Add specific diagnostic when attempting to transmute between equal generic types

[varkor]

Also clarifies the wording of E0512.

Fixes #49793.

[rust-highfive]

r? @matthewjasper

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[bors]

☔ The latest upstream changes (presumably #57108) made this pull request unmergeable. Please resolve the merge conflicts.

[matthewjasper]

@bors r+

[bors]

📌 Commit d178fce has been approved by matthewjasper

[bors]

⌛ Testing commit d178fce with merge 94cca8cc0441e11dc3b8d290d95fec7e632d39fb...

[bors]

💔 Test failed - status-travis

[varkor]

@bors r=matthewjasper

[bors]

📌 Commit 3ed83e59e3ee00d08b473a49a5420d2889460308 has been approved by matthewjasper

[rust-highfive]

The job x86_64-gnu-llvm-6.0 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.

travis_time:end:0e06134c:start=1546163949861786982,finish=1546164007111771877,duration=57249984895
$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout

Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#pull-requests-and-security-restrictions
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
$ export SCCACHE_REGION=us-west-1
Setting environment variables from .travis.yml
$ export IMAGE=x86_64-gnu-llvm-6.0
---
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[01:02:23] ......................................F............................................................. 4900/5211
[01:02:31] ..................................................i................................................. 5200/5211
obj/build/x86_64-unknown-linux-gnu/test/ui/transmute/main/auxiliary" "-A" "unused"
[01:02:32] stdout:
[01:02:32] ------------------------------------------
[01:02:32] ------------------------------------------
[01:02:32] 
[01:02:32] ------------------------------------------
[01:02:32] stderr:
[01:02:32] ------------------------------------------
[01:02:32] {"message":"cannot transmute between types of different sizes, or dependently-sized types","code":{"code":"E0512","explanation":"\nTransmute with two differently sized types was attempted. Erroneous code\nexample:\n\n```compile_fail,E0512\nfn takes_u8(_: u8) {}\n\nfn main() {\n    unsafe { takes_u8(::std::mem::transmute(0u16)); }\n    // error: cannot transmute between types of different sizes,\n    //        or dependently-sized types\n}\n```\n\nPlease use types with same size or use the expected type directly. Example:\n\n```\nfn takes_u8(_: u8) {}\n\nfn main() {\n    unsafe { takes_u8(::std::mem::transmute(0i8)); } // ok!\n    // or:\n    unsafe { takes_u8(0u8); } // ok!\n}\n```\n"},"level":"error","spans":[{"file_name":"/checkout/src/test/ui/transmute/main.rs","byte_start":434,"byte_end":443,"line_start":16,"line_end":16,"column_start":5,"column_end":14,"is_primary":true,"text":[{"text":"    transmute(x) //~ ERROR cannot transmute between types of different sizes","highlight_start":5,"highlight_end":14}],"label":null,"suggested_replacement":null,"suggestion_applicability":null,"expansion":null}],"children":[{"message":"source type: `<C as TypeConstructor<'a>>::T` (size can vary because of <C as TypeConstructor>::T)","code":null,"level":"note","spans":[],"children":[],"rendered":null},{"message":"target type: `<C as TypeConstructor<'b>>::T` (size can vary because of <C as TypeConstructor>::T)","code":null,"level":"note","spans":[],"children":[],"rendered":null}],"rendered":"error[E0512]: cannot transmute between types of different sizes, or dependently-sized types\n  --> /checkout/src/test/ui/transmute/main.rs:16:5\n   |\nLL |     transmute(x) //~ ERROR cannot transmute between types of different sizes\n   |     ^^^^^^^^^\n   |\n   = note: source type: `<C as TypeConstructor<'a>>::T` (size can vary because of <C as TypeConstructor>::T)\n   = note: target type: `<C as TypeConstructor<'b>>::T` (size can vary because of <C as TypeConstructor>::T)\n\n"}
[01:02:32] {"message":"cannot transmute between types of different sizes, or dependently-sized types","code":{"code":"E0512","explanation":"\nTransmute with two differently sized types was attempted. Erroneous code\nexample:\n\n```compile_fail,E0512\nfn takes_u8(_: u8) {}\n\nfn main() {\n    unsafe { takes_u8(::std::mem::transmute(0u16)); }\n    // error: cannot transmute between types of different sizes,\n    //        or dependently-sized types\n}\n```\n\nPlease use types with same size or use the expected type directly. Example:\n\n```\nfn takes_u8(_: u8) {}\n\nfn main() {\n    unsafe { takes_u8(::std::mem::transmute(0i8)); } // ok!\n    // or:\n    unsafe { takes_u8(0u8); } // ok!\n}\n```\n"},"level":"error","spans":[{"file_name":"/checkout/src/test/ui/transmute/main.rs","byte_start":546,"byte_end":555,"line_start":20,"line_end":20,"column_start":17,"column_end":26,"is_primary":true,"text":[{"text":"    let x: u8 = transmute(10u16); //~ ERROR cannot transmute between types of different  
[01:02:32] Build completed unsuccessfully in 0:03:58
[01:02:32] Build completed unsuccessfully in 0:03:58
[01:02:32] Makefile:48: recipe for target 'check' failed
[01:02:32] make: *** [check] Error 1
124968 ./obj/build/x86_64-unknown-linux-gnu/stage0-bootstrap-tools/x86_64-unknown-linux-gnu
124964 ./obj/build/x86_64-unknown-linux-gnu/stage0-bootstrap-tools/x86_64-unknown-linux-gnu/release
119264 ./obj/build/x86_64-unknown-linux-gnu/stage0-bootstrap-tools/x86_64-unknown-linux-gnu/release/deps
115364 ./src/llvm/test/CodeGen

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[bors]

⌛ Testing commit 3ed83e59e3ee00d08b473a49a5420d2889460308 with merge cd09c99c467e4e50d1cb5757ac35784b90dd370d...

[bors]

💔 Test failed - status-travis

[varkor]

(Sorry, completely misread that previous error.)

@bors r=matthewjasper

[bors]

📌 Commit 7d5f6ce has been approved by matthewjasper

[bors]

⌛ Testing commit 7d5f6ce with merge f39bd9b...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: matthewjasper
Pushing f39bd9b to master...