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<h1 class="title">doc/manual/var_t_return</h1>
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<p>Memory safety for returning by <tt class="docutils literal"><span class="pre"><span class="Keyword">var</span> <span class="Identifier">T</span></span></tt> is ensured by a simple borrowing rule: If <tt class="docutils literal"><span class="pre"><span class="Identifier">result</span></span></tt> does not refer to a location pointing to the heap (that is in <tt class="docutils literal"><span class="pre"><span class="Identifier">result</span> <span class="Operator">=</span> <span class="Identifier">X</span></span></tt> the <tt class="docutils literal"><span class="pre"><span class="Identifier">X</span></span></tt> involves a <tt class="docutils literal"><span class="pre"><span class="Keyword">ptr</span></span></tt> or <tt class="docutils literal"><span class="pre"><span class="Keyword">ref</span></span></tt> access) then it has to be derived from the routine's first parameter:</p>
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<p><pre class="listing"><span class="Keyword">proc</span> <span class="Identifier">forward</span><span class="Punctuation">[</span><span class="Identifier">T</span><span class="Punctuation">]</span><span class="Punctuation">(</span><span class="Identifier">x</span><span class="Punctuation">:</span> <span class="Keyword">var</span> <span class="Identifier">T</span><span class="Punctuation">)</span><span class="Punctuation">:</span> <span class="Keyword">var</span> <span class="Identifier">T</span> <span class="Operator">=</span>
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<span class="Identifier">result</span> <span class="Operator">=</span> <span class="Identifier">x</span> <span class="Comment"># ok, derived from the first parameter.</span>
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<span class="Keyword">proc</span> <span class="Identifier">p</span><span class="Punctuation">(</span><span class="Identifier">param</span><span class="Punctuation">:</span> <span class="Keyword">var</span> <span class="Identifier">int</span><span class="Punctuation">)</span><span class="Punctuation">:</span> <span class="Keyword">var</span> <span class="Identifier">int</span> <span class="Operator">=</span>
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<span class="Keyword">var</span> <span class="Identifier">x</span><span class="Punctuation">:</span> <span class="Identifier">int</span>
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<span class="Comment"># we know 'forward' provides a view into the location derived from</span>
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<span class="Comment"># its first argument 'x'.</span>
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<span class="Identifier">result</span> <span class="Operator">=</span> <span class="Identifier">forward</span><span class="Punctuation">(</span><span class="Identifier">x</span><span class="Punctuation">)</span> <span class="Comment"># Error: location is derived from `x`</span>
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<span class="Comment"># which is not p's first parameter and lives</span>
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<span class="Comment"># on the stack.</span></pre></p>
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<p>In other words, the lifetime of what <tt class="docutils literal"><span class="pre"><span class="Identifier">result</span></span></tt> points to is attached to the lifetime of the first parameter and that is enough knowledge to verify memory safety at the call site. </p>
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