ergonomic improvements to the methods in infcx

src/librustc/infer/at.rs

@@ -0,0 +1,310 @@
+// Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! A nice interface for working with the infcx.  The basic idea is to
+//! do `infcx.at(cause, param_env)`, which sets the "cause" of the
+//! operation as well as the surrounding parameter environment.  Then
+//! you can do something like `.sub(a, b)` or `.eq(a, b)` to create a
+//! subtype or equality relationship respectively. The first argument
+//! is always the "expected" output from the POV of diagnostics.
+//!
+//! Examples:
+//!
+//!     infcx.at(cause, param_env).sub(a, b)
+//!     // requires that `a <: b`, with `a` considered the "expected" type
+//!
+//!     infcx.at(cause, param_env).sup(a, b)
+//!     // requires that `b <: a`, with `a` considered the "expected" type
+//!
+//!     infcx.at(cause, param_env).eq(a, b)
+//!     // requires that `a == b`, with `a` considered the "expected" type
+//!
+//! For finer-grained control, you can also do use `trace`:
+//!
+//!     infcx.at(...).trace(a, b).sub(&c, &d)
+//!
+//! This will set `a` and `b` as the "root" values for
+//! error-reporting, but actually operate on `c` and `d`. This is
+//! sometimes useful when the types of `c` and `d` are not traceable
+//! things. (That system should probably be refactored.)
+
+use super::*;
+
+use ty::relate::{Relate, TypeRelation};
+
+pub struct At<'a, 'gcx: 'tcx, 'tcx: 'a> {
+    infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
+    cause: &'a ObligationCause<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+}
+
+pub struct Trace<'a, 'gcx: 'tcx, 'tcx: 'a> {
+    at: At<'a, 'gcx, 'tcx>,
+    a_is_expected: bool,
+    trace: TypeTrace<'tcx>,
+}
+
+impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
+    pub fn at(&'a self,
+              cause: &'a ObligationCause<'tcx>,
+              param_env: ty::ParamEnv<'tcx>)
+              -> At<'a, 'gcx, 'tcx>
+    {
+        At { infcx: self, cause, param_env }
+    }
+}
+
+pub trait ToTrace<'tcx>: Relate<'tcx> + Copy {
+    fn to_trace(cause: &ObligationCause<'tcx>,
+                a_is_expected: bool,
+                a: Self,
+                b: Self)
+                -> TypeTrace<'tcx>;
+}
+
+impl<'a, 'gcx, 'tcx> At<'a, 'gcx, 'tcx> {
+    /// Hacky routine for equating two impl headers in coherence.
+    pub fn eq_impl_headers(self,
+                           expected: &ty::ImplHeader<'tcx>,
+                           actual: &ty::ImplHeader<'tcx>)
+                           -> InferResult<'tcx, ()>
+    {
+        debug!("eq_impl_header({:?} = {:?})", expected, actual);
+        match (expected.trait_ref, actual.trait_ref) {
+            (Some(a_ref), Some(b_ref)) =>
+                self.eq(a_ref, b_ref),
+            (None, None) =>
+                self.eq(expected.self_ty, actual.self_ty),
+            _ =>
+                bug!("mk_eq_impl_headers given mismatched impl kinds"),
+        }
+    }
+
+    /// Make `a <: b` where `a` may or may not be expected
+    pub fn sub_exp<T>(self,
+                      a_is_expected: bool,
+                      a: T,
+                      b: T)
+                      -> InferResult<'tcx, ()>
+        where T: ToTrace<'tcx>
+    {
+        self.trace_exp(a_is_expected, a, b).sub(&a, &b)
+    }
+
+    /// Make `actual <: expected`. For example, if type-checking a
+    /// call like `foo(x)`, where `foo: fn(i32)`, you might have
+    /// `sup(i32, x)`, since the "expected" type is the type that
+    /// appears in the signature.
+    pub fn sup<T>(self,
+                  expected: T,
+                  actual: T)
+                  -> InferResult<'tcx, ()>
+        where T: ToTrace<'tcx>
+    {
+        self.sub_exp(false, actual, expected)
+    }
+
+    /// Make `expected <: actual`
+    pub fn sub<T>(self,
+                  expected: T,
+                  actual: T)
+                  -> InferResult<'tcx, ()>
+        where T: ToTrace<'tcx>
+    {
+        self.sub_exp(true, expected, actual)
+    }
+
+    /// Make `expected <: actual`
+    pub fn eq_exp<T>(self,
+                     a_is_expected: bool,
+                     a: T,
+                     b: T)
+                     -> InferResult<'tcx, ()>
+        where T: ToTrace<'tcx>
+    {
+        self.trace_exp(a_is_expected, a, b).eq(&a, &b)
+    }
+
+    /// Make `expected <: actual`
+    pub fn eq<T>(self,
+                 expected: T,
+                 actual: T)
+                 -> InferResult<'tcx, ()>
+        where T: ToTrace<'tcx>
+    {
+        self.trace(expected, actual).eq(&expected, &actual)
+    }
+
+    /// Compute the least-upper-bound, or mutual supertype, of two
+    /// values. The order of the arguments doesn't matter, but since
+    /// this can result in an error (e.g., if asked to compute LUB of
+    /// u32 and i32), it is meaningful to call one of them the
+    /// "expected type".
+    pub fn lub<T>(self,
+                  expected: T,
+                  actual: T)
+                  -> InferResult<'tcx, T>
+        where T: ToTrace<'tcx>
+    {
+        self.trace(expected, actual).lub(&expected, &actual)
+    }
+
+    /// Compute the greatest-lower-bound, or mutual subtype, of two
+    /// values. As with `lub` order doesn't matter, except for error
+    /// cases.
+    pub fn glb<T>(self,
+                  expected: T,
+                  actual: T)
+                  -> InferResult<'tcx, T>
+        where T: ToTrace<'tcx>
+    {
+        self.trace(expected, actual).glb(&expected, &actual)
+    }
+
+    /// Sets the "trace" values that will be used for
+    /// error-repporting, but doesn't actually perform any operation
+    /// yet (this is useful when you want to set the trace using
+    /// distinct values from those you wish to operate upon).
+    pub fn trace<T>(self,
+                    expected: T,
+                    actual: T)
+                    -> Trace<'a, 'gcx, 'tcx>
+        where T: ToTrace<'tcx>
+    {
+        self.trace_exp(true, expected, actual)
+    }
+
+    /// Like `trace`, but the expected value is determined by the
+    /// boolean argument (if true, then the first argument `a` is the
+    /// "expected" value).
+    pub fn trace_exp<T>(self,
+                        a_is_expected: bool,
+                        a: T,
+                        b: T)
+                        -> Trace<'a, 'gcx, 'tcx>
+        where T: ToTrace<'tcx>
+    {
+        let trace = ToTrace::to_trace(self.cause, a_is_expected, a, b);
+        Trace { at: self, trace: trace, a_is_expected }
+    }
+}
+
+impl<'a, 'gcx, 'tcx> Trace<'a, 'gcx, 'tcx> {
+    /// Make `a <: b` where `a` may or may not be expected (if
+    /// `a_is_expected` is true, then `a` is expected).
+    /// Make `expected <: actual`
+    pub fn sub<T>(self,
+                  a: &T,
+                  b: &T)
+                  -> InferResult<'tcx, ()>
+        where T: Relate<'tcx>
+    {
+        debug!("sub({:?} <: {:?})", a, b);
+        let Trace { at, trace, a_is_expected } = self;
+        at.infcx.commit_if_ok(|_| {
+            let mut fields = at.infcx.combine_fields(trace, at.param_env);
+            fields.sub(a_is_expected)
+                  .relate(a, b)
+                  .map(move |_| InferOk { value: (), obligations: fields.obligations })
+        })
+    }
+
+    /// Make `a == b`; the expectation is set by the call to
+    /// `trace()`.
+    pub fn eq<T>(self,
+                 a: &T,
+                 b: &T)
+                 -> InferResult<'tcx, ()>
+        where T: Relate<'tcx>
+    {
+        debug!("eq({:?} == {:?})", a, b);
+        let Trace { at, trace, a_is_expected } = self;
+        at.infcx.commit_if_ok(|_| {
+            let mut fields = at.infcx.combine_fields(trace, at.param_env);
+            fields.equate(a_is_expected)
+                  .relate(a, b)
+                  .map(move |_| InferOk { value: (), obligations: fields.obligations })
+        })
+    }
+
+    pub fn lub<T>(self,
+                  a: &T,
+                  b: &T)
+                  -> InferResult<'tcx, T>
+        where T: Relate<'tcx>
+    {
+        debug!("lub({:?} \\/ {:?})", a, b);
+        let Trace { at, trace, a_is_expected } = self;
+        at.infcx.commit_if_ok(|_| {
+            let mut fields = at.infcx.combine_fields(trace, at.param_env);
+            fields.lub(a_is_expected)
+                  .relate(a, b)
+                  .map(move |t| InferOk { value: t, obligations: fields.obligations })
+        })
+    }
+
+    pub fn glb<T>(self,
+                  a: &T,
+                  b: &T)
+                  -> InferResult<'tcx, T>
+        where T: Relate<'tcx>
+    {
+        debug!("glb({:?} /\\ {:?})", a, b);
+        let Trace { at, trace, a_is_expected } = self;
+        at.infcx.commit_if_ok(|_| {
+            let mut fields = at.infcx.combine_fields(trace, at.param_env);
+            fields.glb(a_is_expected)
+                  .relate(a, b)
+                  .map(move |t| InferOk { value: t, obligations: fields.obligations })
+        })
+    }
+}
+
+impl<'tcx> ToTrace<'tcx> for Ty<'tcx> {
+    fn to_trace(cause: &ObligationCause<'tcx>,
+                a_is_expected: bool,
+                a: Self,
+                b: Self)
+                -> TypeTrace<'tcx>
+    {
+        TypeTrace {
+            cause: cause.clone(),
+            values: Types(ExpectedFound::new(a_is_expected, a, b))
+        }
+    }
+}
+
+impl<'tcx> ToTrace<'tcx> for ty::TraitRef<'tcx> {
+    fn to_trace(cause: &ObligationCause<'tcx>,
+                a_is_expected: bool,
+                a: Self,
+                b: Self)
+                -> TypeTrace<'tcx>
+    {
+        TypeTrace {
+            cause: cause.clone(),
+            values: TraitRefs(ExpectedFound::new(a_is_expected, a, b))
+        }
+    }
+}
+
+impl<'tcx> ToTrace<'tcx> for ty::PolyTraitRef<'tcx> {
+    fn to_trace(cause: &ObligationCause<'tcx>,
+                a_is_expected: bool,
+                a: Self,
+                b: Self)
+                -> TypeTrace<'tcx>
+    {
+        TypeTrace {
+            cause: cause.clone(),
+            values: PolyTraitRefs(ExpectedFound::new(a_is_expected, a, b))
+        }
+    }
+}