Disallow generic type parameters from appearing within certain constants #68356

BenLewis-Seequent · 2020-01-18T23:14:52Z

This improves the diagonistics/fixes ICEs when generic parameters appear within some constants. For example of the new errors:

use std::mem::size_of;

fn foo<T>() {
    let _ = [0; size_of::<T>()]; // now errors with: type parameters can't appear within an array length expression
}

#[repr(usize)]
enum Enum<T> {
    V1 = size_of::<T>(), // now errors with: type parameters can't appear within an enum discriminant
}

Previously emitted confusing errors like:

error[E0277]: the size for values of type `T` cannot be known at compilation time
   --> src/main.rs:45:27
    |
35  | fn foo<T>() {
    |        - help: consider restricting this bound: `T: std::marker::Sized`
...
45  |     let _ = [0; size_of::<T>()];
    |                           ^ doesn't have a size known at compile-time
    |
    = help: the trait `std::marker::Sized` is not implemented for `T`

Note doesn't disallow referring to Self in either enum discriminants or array length expressions as there are cases where it can currently be sucessfully used, i.e.:

struct Foo;

impl Foo {
    const A: usize = 37;

    fn bar() -> [u8; Self::A]{
        [0; Self::A]
    }
}

I don't think that this disallows any more code that didn't either produce a compiler error before or caused an ICE.

cc #43408
cc #67945 (doesn't actually fully fix it see: #67945 (comment))

…nts. Previously using generic type parameters within array length expressions/enum discriminants caused ICE's or confusing diagnostics.

rust-highfive · 2020-01-18T23:14:57Z

Some changes occurred in diagnostic error codes

cc @GuillaumeGomez

rust-highfive · 2020-01-18T23:14:59Z

r? @eddyb

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

src/librustc_error_codes/error_codes/E0747.md

src/librustc_resolve/diagnostics.rs

src/librustc_resolve/late.rs

Centril · 2020-01-19T00:16:56Z

src/librustc_resolve/lib.rs

@@ -1,3 +1,4 @@
+// ignore-tidy-filelength


Can you split things in a follow up PR?

Centril · 2020-01-19T00:19:08Z

cc @petrochenkov @varkor

rust-highfive · 2020-01-19T00:50:32Z

The job x86_64-gnu-llvm-7 of your PR failed (pretty log, 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.

2020-01-18T23:15:24.5527042Z ========================== Starting Command Output ===========================
2020-01-18T23:15:24.5529323Z [command]/bin/bash --noprofile --norc /home/vsts/work/_temp/9e7d26b4-287b-47c5-a7fb-4c76fb3bfab9.sh
2020-01-18T23:15:24.5529358Z 
2020-01-18T23:15:24.5533115Z ##[section]Finishing: Disable git automatic line ending conversion
2020-01-18T23:15:24.5539162Z ##[section]Starting: Checkout rust-lang/rust@refs/pull/68356/merge to s
2020-01-18T23:15:24.5604709Z Task         : Get sources
2020-01-18T23:15:24.5604743Z Description  : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.
2020-01-18T23:15:24.5604938Z Version      : 1.0.0
2020-01-18T23:15:24.5604967Z Author       : Microsoft
---
2020-01-18T23:15:25.5509316Z ##[command]git remote add origin https://github.com/rust-lang/rust
2020-01-18T23:15:25.5524143Z ##[command]git config gc.auto 0
2020-01-18T23:15:25.5526494Z ##[command]git config --get-all http.https://github.com/rust-lang/rust.extraheader
2020-01-18T23:15:25.5528760Z ##[command]git config --get-all http.proxy
2020-01-18T23:15:25.5535930Z ##[command]git -c http.extraheader="AUTHORIZATION: basic ***" fetch --force --tags --prune --progress --no-recurse-submodules --depth=2 origin +refs/heads/*:refs/remotes/origin/* +refs/pull/68356/merge:refs/remotes/pull/68356/merge
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---
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2020-01-19T00:15:35.8828355Z running 116 tests
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2020-01-19T00:16:00.9220838Z Uplifting stage1 rustc (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
2020-01-19T00:16:00.9221378Z Copying stage2 rustc from stage1 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / x86_64-unknown-linux-gnu)
---
2020-01-19T00:27:47.1161838Z 
2020-01-19T00:27:47.1164776Z    Doc-tests core
2020-01-19T00:27:51.3776093Z 
2020-01-19T00:27:51.3776354Z running 2443 tests
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---
2020-01-19T00:32:34.1225063Z 
2020-01-19T00:32:34.1225316Z running 1003 tests
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2020-01-19T00:33:57.5242560Z test result: ok. 983 passed; 0 failed; 20 ignored; 0 measured; 0 filtered out
2020-01-19T00:33:57.5242948Z 
2020-01-19T00:33:57.5336663Z  finished in 158.434
2020-01-19T00:33:57.5337672Z Testing term stage1 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
---
2020-01-19T00:50:26.2526271Z ---- /checkout/obj/build/x86_64-unknown-linux-gnu/test/error-index.md - Rust_Compiler_Error_Index::E0747 (line 13822) stdout ----
2020-01-19T00:50:26.2526578Z error[E0447]: type parameters can't appear within an array length expression
2020-01-19T00:50:26.2526827Z  --> /checkout/obj/build/x86_64-unknown-linux-gnu/test/error-index.md:13826:27
2020-01-19T00:50:26.2526869Z   |
2020-01-19T00:50:26.2526925Z 6 |     let _ = [0; size_of::<T>()];
2020-01-19T00:50:26.2526964Z   |                           ^ type parameter
2020-01-19T00:50:26.2527024Z error: aborting due to previous error
2020-01-19T00:50:26.2527068Z 
2020-01-19T00:50:26.2527273Z For more information about this error, try `rustc --explain E0447`.
2020-01-19T00:50:26.2527273Z For more information about this error, try `rustc --explain E0447`.
2020-01-19T00:50:26.2527317Z Some expected error codes were not found: ["E0747"]
2020-01-19T00:50:26.2527713Z error[E0308]: mismatched types
2020-01-19T00:50:26.2527953Z   --> /checkout/obj/build/x86_64-unknown-linux-gnu/test/error-index.md:13866:20
2020-01-19T00:50:26.2528012Z    |
2020-01-19T00:50:26.2528012Z    |
2020-01-19T00:50:26.2528050Z 12 |     const X: i32 = size_of::<T>();
2020-01-19T00:50:26.2528098Z    |                    ^^^^^^^^^^^^^^ expected `i32`, found `usize`
2020-01-19T00:50:26.2528176Z error: aborting due to previous error
2020-01-19T00:50:26.2528200Z 
2020-01-19T00:50:26.2528407Z For more information about this error, try `rustc --explain E0308`.
2020-01-19T00:50:26.2528596Z Couldn't compile the test.
---
2020-01-19T00:50:26.2551590Z   local time: Sun Jan 19 00:50:26 UTC 2020
2020-01-19T00:50:26.3707033Z   network time: Sun, 19 Jan 2020 00:50:26 GMT
2020-01-19T00:50:26.3708617Z == end clock drift check ==
2020-01-19T00:50:27.4344569Z 
2020-01-19T00:50:27.4434731Z ##[error]Bash exited with code '1'.
2020-01-19T00:50:27.4446087Z ##[section]Finishing: Run build
2020-01-19T00:50:27.4466368Z ##[section]Starting: Checkout rust-lang/rust@refs/pull/68356/merge to s
2020-01-19T00:50:27.4467998Z Task         : Get sources
2020-01-19T00:50:27.4468038Z Description  : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.
2020-01-19T00:50:27.4468093Z Version      : 1.0.0
2020-01-19T00:50:27.4468128Z Author       : Microsoft
2020-01-19T00:50:27.4468128Z Author       : Microsoft
2020-01-19T00:50:27.4468166Z Help         : [More Information](https://go.microsoft.com/fwlink/?LinkId=798199)
2020-01-19T00:50:27.4468224Z ==============================================================================
2020-01-19T00:50:27.8648595Z Cleaning any cached credential from repository: rust-lang/rust (GitHub)
2020-01-19T00:50:27.8686395Z ##[section]Finishing: Checkout rust-lang/rust@refs/pull/68356/merge to s
2020-01-19T00:50:27.8797450Z Cleaning up task key
2020-01-19T00:50:27.8798091Z Start cleaning up orphan processes.
2020-01-19T00:50:27.8899445Z Terminate orphan process: pid (4513) (python)
2020-01-19T00:50:27.9176644Z ##[section]Finishing: Finalize Job

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rust-highfive · 2020-01-19T05:31:34Z

The job x86_64-gnu-llvm-7 of your PR failed (pretty log, 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.

2020-01-19T03:36:14.5508219Z ========================== Starting Command Output ===========================
2020-01-19T03:36:14.5510082Z [command]/bin/bash --noprofile --norc /home/vsts/work/_temp/105aa98e-f14c-4ebc-aa2d-c7baa0ab4d55.sh
2020-01-19T03:36:14.5510124Z 
2020-01-19T03:36:14.5513966Z ##[section]Finishing: Disable git automatic line ending conversion
2020-01-19T03:36:14.5525360Z ##[section]Starting: Checkout rust-lang/rust@refs/pull/68356/merge to s
2020-01-19T03:36:14.5527287Z Task         : Get sources
2020-01-19T03:36:14.5527324Z Description  : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.
2020-01-19T03:36:14.5527411Z Version      : 1.0.0
2020-01-19T03:36:14.5527447Z Author       : Microsoft
---
2020-01-19T03:36:15.5999060Z ##[command]git remote add origin https://github.com/rust-lang/rust
2020-01-19T03:36:15.6017814Z ##[command]git config gc.auto 0
2020-01-19T03:36:15.6024847Z ##[command]git config --get-all http.https://github.com/rust-lang/rust.extraheader
2020-01-19T03:36:15.6030332Z ##[command]git config --get-all http.proxy
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---
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2020-01-19T04:49:35.8851614Z  finished in 31.439
2020-01-19T04:49:35.8859596Z Uplifting stage1 rustc (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
2020-01-19T04:49:35.8859993Z Copying stage2 rustc from stage1 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / x86_64-unknown-linux-gnu)
---
2020-01-19T05:04:00.4056304Z 
2020-01-19T05:04:00.4060374Z    Doc-tests core
2020-01-19T05:04:05.4878521Z 
2020-01-19T05:04:05.4879431Z running 2443 tests
2020-01-19T05:04:15.5594767Z ......iiiii......................................................................................... 100/2443
2020-01-19T05:04:25.2949324Z ..................................................................................ii................ 200/2443
2020-01-19T05:04:48.1866270Z .................i.................................................................................. 400/2443
2020-01-19T05:04:48.1866270Z .................i.................................................................................. 400/2443
2020-01-19T05:04:58.5037701Z ..................................................................i..i..................iiii........ 500/2443
2020-01-19T05:05:16.0515727Z .................................................................................................... 700/2443
2020-01-19T05:05:25.2058010Z .................................................................................................... 800/2443
2020-01-19T05:05:34.4588912Z .................................................................................................... 900/2443
2020-01-19T05:05:43.5768343Z .................................................................................................... 1000/2443
---
2020-01-19T05:09:24.5482990Z .................................................................................................... 500/760
2020-01-19T05:09:24.5828850Z .................thread '<unnamed>' panicked at 'called `Result::unwrap()` on an `Err` value: RecvError', src/libstd/sync/mpsc/mod.rs:2741:22
2020-01-19T05:09:24.5856658Z ....thread '<unnamed>' panicked at 'called `Result::unwrap()` on an `Err` value: RecvError', src/libstd/sync/mpsc/mod.rs:2778:21
2020-01-19T05:09:24.5861393Z thread '<unnamed>' panicked at 'called `Result::unwrap()` on an `Err` value: "SendError(..)"', src/libstd/sync/mpsc/mod.rs:2766:17
2020-01-19T05:09:24.5876275Z .......thread '<unnamed>' panicked at 'called `Result::unwrap()` on an `Err` value: RecvError', src/libstd/sync/mpsc/mod.rs:2645:13
2020-01-19T05:09:24.8848174Z ..........................................thread '<unnamed>' panicked at 'called `Result::unwrap()` on an `Err` value: RecvError', src/libstd/sync/mpsc/mod.rs:1997:22
2020-01-19T05:09:24.8869408Z ....thread '<unnamed>' panicked at 'called `Result::unwrap()` on an `Err` value: "SendError(..)"', src/libstd/sync/mpsc/mod.rs:2022:17
2020-01-19T05:09:24.8895272Z ...thread '<unnamed>' panicked at '.called `Result::unwrap()` on an `Err` value: RecvError', src/libstd/sync/mpsc/mod.rs:2034:21
2020-01-19T05:09:24.8917023Z ...thread '<unnamed>' panicked at 'called `Result::unwrap()` on an `Err` value: RecvError', src/libstd/sync/mpsc/mod.rs:1916:13
2020-01-19T05:09:26.9588672Z ..................thread '<unnamed>' panicked at 'explicit panic', src/libstd/sync/mutex.rs:633:13
2020-01-19T05:09:26.9591859Z ....thread '<unnamed>' panicked at 'test panic in inner thread to poison mutex', src/libstd/sync/mutex.rs:587:13
2020-01-19T05:09:26.9600667Z ....thread '<unnamed>' panicked at 'test panic in inner thread to poison mutex', src/libstd/sync/mutex.rs:563:13
2020-01-19T05:09:26.9601750Z thread '<unnamed>' panicked at 'explicit panic', src/libstd/sync/mutex.rs:694:13
---
2020-01-19T05:09:36.3026284Z 
2020-01-19T05:09:36.3027235Z running 1003 tests
2020-01-19T05:09:57.5943507Z i................................................................................................... 100/1003
2020-01-19T05:10:08.6498113Z .................................................................................................... 200/1003
2020-01-19T05:10:16.4724817Z ..................iii......i......i...i......i...................................................... 300/1003
2020-01-19T05:10:22.0066254Z .................................................................................................... 400/1003
2020-01-19T05:10:29.5282339Z ..........................................i..i.....................................ii............... 500/1003
2020-01-19T05:10:43.1726682Z .................................................................................................... 700/1003
2020-01-19T05:10:43.1726682Z .................................................................................................... 700/1003
2020-01-19T05:10:50.3315828Z .............................iiii................................................................... 800/1003
2020-01-19T05:11:05.9368915Z .................................................................................................... 900/1003
2020-01-19T05:11:13.8120024Z ...................................................iiii............................................. 1000/1003
2020-01-19T05:11:13.8795350Z test result: ok. 983 passed; 0 failed; 20 ignored; 0 measured; 0 filtered out
2020-01-19T05:11:13.8796828Z 
2020-01-19T05:11:13.8930138Z  finished in 186.914
2020-01-19T05:11:13.8944529Z Testing term stage1 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
---
2020-01-19T05:31:27.6512766Z ---- /checkout/obj/build/x86_64-unknown-linux-gnu/test/error-index.md - Rust_Compiler_Error_Index::E0747 (line 13856) stdout ----
2020-01-19T05:31:27.6512949Z error[E0308]: mismatched types
2020-01-19T05:31:27.6513248Z   --> /checkout/obj/build/x86_64-unknown-linux-gnu/test/error-index.md:13866:20
2020-01-19T05:31:27.6513319Z    |
2020-01-19T05:31:27.6513363Z 12 |     const X: i32 = size_of::<T>();
2020-01-19T05:31:27.6513413Z    |                    ^^^^^^^^^^^^^^ expected `i32`, found `usize`
2020-01-19T05:31:27.6513585Z error: aborting due to previous error
2020-01-19T05:31:27.6513621Z 
2020-01-19T05:31:27.6513898Z For more information about this error, try `rustc --explain E0308`.
2020-01-19T05:31:27.6514126Z Couldn't compile the test.
---
2020-01-19T05:31:27.6519418Z   local time: Sun Jan 19 05:31:27 UTC 2020
2020-01-19T05:31:28.1744112Z   network time: Sun, 19 Jan 2020 05:31:28 GMT
2020-01-19T05:31:28.1744228Z == end clock drift check ==
2020-01-19T05:31:28.5489496Z 
2020-01-19T05:31:28.5609536Z ##[error]Bash exited with code '1'.
2020-01-19T05:31:28.5622415Z ##[section]Finishing: Run build
2020-01-19T05:31:28.5650151Z ##[section]Starting: Checkout rust-lang/rust@refs/pull/68356/merge to s
2020-01-19T05:31:28.5672720Z Task         : Get sources
2020-01-19T05:31:28.5672779Z Description  : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.
2020-01-19T05:31:28.5672822Z Version      : 1.0.0
2020-01-19T05:31:28.5672861Z Author       : Microsoft
2020-01-19T05:31:28.5672861Z Author       : Microsoft
2020-01-19T05:31:28.5672912Z Help         : [More Information](https://go.microsoft.com/fwlink/?LinkId=798199)
2020-01-19T05:31:28.5672960Z ==============================================================================
2020-01-19T05:31:29.0309085Z Cleaning any cached credential from repository: rust-lang/rust (GitHub)
2020-01-19T05:31:29.0313882Z ##[section]Finishing: Checkout rust-lang/rust@refs/pull/68356/merge to s
2020-01-19T05:31:29.0456825Z Cleaning up task key
2020-01-19T05:31:29.0463294Z Start cleaning up orphan processes.
2020-01-19T05:31:29.0610370Z Terminate orphan process: pid (3582) (python)
2020-01-19T05:31:29.1026900Z ##[section]Finishing: Finalize Job

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)

eddyb · 2020-01-19T14:41:15Z

I disagree with doing this in rustc_resolve, and the phrasing of the errors.
This is an implementation bug, they should be working (and they kind of do already with #![feature(const_generics)], but that's a bit of a hack).

The reason I didn't want this in rustc_resolve is because I didn't want to risk introducing forwards-compatibility hazards - the behavior of rustc_resolve today is the one we want to keep, if we want to error we should probably do it in rustc_typeck::astconv, perhaps by checking any generic parameter hir::Ty we're converting to Ty actually exists in the generics currently in scope (as opposed to looking it up in the parameter's parent).

(there's probably one exception to this, and that would be the enum discriminant case, but we can allow name resolution on it and error later when trying to evaluate discriminants)

For improving errors, I would prefer doing a crater run with this changed to always be Some:
(now that we've special-cased literals, it might always work in practice)

rust/src/librustc_typeck/collect.rs

Lines 1022 to 1033 in 6250d56

    
           // FIXME(#43408) enable this always when we get lazy normalization. 
        
           Node::AnonConst(_) => { 
        
               // HACK(eddyb) this provides the correct generics when 
        
               // `feature(const_generics)` is enabled, so that const expressions 
        
               // used with const generics, e.g. `Foo<{N+1}>`, can work at all. 
        
               if tcx.features().const_generics { 
        
                   let parent_id = tcx.hir().get_parent_item(hir_id); 
        
                   Some(tcx.hir().local_def_id(parent_id)) 
        
               } else { 
        
                   None 
        
               } 
        
           }

Disallow generic type parameters from appearing within certain consta…

05a50c5

…nts. Previously using generic type parameters within array length expressions/enum discriminants caused ICE's or confusing diagnostics.

rust-highfive assigned eddyb Jan 18, 2020

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jan 18, 2020

Centril reviewed Jan 19, 2020

View reviewed changes

Centril added A-lazy-normalization Area: lazy normalization (tracking issue: #60471) F-const_generics `#![feature(const_generics)]` labels Jan 19, 2020

Code review changes.

51e5f3b

BenLewis-Seequent closed this Jan 22, 2020

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Disallow generic type parameters from appearing within certain constants #68356

Disallow generic type parameters from appearing within certain constants #68356

BenLewis-Seequent commented Jan 18, 2020

rust-highfive commented Jan 18, 2020

rust-highfive commented Jan 18, 2020

Centril Jan 19, 2020

Centril commented Jan 19, 2020

rust-highfive commented Jan 19, 2020

rust-highfive commented Jan 19, 2020

eddyb commented Jan 19, 2020

Disallow generic type parameters from appearing within certain constants #68356

Disallow generic type parameters from appearing within certain constants #68356

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BenLewis-Seequent commented Jan 18, 2020

rust-highfive commented Jan 18, 2020

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Centril commented Jan 19, 2020

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rust-highfive commented Jan 19, 2020

eddyb commented Jan 19, 2020