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lifetime extension: mention how projections are handled #63

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Jan 3, 2021
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lifetime extension: mention how projections are handled #63

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6 changes: 4 additions & 2 deletions promotion.md
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Expand Up @@ -13,8 +13,10 @@ There are a few different contexts where promotion is beneficial.
"Lifetime extension" is a mechanism that affects code like `&3`:
Instead of putting it on the stack, the `3` is allocated in global static memory
and a reference with lifetime `'static` is provided. This is essentially an
automatic transformation turning `&EXPR` into
`{ const _PROMOTED = &EXPR; _PROMOTED }`, but only if `EXPR` qualifies.
automatic transformation turning `&EXPR` into `{ const _PROMOTED = &EXPR;
_PROMOTED }`, but only if `EXPR` qualifies. Topmost projections are not
promoted, so `&EXPR.proj1.proj2` turns into `{ const _PROMOTED = &EXPR;
&(*_PROMOTED).proj1.proj2 }`.

Note that promotion happens on the MIR, not on surface-level syntax. This is
relevant when discussing e.g. handling of panics caused by overflowing
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