Partially implement type ascription #30184

petrochenkov · Dec 3, 2015

This PR is a rebase of the original PR by @eddyb #21836 with some unrebasable parts manually reapplied, feature gate added + type equality restriction added as described below.

This implementation is partial because the type equality restriction is applied to all type ascription expressions and not only those in lvalue contexts. Thus, all difficulties with detection of these contexts and translation of coercions having effect in runtime are avoided.
So, you can't write things with coercions like let slice = &[1, 2, 3]: &[u8];. It obviously makes type ascription less useful than it should be, but it's still much more useful than not having type ascription at all.
In particular, things like let v = something.iter().collect(): Vec<_>; and let u = t.into(): U; work as expected and I'm pretty happy with these improvements alone.

Part of #23416

petrochenkov · Dec 3, 2015

Hm, @rust-highfive failed to assign a reviewer.

r? @eddyb

eddyb · Dec 3, 2015

I wrote the original version of this code, so I'd rather have someone else look over it.
r? @nikomatsakis

oli-obk · Dec 3, 2015

oli-obk reviewed Dec 3, 2015

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oli-obk · Dec 3, 2015

src/librustc_typeck/check/mod.rs

@@ -2671,6 +2671,14 @@ fn check_lit<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
    }
 }

+fn check_expr_eq_type<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
+                                     expr: &'tcx hir::Expr,


wrong indentation

bors · Dec 4, 2015

☔️ The latest upstream changes (presumably #29850) made this pull request unmergeable. Please resolve the merge conflicts.

petrochenkov · Dec 4, 2015

Rebased.

bors · Dec 16, 2015

☔️ The latest upstream changes (presumably #30206) made this pull request unmergeable. Please resolve the merge conflicts.

petrochenkov · Dec 16, 2015

Rebased.

nikomatsakis · Dec 17, 2015

Sorry for the delay! I'll get to this soon.

nikomatsakis · Dec 18, 2015

nikomatsakis reviewed Dec 18, 2015

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nikomatsakis · Dec 18, 2015

src/librustc_mir/hair/cx/expr.rs

@@ -320,6 +320,8 @@ impl<'tcx> Mirror<'tcx> for &'tcx hir::Expr {
                                  name: Field::new(index.node as usize) },
            hir::ExprCast(ref source, _) =>
                ExprKind::Cast { source: source.to_ref() },
+            hir::ExprType(ref source, _) =>
+                return source.make_mirror(cx),


This doesn't seem right. I think we probably want to convert this to a ExprKind::Cast.

/me reconsiders. Actually, let me read a bit more and get back to this point!

nikomatsakis · Dec 18, 2015

@petrochenkov I didn't notice at first that you skipped over the coercion aspect. Makes sense for a first pass, this certainly simplifies things. The code looks great. The only thing I would want to add are some parsing tests that test the relative precedence of : with other operators. Can you do that?

nikomatsakis · Dec 18, 2015

Some examples tests for precedence:

&[1, 2, 3]: &[u8; 3] => (&[1,2,3]): &[u8: 3]
x:u8+y => (22:u8)+42
x+y:u8 => x+(y:u8)

The only tricky part is how to write such at est. I envisioned testing this by setting up some scenarios that will fail to type-check if the precedence is not correct? I'm not sure if there is a better way.

petrochenkov · Dec 18, 2015

Sorry for the delay! I'll get to this soon.

Thanks! In fact my free time is coming to an end rapidly, so I have to wrap up my work on rustc ASAP (at least for now).
I'll add tests for operator precedence.

nikomatsakis · Dec 18, 2015

@petrochenkov

Thanks! In fact my free time is coming to an end rapidly, so I have to wrap up my work on rustc ASAP (at least for now).

:( Let me know if there is work you think you will not have time for! I'd be happy to land this PR (or privacy) and then do the followup work myself.

petrochenkov · Dec 18, 2015

Updated with the operator precedence tests.

nikomatsakis · Dec 19, 2015

@bors r+

Another nice PR. Thanks @petrochenkov!

bors · Dec 19, 2015

📌 Commit 95fdaf2 has been approved by nikomatsakis

bors · Dec 19, 2015

⌛️ Testing commit 95fdaf2 with merge 440ef8b...

bors · Dec 19, 2015

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt

petrochenkov · Dec 19, 2015

This should probably be marked with relnotes

brson · Jan 8, 2016

This ended up causing breakage in macros with multiple patterns.

rust-highfive assigned eddyb Dec 3, 2015

rust-highfive assigned nikomatsakis and unassigned eddyb Dec 3, 2015

petrochenkov force-pushed the petrochenkov:ascr branch from 8fd1579 to 96d2791 Dec 4, 2015

eddyb and others added some commits Feb 1, 2015

Implement type ascription.

b8157cc

Add ExprType to HIR and make everything compile
+ Apply parser changes manually + Add feature gate

e0ceef5

petrochenkov force-pushed the petrochenkov:ascr branch from 96d2791 to 7afc5e2 Dec 16, 2015

Require exact type equality + add tests
+ Rebase fixes

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95fdaf2

petrochenkov force-pushed the petrochenkov:ascr branch from 7afc5e2 to 95fdaf2 Dec 18, 2015

bors merged commit 95fdaf2 into rust-lang:master Dec 19, 2015
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Improve ELF TLS detection on OSX #30417

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Implement type ascription. #21836

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Handle type ascription #848

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Improve ELF TLS detection on OSX #30417

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Implement type ascription. #21836

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Handle type ascription #848

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petrochenkov force-pushed the petrochenkov:ascr branch from `8fd1579` to `96d2791` Dec 4, 2015

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bors merged commit `95fdaf2` into rust-lang:master Dec 19, 2015
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