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lvalues are called places in Rust/MIR #40

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lvalues are called places in Rust/MIR #40

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4 changes: 2 additions & 2 deletions reference/src/introduction.md
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Expand Up @@ -8,7 +8,7 @@
* *niche*
* *layout*
* *tag*
* *rvalue*
* *lvalue*
* *place* (or *lvalue* in C/C++ speak)
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These are called "lvalues" in C, and "glvalues" in C++.

* *rvalue* (maybe we can come up with a Rust term for this as well?)
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These are called "values of the expression" in C, "prvalue" in C++.

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(and we should just call them "values")

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Isn't there a term to distinguish expressions like "3+2" from expressions like "x", where the latter can be used as lvalues but the former cannot?

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@strega-nil strega-nil Nov 3, 2018 •
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Yes, the latter is a (Rust) place expression, (C) lvalue expression, or (C++) glvalue expression, whereas the former is a (Rust) value expression, (C++) prvalue expression, and C doesn't have a special name for it.

In Rust, there's an automatic coercion from value expressions to place expressions in some specific places; and in C++, there's an automatic coercion from prvalue expressions to xvalue expressions (a type of glvalue expression).

If you want to convert from a place expression to a value expression it depends on what the kind of the place expression is:

  • in C it doesn't matter, you can always convert an lvalue expression to the value of the expression.
  • in Rust
    • if the type of the place expression is immovable (!Move), you cannot convert, in general, to a value expression.
    • otherwise, if the type of the place expression is !Copy, then the place expression must be owning in order to convert to a value expression
    • otherwise, the type is Copy, and you can always go from the place expression to the value expression.
  • in C++, a glvalue expression is either an lvalue or an xvalue
    • if the type is trivially copyable, then the value is just copied out of the glvalue expression
    • otherwise, if the expression is an lvalue, then the type must be copyable, and the copy constructor will be called; otherwise, compilation will fail
    • otherwise, if the expression is an xvalue, and the move constructor exists, then that is called
    • otherwise, the expression is an xvalue and the move constructor does not exist. If the copy constructor exists and takes a type const&, then that is called; otherwise, compilation fails.


## Unsafe abstraction