout/src/libstd)
[00:03:23] error[E0547]: missing 'issue'
[00:03:23]    --> libcore/num/f32.rs:132:5
[00:03:23]     |
[00:03:23] 132 |     #[unstable(feature = "extra_log_consts")]
[00:03:23] 
[00:03:23] error[E0547]: missing 'issue'
[00:03:23]    --> libcore/num/f32.rs:140:5
[00:03:23]     |
[00:03:23]     |
[00:03:23] 140 |     #[unstable(feature = "extra_log_consts")]
[00:03:23] 
[00:03:23] error[E0547]: missing 'issue'
[00:03:23]    --> libcore/num/f64.rs:128:5
[00:03:23]     |
[00:03:23]     |
[00:03:23] 128 |     #[unstable(feature = "extra_log_consts")]
[00:03:23] 
[00:03:23] error[E0547]: missing 'issue'
[00:03:23]    --> libcore/num/f64.rs:136:5
[00:03:23]     |
[00:03:23]     |
[00:03:23] 136 |     #[unstable(feature = "extra_log_consts")]
[00:03:23] 
[00:03:26]    Compiling compiler_builtins v0.0.0 (file:///checkout/src/rustc/compiler_builtins_shim)
[00:03:26]    Compiling cmake v0.1.30
[00:03:26]    Compiling alloc_jemalloc v0.0.0 (file:///checkout/src/liballoc_jemalloc)
---
[00:03:36] Caused by:
[00:03:36]   process didn't exit successfully: `/checkout/obj/build/bootstrap/debug/rustc --crate-name core libcore/lib.rs --color always --error-format json --crate-type lib --emit=dep-info,link -C opt-level=3 -C metadata=fb1e36473ec4786e -C extra-filename=-fb1e36473ec4786e --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` (exit code: 101)
[00:03:36] warning: build failed, waiting for other jobs to finish...
[00:03:45] error: build failed
[00:03:45] 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"
[00:03:45] expected success, got: exit code: 101
[00:03:45] thread 'main' panicked at 'cargo must succeed', bootstrap/compile.rs:1091:9
[00:03:45] travis_fold:end:stage0-std

[00:03:45] travis_time:end:stage0-std:start=1525797730516430972,finish=1525797757743978115,duration=27227547143


[00:03:45] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
[00:03:45] Build completed unsuccessfully in 0:00:29
[00:03:45] Makefile:79: recipe for target 'tidy' failed
[00:03:45] make: *** [tidy] Error 1

The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:34322484
$ 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)

[dtolnay]

Looking for some precedent I found that Go provides only the four we already have while D provides all six.

I would be on board with landing these unstable. Please file a tracking issue and include the issue number in the PR. Thanks!

[clarfonthey]

Done!

[dtolnay]

Please use the tracking issue number and not the PR number.

[clarfonthey]

My mistake; that's fixed now.

[dtolnay]

@bors r+

[bors]

📌 Commit 23b6e46 has been approved by dtolnay